1
|
Calebiro D, Koszegi Z, Lanoiselée Y, Miljus T, O'Brien S. G protein-coupled receptor-G protein interactions: a single-molecule perspective. Physiol Rev 2020; 101:857-906. [PMID: 33331229 DOI: 10.1152/physrev.00021.2020] [Citation(s) in RCA: 50] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.
Collapse
Affiliation(s)
- Davide Calebiro
- Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom; Centre of Membrane Proteins and Receptors (COMPARE), Universities of Nottingham and Birmingham, Birmingham, United Kingdom
| | - Zsombor Koszegi
- Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom; Centre of Membrane Proteins and Receptors (COMPARE), Universities of Nottingham and Birmingham, Birmingham, United Kingdom
| | - Yann Lanoiselée
- Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom; Centre of Membrane Proteins and Receptors (COMPARE), Universities of Nottingham and Birmingham, Birmingham, United Kingdom
| | - Tamara Miljus
- Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom; Centre of Membrane Proteins and Receptors (COMPARE), Universities of Nottingham and Birmingham, Birmingham, United Kingdom
| | - Shannon O'Brien
- Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom; Centre of Membrane Proteins and Receptors (COMPARE), Universities of Nottingham and Birmingham, Birmingham, United Kingdom
| |
Collapse
|
2
|
Conley SM, Al-Ubaidi MR, Han Z, Naash MI. Rim formation is not a prerequisite for distribution of cone photoreceptor outer segment proteins. FASEB J 2014; 28:3468-79. [PMID: 24736412 DOI: 10.1096/fj.14-251397] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Retinal degeneration slow (RDS/PRPH2) is critical for the formation of the disc/lamella rim in photoreceptor outer segments (OSs), but plays a different role in rods vs. cones. Without RDS, rods fail to form OSs, however, cones lacking RDS (in the rds(-/-)/Nrl(-/-)) exhibit balloon-like OSs devoid of lamellae. We show that distribution of most proteins in the lamella and PM domains is preserved even in the absence of RDS, rim, and lamella structures. However, the rim protein prominin-1 exhibits altered trafficking and OS localization, suggesting that proper targeting and distribution of rim proteins may require RDS. Our ultrastructural studies show that in cones, OS formation is initiated by the growth of opsin-containing membrane with RDS-mediated rim formation as a secondary step. This is directly opposite to rods and significantly advances our understanding of the role of the rim in cone OS morphogenesis. Furthermore, our results suggest that the unique folded lamella architecture of the cone OS may maximize density or proximity of phototransduction proteins, but is not required for OS function or for protein distribution and retention in different membrane domains.
Collapse
Affiliation(s)
- Shannon M Conley
- Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, USA
| | - Muayyad R Al-Ubaidi
- Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, USA
| | - Zongchao Han
- Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, USA
| | - Muna I Naash
- Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, USA
| |
Collapse
|
3
|
Skiba NP, Spencer WJ, Salinas RY, Lieu EC, Thompson JW, Arshavsky VY. Proteomic identification of unique photoreceptor disc components reveals the presence of PRCD, a protein linked to retinal degeneration. J Proteome Res 2013; 12:3010-8. [PMID: 23672200 DOI: 10.1021/pr4003678] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Visual signal transduction takes place on the surface of flat membrane vesicles called photoreceptor discs, which reside inside the light-sensitive outer segment organelle of vertebrate photoreceptor cells. Although biochemical studies have indicated that discs are built with a handful of highly specialized proteins, proteomic studies have yielded databases consisting of hundreds of entries. We addressed this controversy by employing protein correlation profiling, which allows identification of unique components of organelles that can be fractionated but not purified to absolute homogeneity. We subjected discs to sequential steps of fractionation and identified the relative amounts of proteins in each fraction by label-free quantitative mass spectrometry. This analysis demonstrated that the photoreceptor disc proteome contains only eleven components, which satisfy the hallmark criterion for being unique disc-resident components: the retention of a constant molar ratio among themselves across fractionation steps. Remarkably, one of them is PRCD, a protein whose mutations have been shown to cause blindness, yet cellular localization remained completely unknown. Identification of PRCD as a novel disc-specific protein facilitates understanding its functional role and the pathobiological significance of its mutations. Our study provides a striking example how protein correlation profiling allows a distinction between constitutive components of cellular organelles and their inevitable contaminants.
Collapse
Affiliation(s)
- Nikolai P Skiba
- Albert Eye Research Institute, 2Institute for Genome Sciences & Policy, Duke University School of Medicine, Durham, North Carolina 27710, United States
| | | | | | | | | | | |
Collapse
|
4
|
Mao W, Miyagishima KJ, Yao Y, Soreghan B, Sampath AP, Chen J. Functional comparison of rod and cone Gα(t) on the regulation of light sensitivity. J Biol Chem 2013; 288:5257-67. [PMID: 23288843 DOI: 10.1074/jbc.m112.430058] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
The signaling cascades mediated by G protein-coupled receptors (GPCRs) exhibit a wide spectrum of spatial and temporal response properties to fulfill diverse physiological demands. However, the mechanisms that shape the signaling response of the GPCR are not well understood. In this study, we replaced cone transducin α (cTα) for rod transducin α (rTα) in rod photoreceptors of transgenic mice, which also express S opsin, to evaluate the role of Gα subtype on signal amplification from different GPCRs in the same cell; such analysis may explain functional differences between retinal rod and cone photoreceptors. We showed that ectopically expressed cTα 1) forms a heterotrimeric complex with rod Gβ(1)γ(1), 2) substitutes equally for rTα in generating photoresponses initiated by either rhodopsin or S-cone opsin, and 3) exhibited similar light-activated translocation as endogenous rTα in rods and endogenous cTα in cones. Thus, rTα and cTα appear functionally interchangeable. Interestingly, light sensitivity appeared to correlate with the concentration of cTα when expression is reduced below 35% of normal. However, quantification of endogenous cTα concentration in cones showed a higher level to rTα in rods. Thus, reduced sensitivity in cones cannot be explained by reduced coupling efficiency between the GPCR and G protein or a lower concentration of G protein in cones versus rods.
Collapse
Affiliation(s)
- Wen Mao
- Department of Cell and Neurobiology, Zilkha Neurogenetic Institute, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, USA
| | | | | | | | | | | |
Collapse
|
5
|
Inagaki S, Ghirlando R, White JF, Gvozdenovic-Jeremic J, Northup JK, Grisshammer R. Modulation of the interaction between neurotensin receptor NTS1 and Gq protein by lipid. J Mol Biol 2012; 417:95-111. [PMID: 22306739 DOI: 10.1016/j.jmb.2012.01.023] [Citation(s) in RCA: 87] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2011] [Revised: 01/09/2012] [Accepted: 01/13/2012] [Indexed: 12/11/2022]
Abstract
Membrane lipids have been implicated to influence the activity of G-protein-coupled receptors (GPCRs). Almost all of our knowledge on the role of lipids on GPCR and G protein function comes from work on the visual pigment rhodopsin and its G protein transducin, which reside in a highly specialized membrane environment. Thus, insight gained from rhodopsin signaling may not be simply translated to other nonvisual GPCRs. Here, we investigated the effect of lipid head group charges on the signal transduction properties of the class A GPCR neurotensin (NT) receptor 1 (NTS1) under defined experimental conditions, using self-assembled phospholipid nanodiscs prepared with the zwitter-ionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), the negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), or a POPC/POPG mixture. A combination of dynamic light scattering and sedimentation velocity showed that NTS1 was monomeric in POPC-, POPC/POPG-, and POPG-nanodiscs. Binding of the agonist NT to NTS1 occurred with similar affinities and was essentially unaffected by the phospholipid composition. In contrast, Gq protein coupling to NTS1 in various lipid nanodiscs was significantly different, and the apparent affinity of Gαq and Gβ(1)γ(1) to activated NTS1 increased with increasing POPG content. NTS1-catalyzed GDP/GTPγS nucleotide exchange at Gαq in the presence of Gβ(1)γ(1) and NT was crucially affected by the lipid type, with exchange rates higher by 1 or 2 orders of magnitude in POPC/POPG- and POPG-nanodiscs, respectively, compared to POPC-nanodiscs. Our data demonstrate that negatively charged lipids in the immediate vicinity of a nonvisual GPCR modulate the G-protein-coupling step.
Collapse
Affiliation(s)
- Sayaka Inagaki
- Membrane Protein Structure Function Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Department of Health and Human Services, Rockville, MD 20852, USA
| | | | | | | | | | | |
Collapse
|
6
|
Arshavsky VY, Burns ME. Photoreceptor signaling: supporting vision across a wide range of light intensities. J Biol Chem 2011; 287:1620-6. [PMID: 22074925 DOI: 10.1074/jbc.r111.305243] [Citation(s) in RCA: 148] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
For decades, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and biochemistry and for understanding many facets of neuronal cell biology. In recent years, new knowledge of the kinetics of signaling and the large-scale movements of proteins underlying signaling has led to a deeper appreciation of the photoreceptor's unique challenge in mediating the first steps in vision over a wide range of light intensities.
Collapse
Affiliation(s)
- Vadim Y Arshavsky
- Departments of Ophthalmology and Pharmacology, Duke University, Durham, North Carolina 27710, USA.
| | | |
Collapse
|
7
|
Abstract
A fundamental question of cell signaling biology is how faint external signals produce robust physiological responses. One universal mechanism relies on signal amplification via intracellular cascades mediated by heterotrimeric G-proteins. This high amplification system allows retinal rod photoreceptors to detect single photons of light. Although much is now known about the role of the α-subunit of the rod-specific G-protein transducin in phototransduction, the physiological function of the auxiliary βγ-complex in this process remains a mystery. Here, we show that elimination of the transducin γ-subunit drastically reduces signal amplification in intact mouse rods. The consequence is a striking decline in rod visual sensitivity and severe impairment of nocturnal vision. Our findings demonstrate that transducin βγ-complex controls signal amplification of the rod phototransduction cascade and is critical for the ability of rod photoreceptors to function in low light conditions.
Collapse
|
8
|
Gopalakrishna KN, Doddapuneni K, Boyd KK, Masuho I, Martemyanov KA, Artemyev NO. Interaction of transducin with uncoordinated 119 protein (UNC119): implications for the model of transducin trafficking in rod photoreceptors. J Biol Chem 2011; 286:28954-28962. [PMID: 21712387 DOI: 10.1074/jbc.m111.268821] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The key visual G protein, transducin undergoes bi-directional translocations between the outer segment (OS) and inner compartments of rod photoreceptors in a light-dependent manner thereby contributing to adaptation and neuroprotection of rods. A mammalian uncoordinated 119 protein (UNC119), also known as Retina Gene 4 protein (RG4), has been recently implicated in transducin transport to the OS in the dark through its interaction with the N-acylated GTP-bound transducin-α subunit (Gα(t1)). Here, we demonstrate that the interaction of human UNC119 (HRG4) with transducin is dependent on the N-acylation, but does not require the GTP-bound form of Gα(t1). The lipid specificity of UNC119 is unique: UNC119 bound the myristoylated N terminus of Gα(t1) with much higher affinity than a prenylated substrate, whereas the homologous prenyl-binding protein PrBP/δ did not interact with the myristoylated peptide. UNC119 was capable of interacting with Gα(t1)GDP as well as with heterotrimeric transducin (G(t)). This interaction of UNC119 with G(t) led to displacement of Gβ(1)γ(1) from the heterotrimer. Furthermore, UNC119 facilitated solubilization of G(t) from dark-adapted rod OS membranes. Consistent with these observations, UNC119 inhibited rhodopsin-dependent activation of G(t), but had no effect on the GTP-hydrolysis by Gα(t1). A model for the role of UNC119 in the IS→OS translocation of G(t) is proposed based on the UNC119 ability to dissociate G(t) subunits from each other and the membrane. We also found that UNC119 inhibited activation of G(o) by D2 dopamine receptor in cultured cells. Thus, UNC119 may play conserved inhibitory role in regulation of GPCR-G protein signaling in non-visual tissues.
Collapse
Affiliation(s)
- Kota N Gopalakrishna
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, Iowa 52242
| | - Krishnarao Doddapuneni
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, Iowa 52242
| | - Kimberly K Boyd
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, Iowa 52242
| | - Ikuo Masuho
- Department of Ophthalmology and Visual Sciences, University of Iowa, Iowa City, Iowa 52242 and
| | - Kirill A Martemyanov
- Department of Ophthalmology and Visual Sciences, University of Iowa, Iowa City, Iowa 52242 and
| | - Nikolai O Artemyev
- Department of Molecular Physiology and Biophysics, University of Iowa, Iowa City, Iowa 52242; Department of Neuroscience, The Scripps Research Institute, Jupiter, Florida 33458.
| |
Collapse
|
9
|
Diffusion and light-dependent compartmentalization of transducin. Mol Cell Neurosci 2010; 46:340-6. [PMID: 21044685 DOI: 10.1016/j.mcn.2010.10.006] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2010] [Revised: 09/17/2010] [Accepted: 10/26/2010] [Indexed: 11/21/2022] Open
Abstract
Diffusion and light-dependent compartmentalization of transducin are essential for phototransduction and light adaptation of rod photoreceptors. Here, transgenic Xenopus laevis models were designed to probe the roles of transducin/rhodopsin interactions and lipid modifications in transducin compartmentalization, membrane mobility, and light-induced translocation. Localization and diffusion of EGFP-fused rod transducin-α subunit (Gα(t1)), mutant Gα(t1) that is predicted to be N-acylated and S-palmitoylated (Gα(t1)A3C), and mutant Gα(t1) uncoupled from light-activated rhodopsin (Gα(t1)-Ctα(s)), were examined by EGFP-fluorescence imaging and fluorescence recovery after photobleaching (FRAP). Similar to Gα(t1), Gα(t1)A3C and Gα(t1)-Ctα(s) were correctly targeted to the rod outer segments in the dark, however the light-dependent translocation of both mutants was markedly impaired. Our analysis revealed a moderate acceleration of the lateral diffusion for the activated Gα(t1) consistent with the diffusion of the separated Gα(t1)GTP and Gβ(1)γ(1) on the membrane surface. Unexpectedly, the kinetics of longitudinal diffusion were comparable for Gα(t1)GTP with a single lipid anchor and heterotrimeric Gα(t1)β(1)γ(1) or Gα(t1)-Ctα(s)β(1)γ(1) with two lipid modifications. This contrasted the lack of the longitudinal diffusion of the Gα(t1)A3C mutant apparently caused by its stable two lipid attachment to the membrane and suggests the existence of a mechanism that facilitates axial diffusion of Gα(t1)β(1)γ(1).
Collapse
|
10
|
|
11
|
Kosloff M, Alexov E, Arshavsky VY, Honig B. Electrostatic and lipid anchor contributions to the interaction of transducin with membranes: mechanistic implications for activation and translocation. J Biol Chem 2008; 283:31197-207. [PMID: 18782760 PMCID: PMC2576562 DOI: 10.1074/jbc.m803799200] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The heterotrimeric G protein transducin is a key component of the
vertebrate phototransduction cascade. Transducin is peripherally attached to
membranes of the rod outer segment, where it interacts with other proteins at
the membrane-cytosol interface. However, upon sustained activation by light,
the dissociated Gtα and
Gβ1γ1 subunits of transducin translocate from
the outer segment to other parts of the rod cell. Here we used a computational
approach to analyze the interaction strength of transducin and its subunits
with acidic lipid bilayers, as well as the range of orientations that they are
allowed to occupy on the membrane surface. Our results suggest that the
combined constraints of electrostatics and lipid anchors substantially limit
the rotational degrees of freedom of the membrane-bound transducin
heterotrimer. This may contribute to a faster transducin activation rate by
accelerating transducin-rhodopsin complex formation. Notably, the membrane
interactions of the dissociated transducin subunits are very different from
those of the heterotrimer. As shown previously,
Gβ1γ1 experiences significant attractive
interactions with negatively charged membranes, whereas our new results
suggest that Gtα is electrostatically repelled by such
membranes. We suggest that this repulsion could facilitate the membrane
dissociation and intracellular translocation of Gtα.
Moreover, based on similarities in sequence and electrostatic properties, we
propose that the properties described for transducin are common to its
homologs within the Gi subfamily. In a broader view, this work
exemplifies how the activity-dependent association and dissociation of a G
protein can change both the affinity for membranes and the range of allowed
orientations, thereby modulating G protein function.
Collapse
Affiliation(s)
- Mickey Kosloff
- Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY, USA
| | | | | | | |
Collapse
|
12
|
Preininger AM, Parello J, Meier SM, Liao G, Hamm HE. Receptor-mediated changes at the myristoylated amino terminus of Galpha(il) proteins. Biochemistry 2008; 47:10281-93. [PMID: 18771287 DOI: 10.1021/bi800741r] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
G protein-coupled receptors (GPCRs) catalyze nucleotide release in heterotrimeric G proteins, the slow step in G protein activation. G i/o family proteins are permanently, cotranslationally myristoylated at the extreme amino terminus. While myristoylation of the amino terminus has long been known to aid in anchoring G i proteins to the membrane, the role of myristoylation with regard to interaction with activated receptors is not known. Previous studies have characterized activation-dependent changes in the amino terminus of Galpha proteins in solution [Medkova, M. (2002) Biochemistry 41, 9963-9972; Preininger, A. M. (2003) Biochemistry 42, 7931-7941], but changes in the environment of specific residues within the Galpha i1 amino terminus during receptor-mediated G i activation have not been reported. Using site-specific fluorescence labeling of individual residues along a stretch of the Galpha il amino terminus, we found specific changes in the environment of these residues upon interaction with the activated receptor and following GTPgammaS binding. These changes map to a distinct surface of the amino-terminal helix opposite the Gbetagamma binding interface. The receptor-dependent fluorescence changes are consistent with a myristoylated amino terminus in the proximity of the membrane and/or receptor. Myristoylation affects both the rate and intensity of receptor activation-dependent changes detected at several residues along the amino terminus (with no significant effect on the rate of receptor-mediated GTPgammaS binding). This work demonstrates that the myristoylated amino terminus of Galpha il proteins undergoes receptor-mediated changes during the dynamic process of G protein signaling.
Collapse
Affiliation(s)
- Anita M Preininger
- Department of Pharmacology, Vanderbilt University Medical Center, Nashville, Tennessee 37232-6600, USA
| | | | | | | | | |
Collapse
|
13
|
Vögler O, Barceló JM, Ribas C, Escribá PV. Membrane interactions of G proteins and other related proteins. BIOCHIMICA ET BIOPHYSICA ACTA 2008; 1778:1640-52. [PMID: 18402765 DOI: 10.1016/j.bbamem.2008.03.008] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/11/2007] [Revised: 03/01/2008] [Accepted: 03/12/2008] [Indexed: 01/25/2023]
Abstract
Guanine nucleotide-binding proteins, G proteins, propagate incoming messages from receptors to effector proteins. They switch from an inactive to active state by exchanging a GDP molecule for GTP, and they return to the inactive form by hydrolyzing GTP to GDP. Small monomeric G proteins, such as Ras, are involved in controlling cell proliferation, differentiation and apoptosis, and they interact with membranes through isoprenyl moieties, fatty acyl moieties, and electrostatic interactions. This protein-lipid binding facilitates productive encounters of Ras and Raf proteins in defined membrane regions, so that signals can subsequently proceed through MEK and ERK kinases, which constitute the canonical MAP kinase signaling cassette. On the other hand, heterotrimeric G proteins undergo co/post-translational modifications in the alpha (myristic and/or palmitic acid) and the gamma (farnesol or geranylgeraniol) subunits. These modifications not only assist the G protein to localize to the membrane but they also help distribute the heterotrimer (Galphabetagamma) and the subunits generated upon activation (Galpha and Gbetagamma) to appropriate membrane microdomains. These proteins transduce messages from ubiquitous serpentine receptors, which control important functions such as taste, vision, blood pressure, body weight, cell proliferation, mood, etc. Moreover, the exchange of GDP by GTP is triggered by nucleotide exchange factors. Membrane receptors that activate G proteins can be considered as such, but other cytosolic, membranal or amphitropic proteins can accelerate the rate of G protein exchange or even activate this process in the absence of receptor-mediated activation. These and other protein-protein interactions of G proteins with other signaling proteins are regulated by their lipid preferences. Thus, G protein-lipid interactions control the features of messages and cell physiology.
Collapse
Affiliation(s)
- Oliver Vögler
- Molecular Cell Biomedicine, Department of Biology-IUNICS, Universitat de les Illes Balears, Palma de Mallorca, Spain
| | | | | | | |
Collapse
|
14
|
Wensel TG. Signal transducing membrane complexes of photoreceptor outer segments. Vision Res 2008; 48:2052-61. [PMID: 18456304 DOI: 10.1016/j.visres.2008.03.010] [Citation(s) in RCA: 97] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2008] [Revised: 03/17/2008] [Accepted: 03/19/2008] [Indexed: 11/25/2022]
Abstract
Signal transduction in outer segments of vertebrate photoreceptors is mediated by a series of reactions among multiple polypeptides that form protein-protein complexes within or on the surface of the disk and plasma membranes. The individual components in the activation reactions include the photon receptor rhodopsin and the products of its absorption of light, the three subunits of the G protein, transducin, the four subunits of the cGMP phosphodiesterase, PDE6 and the four subunits of the cGMP-gated cation channel. Recovery involves membrane complexes with additional polypeptides including the Na(+)/Ca(2+), K(+) exchanger, NCKX2, rhodopsin kinases RK1 and RK7, arrestin, guanylate cyclases, guanylate cyclase activating proteins, GCAP1 and GCAP2, and the GTPase accelerating complex of RGS9-1, G(beta5L), and membrane anchor R9AP. Modes of membrane binding by these polypeptides include transmembrane helices, fatty acyl or isoprenyl modifications, polar interactions with lipid head groups, non-polar interactions of hydrophobic side chains with lipid hydrocarbon phase, and both polar and non-polar protein-protein interactions. In the course of signal transduction, complexes among these polypeptides form and dissociate, and undergo structural rearrangements that are coupled to their interactions with and catalysis of reactions by small molecules and ions, including guanine nucleotides, ATP, Ca(2+), Mg(2+), and lipids. The substantial progress that has been made in understanding the composition and function of these complexes is reviewed, along with the more preliminary state of our understanding of the structures of these complexes and the challenges and opportunities that present themselves for deepening our understanding of these complexes, and how they work together to convert a light signal into an electrical signal.
Collapse
Affiliation(s)
- Theodore G Wensel
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA.
| |
Collapse
|
15
|
Light-dependent compartmentalization of transducin in rod photoreceptors. Mol Neurobiol 2008; 37:44-51. [PMID: 18425604 DOI: 10.1007/s12035-008-8015-2] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2008] [Accepted: 03/17/2008] [Indexed: 10/22/2022]
Abstract
Three major visual signaling proteins, transducin, arrestin, and recoverin undergo bidirectional translocations between the outer segment and inner compartments of rod photoreceptors in a light-dependent manner. The light-dependent translocation of proteins is believed to contribute to adaptation and neuroprotection of photoreceptor cells. The potential physiological significance and mechanisms of light-controlled protein translocations are at the center of current discussion. In this paper, I outline the latest advances in understanding the mechanisms of bidirectional translocation of transducin and determinants of its steady-state distribution in dark- and light-adapted photoreceptor cells.
Collapse
|
16
|
Strickfaden SC, Pryciak PM. Distinct roles for two Galpha-Gbeta interfaces in cell polarity control by a yeast heterotrimeric G protein. Mol Biol Cell 2007; 19:181-97. [PMID: 17978098 DOI: 10.1091/mbc.e07-04-0385] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Saccharomyces cerevisiae mating pheromones trigger dissociation of a heterotrimeric G protein (Galphabetagamma) into Galpha-guanosine triphosphate (GTP) and Gbetagamma. The Gbetagamma dimer regulates both mitogen-activated protein (MAP) kinase cascade signaling and cell polarization. Here, by independently activating the MAP kinase pathway, we studied the polarity role of Gbetagamma in isolation from its signaling role. MAP kinase signaling alone could induce cell asymmetry but not directional growth. Surprisingly, active Gbetagamma, either alone or with Galpha-GTP, could not organize a persistent polarization axis. Instead, following pheromone gradients (chemotropism) or directional growth without pheromone gradients (de novo polarization) required an intact receptor-Galphabetagamma module and GTP hydrolysis by Galpha. Our results indicate that chemoattractant-induced cell polarization requires continuous receptor-Galphabetagamma communication but not modulation of MAP kinase signaling. To explore regulation of Gbetagamma by Galpha, we mutated Gbeta residues in two structurally distinct Galpha-Gbeta binding interfaces. Polarity control was disrupted only by mutations in the N-terminal interface, and not the Switch interface. Incorporation of these mutations into a Gbeta-Galpha fusion protein, which enforces subunit proximity, revealed that Switch interface dissociation regulates signaling, whereas the N-terminal interface may govern receptor-Galphabetagamma coupling. These findings raise the possibility that the Galphabetagamma heterotrimer can function in a partially dissociated state, tethered by the N-terminal interface.
Collapse
Affiliation(s)
- Shelly C Strickfaden
- Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, MA 01605, USA
| | | |
Collapse
|
17
|
Ernst OP, Gramse V, Kolbe M, Hofmann KP, Heck M. Monomeric G protein-coupled receptor rhodopsin in solution activates its G protein transducin at the diffusion limit. Proc Natl Acad Sci U S A 2007; 104:10859-64. [PMID: 17578920 PMCID: PMC1904172 DOI: 10.1073/pnas.0701967104] [Citation(s) in RCA: 180] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2007] [Indexed: 11/18/2022] Open
Abstract
G protein-coupled receptors mediate biological signals by stimulating nucleotide exchange in heterotrimeric G proteins (Galphabetagamma). Receptor dimers have been proposed as the functional unit responsible for catalytic interaction with Galphabetagamma. To investigate whether a G protein-coupled receptor monomer can activate Galphabetagamma, we used the retinal photoreceptor rhodopsin and its cognate G protein transducin (G(t)) to determine the stoichiometry of rhodopsin/G(t) binding and the rate of catalyzed nucleotide exchange in G(t). Purified rhodopsin was prepared in dodecyl maltoside detergent solution. Rhodopsin was monomeric as concluded from fluorescence resonance energy transfer, copurification studies with fluorescent labeled and unlabeled rhodopsin, size exclusion chromatography, and multiangle laser light scattering. A 1:1 complex between light-activated rhodopsin and G(t) was found in the elution profiles, and one molecule of GDP was released upon complex formation. Analysis of the speed of catalytic rhodopsin/G(t) interaction yielded a maximum of approximately 50 G(t) molecules per second and molecule of activated rhodopsin. The bimolecular rate constant is close to the diffusion limit in the diluted system. The results show that the interaction of G(t) with an activated rhodopsin monomer is sufficient for fully functional G(t) activation. Although the activation rate in solution is at the physically possible limit, the rate in the native membrane is still 10-fold higher. This is likely attributable to the precise orientation of the G protein to the membrane surface, which enables a fast docking process preceding the actual activation step. Whether docking in membranes involves the formation of rhodopsin dimers or oligomers remains to be elucidated.
Collapse
Affiliation(s)
- Oliver P Ernst
- Institut für Medizinische Physik und Biophysik, Charité-Universitätsmedizin Berlin, Charitéplatz 1, D-10117 Berlin, Germany.
| | | | | | | | | |
Collapse
|
18
|
Rosenzweig DH, Nair KS, Wei J, Wang Q, Garwin G, Saari JC, Chen CK, Smrcka AV, Swaroop A, Lem J, Hurley JB, Slepak VZ. Subunit dissociation and diffusion determine the subcellular localization of rod and cone transducins. J Neurosci 2007; 27:5484-94. [PMID: 17507570 PMCID: PMC2655354 DOI: 10.1523/jneurosci.1421-07.2007] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
Activation of rod photoreceptors by light induces a massive redistribution of the heterotrimeric G-protein transducin. In darkness, transducin is sequestered within the membrane-enriched outer segments of the rod cell. In light, it disperses throughout the entire neuron. We show here that redistribution of rod transducin by light requires activation, but it does not require ATP. This observation rules out participation of molecular motors in the redistribution process. In contrast to the light-stimulated redistribution of rod transducin in rods, cone transducin in cones does not redistribute during activation. Remarkably, when cone transducin is expressed in rods, it does undergo light-stimulated redistribution. We show here that the difference in subcellular localization of activated rod and cone G-proteins correlates with their affinity for membranes. Activated rod transducin releases from membranes, whereas activated cone transducin remains bound to membranes. A synthetic peptide that dissociates G-protein complexes independently of activation facilitates dispersion of both rod and cone transducins within the cells. This peptide also facilitates detachment of both G-proteins from the membranes. Together, these results show that it is the dissociation state of transducin that determines its localization in photoreceptors. When rod transducin is stimulated, its subunits dissociate, leave outer segment membranes, and equilibrate throughout the cell. Cone transducin subunits do not dissociate during activation and remain sequestered within the outer segment. These findings indicate that the subunits of some heterotrimeric G-proteins remain associated during activation in their native environments.
Collapse
Affiliation(s)
- Derek H. Rosenzweig
- Department of Molecular and Cellular Pharmacology and Neuroscience Program, University of Miami Miller School of Medicine, Miami, Florida 33136
| | - K. Saidas Nair
- Department of Molecular and Cellular Pharmacology and Neuroscience Program, University of Miami Miller School of Medicine, Miami, Florida 33136
| | | | - Qiang Wang
- Department of Molecular and Cellular Pharmacology and Neuroscience Program, University of Miami Miller School of Medicine, Miami, Florida 33136
| | - Greg Garwin
- Ophthalmology, University of Washington, Seattle, Washington 98195
| | - John C. Saari
- Ophthalmology, University of Washington, Seattle, Washington 98195
| | - Ching-Kang Chen
- Department of Biochemistry, Virginia Commonwealth University, Richmond, Virginia 23284
| | - Alan V. Smrcka
- Department of Pharmacology and Physiology, University of Rochester School of Medicine and Dentistry, Rochester, New York 14603
| | - Anand Swaroop
- Departments of Ophthalmology and Visual Sciences, and Human Genetics, University of Michigan, Ann Arbor, Michigan 48109, and
| | - Janis Lem
- Molecular Cardiology Research Institute, Tufts–New England Medical Center, Boston, Massachusetts 02111
| | | | - Vladlen Z. Slepak
- Department of Molecular and Cellular Pharmacology and Neuroscience Program, University of Miami Miller School of Medicine, Miami, Florida 33136
| |
Collapse
|
19
|
Herrmann R, Heck M, Henklein P, Hofmann KP, Ernst OP. Signal Transfer from GPCRs to G Proteins. J Biol Chem 2006; 281:30234-41. [PMID: 16847064 DOI: 10.1074/jbc.m600797200] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Catalysis of nucleotide exchange in heterotrimeric G proteins (Galphabetagamma) is a key step in cellular signal transduction mediated by G protein-coupled receptors. The Galpha N terminus with its helical stretch is thought to be crucial for G protein/activated receptor (R(*)) interaction. The N-terminal fatty acylation of Galpha is important for membrane targeting of G proteins. By applying biophysical techniques to the rhodopsin/transducin model system, we studied the effect of N-terminal truncations in Galpha. In Galphabetagamma, lack of the fatty acid and Galpha truncations up to 33 amino acids had little effect on R(*) binding and R(*)-catalyzed nucleotide exchange, implying that this region is not mandatory for R(*)/Galphabetagamma interaction. However, when the other hydrophobic modification of Galphabetagamma, the Ggamma C-terminal farnesyl moiety, is lacking, R(*) interaction requires the fatty acylated Galpha N terminus. This suggests that the two hydrophobic extensions can replace each other in the interaction of Galphabetagamma with R(*). We propose that in native Galphabetagamma, these two terminal regions are functionally redundant and form a microdomain that serves both to anchor the G protein to the membrane and to establish an initial docking complex with R(*). Accordingly, we find that the native fatty acylated Galpha is competent to interact with R(*) even in the absence of Gbetagamma, whereas nonacylated Galpha requires Gbetagamma for interaction. Experiments with N-terminally truncated Galpha subunits suggest that in the second step of the catalytic process, the receptor binds to the alphaN/beta1-loop region of Galpha to reduce nucleotide affinity and to make the Galpha C terminus available for subsequent interaction with R(*).
Collapse
Affiliation(s)
- Rolf Herrmann
- Institut für Medizinische Physik und Biophysik, Charité-Universitätsmedizin Berlin, Schumannstrasse 20/21, D-10098 Berlin, Germany
| | | | | | | | | |
Collapse
|
20
|
Mittal V, Linder M. Biochemical characterization of RGS14: RGS14 activity towards G-protein alpha subunits is independent of its binding to Rap2A. Biochem J 2006; 394:309-15. [PMID: 16246175 PMCID: PMC1386029 DOI: 10.1042/bj20051086] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
RGS (regulators of G-protein signalling) modulate signalling by acting as GAPs (GTPase-activating proteins) for alpha subunits of heterotrimeric G-proteins. RGS14 accelerates GTP hydrolysis by G(ialpha) family members through its RGS domain and suppresses guanine nucleotide dissociation from G(ialpha1) and G(ialpha3) subunits through its C-terminal GoLoco domain. Additionally, RGS14 binds the activated forms of the small GTPases Rap1 and Rap2 by virtue of tandem RBDs (Raf-like Ras/Rap binding domains). RGS14 was identified in a screen for Rap2 effectors [Traver, Splingard, Gaudriault and De Gunzburg (2004) Biochem. J. 379, 627-632]. In the present study, we tested whether Rap binding regulates RGS14's biochemical activities. We found that RGS14 activity towards heterotrimeric G-proteins, as either a GAP or a GDI (guanine nucleotide dissociation inhibitor), was unaffected by Rap binding. Extending our biochemical characterization of RGS14, we also examined whether RGS14 can suppress guanine nucleotide exchange on G(ialpha1) in the context of the heterotrimer. We found that a heterotrimer composed of N-myristoylated G(ialpha1) and prenylated G(betagamma) is resistant to the GDI activity of the GoLoco domain of RGS14. This is consistent with models of GoLoco domain action on free G(alpha) and suggests that RGS14 alone cannot induce subunit dissociation to promote receptor-independent activation of G(betagamma)-mediated signalling pathways.
Collapse
Affiliation(s)
- Vivek Mittal
- Department of Cell Biology and Physiology, Washington University School of Medicine, 660 South Euclid Ave., Campus Box 8228, St. Louis, MO 63110, U.S.A
| | - Maurine E. Linder
- Department of Cell Biology and Physiology, Washington University School of Medicine, 660 South Euclid Ave., Campus Box 8228, St. Louis, MO 63110, U.S.A
- To whom correspondence should be addressed (email )
| |
Collapse
|
21
|
Grant JE, Guo LW, Vestling MM, Martemyanov KA, Arshavsky VY, Ruoho AE. The N terminus of GTP gamma S-activated transducin alpha-subunit interacts with the C terminus of the cGMP phosphodiesterase gamma-subunit. J Biol Chem 2006; 281:6194-202. [PMID: 16407279 DOI: 10.1074/jbc.m509511200] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Dynamic regulation of G-protein signaling in the phototransduction cascade ensures the high temporal resolution of vision. In a key step, the activated alpha-subunit of transducin (Galphat-GTP) activates the cGMP phosphodiesterase (PDE) by binding the inhibitory gamma-subunit (PDEgamma). Significant progress in understanding the interaction between Galphat and PDEgamma was achieved by solving the crystal structure of the PDEgamma C-terminal peptide bound to Galphat in the transition state for GTP hydrolysis (Slep, K. C., Kercher, M. A., He, W., Cowan, C. W., Wensel, T. G., and Sigler, P. B. (2001) Nature 409, 1071-1077). However, some of the structural elements of each molecule were absent in the crystal structure. We have probed the binding surface between the PDEgamma C terminus and activated Galphat bound to guanosine 5'-O-(3-thio)-triphosphate (GTPgammaS) using a series of full-length PDEgamma photoprobes generated by intein-mediated expressed protein ligation. For each of seven PDEgamma photoprobe species, expressed protein ligation allowed one benzoyl-L-phenylalaine substitution at selected hydrophobic C-terminal positions, and the addition of a biotin affinity tag at the extreme C terminus. We have detected photocross-linking from several PDEgamma C-terminal positions to the Galphat-GTPgammaS N terminus, particularly from PDEgamma residue 73. The overall percentage of cross-linking to the Galphat-GTPgammaSN terminus was analyzed using a far Western method for examining Galphat-GTPgammaS proteolytic digestion patterns. Furthermore, mass spectrometric analysis of cross-links to Galphat from a benzoyl-phenylalanine replacement at PDEgamma position 86 localized the region of photoinsertion to Galphat N-terminal residues Galphat-(22-26). This novel Galphat/PDEgamma interaction suggests that the transducin N terminus plays an active role in signal transduction.
Collapse
Affiliation(s)
- Jennifer E Grant
- Department of Pharmacology, University of Wisconsin Medical School, Madison, 53706, USA
| | | | | | | | | | | |
Collapse
|
22
|
Sokolov M, Strissel KJ, Leskov IB, Michaud NA, Govardovskii VI, Arshavsky VY. Phosducin facilitates light-driven transducin translocation in rod photoreceptors. Evidence from the phosducin knockout mouse. J Biol Chem 2004; 279:19149-56. [PMID: 14973130 DOI: 10.1074/jbc.m311058200] [Citation(s) in RCA: 96] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Phosducin is a photoreceptor-specific protein known to interact with the beta gamma subunits of G proteins. In pursuit of the function of phosducin, we tested the hypothesis that it regulates the light-driven translocation of G protein transducin from the outer segments of rod photoreceptors to other compartments of the rod cell. Transducin translocation has been previously shown to contribute to rod adaptation to bright illumination, yet the molecular mechanisms underlying the translocation phenomenon remain unknown. In this study we provide two major lines of evidence in support of the role of phosducin in transducin translocation. First, we have demonstrated that transducin beta gamma subunits interact with phosducin along their entire intracellular translocation route, as evident from their co-precipitation in serial tangential sections from light-adapted but not dark-adapted retinas. Second, we generated a phosducin knockout mouse and found that the degree of light-driven transducin translocation in the rods of these mice was significantly reduced as compared with that observed in the rods of wild type animals. In knockout animals the translocation of transducin beta gamma subunits was affected to a larger degree than the translocation of the alpha subunit. We also found that the amount of phosducin in rods is sufficient to interact with practically all of the transducin present in these cells and that the subcellular distribution of phosducin is consistent with that of a soluble protein evenly distributed throughout the entire rod cytoplasm. Together, these data indicate that phosducin binding to transducin beta gamma subunits facilitates transducin translocation. We suggest that the mechanism of phosducin action is based on the reduction of transducin affinity to the membranes of rod outer segments, achieved by keeping the transducin beta gamma subunits apart from the alpha subunit. This increased solubility of transducin would make it more susceptible to translocation from the outer segments.
Collapse
Affiliation(s)
- Maxim Sokolov
- Department of Ophthalmology, Harvard Medical School and the Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, USA
| | | | | | | | | | | |
Collapse
|
23
|
Dietrich A, Scheer A, Illenberger D, Kloog Y, Henis YI, Gierschik P. Studies on G-protein alpha.betagamma heterotrimer formation reveal a putative S-prenyl-binding site in the alpha subunit. Biochem J 2003; 376:449-56. [PMID: 12952523 PMCID: PMC1223783 DOI: 10.1042/bj20030578] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2003] [Revised: 08/14/2003] [Accepted: 09/03/2003] [Indexed: 11/17/2022]
Abstract
The alpha and betagamma subunits of heterotrimeric G-proteins contain specific lipid modifications, which are required for their biological function. However, the relevance of these modifications to the interactions within the heterotrimeric G-protein is not fully understood. In order to explore the role of the S-prenyl moiety of the isoprenylated betagamma dimer of retinal transducin, betagamma(t), in the formation of the heterotrimeric complex with the corresponding N-acylated alpha subunit, alpha(t), we employed purified fully processed subunits, which are soluble in aqueous solutions without detergents. Pertussis-toxin-mediated [(32)P]ADP-ribosylation of alpha(t) is strongly stimulated (approximately 10-fold) in the presence of betagamma(t) and can thus serve as a measure for heterotrimer formation. Using this assay, preincubation of alpha(t) with S-prenyl analogues containing farnesyl or geranylgeranyl moieties was found to inhibit heterotrimer formation in a dose-dependent manner. The inhibition was competitive and reversible, as indicated by its reversal upon increase of the betagamma(t) dimer concentration or by removal of the S-prenyl analogue using gel filtration. The competitive nature of the inhibition is supported by the marked attenuation of the inhibition when the S-prenyl analogue was added to alpha(t) together with or after betagamma(t). The inhibition does not involve interaction with the alpha(t) acyl group, since an S-prenyl analogue inhibited the [(32)P]ADP-ribosylation of an unlipidated alpha(t) mutant. These data suggest the existence of a hitherto unrecognized S-prenyl-binding site in alpha(t), which is critical for its interaction with prenylated betagamma(t).
Collapse
Affiliation(s)
- Alexander Dietrich
- Department of Pharmacology and Toxicology, University of Ulm, 89069 Ulm, Germany
| | | | | | | | | | | |
Collapse
|
24
|
Wolfrum U, Giessl A, Pulvermüller A. Centrins, a novel group of Ca2+-binding proteins in vertebrate photoreceptor cells. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2003; 514:155-78. [PMID: 12596921 DOI: 10.1007/978-1-4615-0121-3_10] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/24/2023]
Abstract
Changes in the intracellular Ca2+-concentration affect the visual signal transduction cascade directly or more often indirectly through Ca2+-binding proteins. Here we review recent findings on centrins in photoreceptor cells of the mammalian retina. Centrins are members of a highly conserved subgroup of the EF-hand superfamily of Ca2+-binding proteins commonly associated with centrosome-related structures. In vertebrate photoreceptor cells, centrins are also prominent components in the connecting cilium linking the light sensitive outer segment with the biosynthetically active inner segment compartment. Recent findings demonstrate that Ca2+-activated centrin forms a complex with the visual G-protein transducin in photoreceptor cells. This Ca2+-dependent assembly of G-proteins with centrin is a novel aspect of the supply of signaling proteins in sensory cells, and a potential link between molecular translocations and signal transduction in general.
Collapse
Affiliation(s)
- Uwe Wolfrum
- Institut für Zoologie, Johannes Gutenberg-Universitit Mainz, 55099 Mainz, Germany.
| | | | | |
Collapse
|
25
|
Hessel E, Heck M, Müller P, Herrmann A, Hofmann KP. Signal transduction in the visual cascade involves specific lipid-protein interactions. J Biol Chem 2003; 278:22853-60. [PMID: 12676942 DOI: 10.1074/jbc.m302747200] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In retinal rod photoreceptor cells, transducin (Gt) and cyclic GMP phosphodiesterase (PDE) are peripherally anchored to the cytoplasmic surface of the disk saccules. We have examined the role of specific phospholipids in the interaction of these proteins with native osmotically intact disk vesicles, employing spin-labeled phospholipid analogues (2% of total phospholipids) and bovine serum albumin back-exchange assay. Inactive GDP-bound transducin exclusively reduced the extraction of negatively charged phosphatidylserine. The effect disappeared upon activation of the G-protein with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). PDE affected the extraction of the zwitterionic phosphatidylcholine and, to a smaller extent, of phosphatidylethanolamine. When active GtGTPgammaS interacted with the PDE to form the active effector, the interaction with phosphatidylcholine was specifically enhanced. Each copy of the G-protein bound 3 +/- 1 molecules of phosphatidylserine, whereas the PDE bound a much larger amount (70 +/- 10) of a mixture of phosphatidylcholine and ethanolamine. The results are interpreted as a head group-specific and state-dependent interaction of the signaling proteins with the phospholipids of the photoreceptor membrane.
Collapse
Affiliation(s)
- Elke Hessel
- Institut für Medizinische Physik und Biophysik, Universitätsklinikum Charité, Humboldt Universität zu Berlin, Ziegelstrasse 5-9, 10098 Berlin, Germany.
| | | | | | | | | |
Collapse
|
26
|
Navarro J, Landau EM, Fahmy K. Receptor-dependent G-protein activation in lipidic cubic phase. Biopolymers 2002; 67:167-77. [PMID: 11979595 DOI: 10.1002/bip.10066] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The primary step in cellular signaling by G-protein-coupled receptors (GPCRs) is the interaction of the agonist-activated transmembrane receptor with an intracellular G-protein. Understanding the underlying molecular mechanisms requires the structural determination of receptor G-protein complexes that are not yet achieved. The crystal structure of the bovine photoreceptor rhodopsin, a prototypical GPCR, was solved recently and the structures of different states of engineered G-proteins were reported. Posttranslational hydrophobic modifications of G-proteins are in most cases removed for crystallization but play functional roles for interactions among G-protein subunits with receptors, as well as membranes. Bovine rhodopsin is reconstituted into lipidic cubic phases to assess their potential for crystallization of receptor G-protein complexes under conditions that may preserve the structural and functional roles of hydrophobic protein modifications. Three-dimensional bilayers of a bicontinuous lipidic cubic phase are successfully employed for crystallization of membrane and soluble proteins. UV-visible absorption and attenuated total reflection Fourier transform IR difference spectroscopy reveal that light activation of cubic phase reconstituted rhodopsin results in the generation of a metarhodopsin II-like state. Via diffusion along aqueous channels, transducin couples efficiently to this photoproduct as evidenced by the nucleotide-dependent increase of transducin fluorescence. Thus, rhodopsin transducin interactions do not crucially depend on the presence of sn1 and sn2 acyl chains, phospholipid head groups, or membrane planarity. Because lipidic cubic phases preserve the essential functional and structural properties of native rhodopsin and transducin, they appear suitable for the detergent-free crystallization of receptor G-protein complexes carrying a normal pattern of hydrophobic modifications.
Collapse
Affiliation(s)
- Javier Navarro
- Department of Physiology and Biophysics, University of Texas Medical Branch, 301 University Boulevard, Galveston, Texas 77555-0641, USA
| | | | | |
Collapse
|
27
|
Pulvermüller A, Giessl A, Heck M, Wottrich R, Schmitt A, Ernst OP, Choe HW, Hofmann KP, Wolfrum U. Calcium-dependent assembly of centrin-G-protein complex in photoreceptor cells. Mol Cell Biol 2002; 22:2194-203. [PMID: 11884606 PMCID: PMC133667 DOI: 10.1128/mcb.22.7.2194-2203.2002] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Photoexcitation of rhodopsin activates a heterotrimeric G-protein cascade leading to cyclic GMP hydrolysis in vertebrate photoreceptors. Light-induced exchanges of the visual G-protein transducin between the outer and inner segment of rod photoreceptors occur through the narrow connecting cilium. Here we demonstrate that transducin colocalizes with the Ca(2+)-binding protein centrin 1 in a specific domain of this cilium. Coimmunoprecipitation, centrifugation, centrin overlay, size exclusion chromatography, and kinetic light-scattering experiments indicate that Ca(2+)-activated centrin 1 binds with high affinity and specificity to transducin. The assembly of centrin-G-protein complex is mediated by the betagamma-complex. The Ca(2+)-dependent assembly of a G protein with centrin is a novel aspect of the supply of signaling proteins in sensory cells and a potential link between molecular translocations and signal transduction in general.
Collapse
Affiliation(s)
- Alexander Pulvermüller
- Institut für Medizinische Physik und Biophysik, Humboldt-Universität zu Berlin, Germany.
| | | | | | | | | | | | | | | | | |
Collapse
|
28
|
Murray D, McLaughlin S, Honig B. The role of electrostatic interactions in the regulation of the membrane association of G protein beta gamma heterodimers. J Biol Chem 2001; 276:45153-9. [PMID: 11557749 DOI: 10.1074/jbc.m101784200] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In this paper we report calculations of electrostatic interactions between the transducin (G(t)) betagamma heterodimer (G(t)betagamma) and phospholipid membranes. Although membrane association of G(t)betagamma is due primarily to the hydrophobic penetration into the membrane interior of a farnesyl chain attached to the gamma subunit, structural studies have revealed that there is a prominent patch of basic residues on the surface of the beta subunit surrounding the site of farnesylation that is exposed upon dissociation from the G(t)alpha subunit. Moreover, phosducin, which produces dissociation of G(t)betagamma from membranes, interacts directly with G(t)betagamma and introduces a cluster of acidic residues into this region. The calculations, which are based on the finite difference Poisson-Boltzmann method, account for a number of experimental observations and suggest that charged residues play a role in mediating protein-membrane interactions. Specifically, the calculations predict the following. 1) Favorable electrostatic interactions enhance the membrane partitioning due to the farnesyl group by an order of magnitude although G(t)betagamma has a large net negative charge (-12). 2) This electrostatic attraction positions G(t)betagamma so that residues implicated in mediating the interaction of G(t)betagamma with its membrane-bound effectors are close to the membrane surface. 3) The binding of phosducin to G(t)betagamma diminishes the membrane partitioning of G(t)betagamma by an order of magnitude. 4) Lowering the ionic strength of the solution converts the electrostatic attraction into a repulsion. Sequence analysis and homology model building suggest that our conclusions may be generalized to other Gbetagamma and phosducin isoforms as well.
Collapse
Affiliation(s)
- D Murray
- Department of Biochemistry, Howard Hughes Medical Institute, Columbia University, New York, New York 10032, USA
| | | | | |
Collapse
|
29
|
Abstract
The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.
Collapse
Affiliation(s)
- T Ebrey
- University of Washington, Seattle 98195, USA
| | | |
Collapse
|
30
|
Min KC, Gravina SA, Sakmar TP. Reconstitution of the vertebrate visual cascade using recombinant heterotrimeric transducin purified from Sf9 cells. Protein Expr Purif 2000; 20:514-26. [PMID: 11087692 DOI: 10.1006/prep.2000.1326] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
For reconstitution studies with rhodopsin and cGMP phosphodiesterase (PDE), all three subunits of heterotrimeric transducin (T alpha beta gamma) were simultaneously expressed in Sf9 cells at high levels using a baculovirus expression system and purified to homogeneity. Light-activated rhodopsin catalyzed the loading of purified recombinant T alpha with GTP gamma S. In vitro reconstitution of rhodopsin, recombinant transducin, and PDE in detergent solution resulted in cGMP hydrolysis upon illumination, demonstrating that recombinant transducin was able to activate PDE. The rate of cGMP hydrolysis by PDE as a function of GTP gamma S-loaded recombinant transducin (T(*)) concentration gave a Hill coefficient of approximately 2, suggesting that the activation of PDE by T(*) was cooperatively regulated. Furthermore, the kinetic rate constants for the activation of PDE by T(*) suggested that only the complex of PDE with two T(*) molecules, PDE. T(2)(*), was significantly catalytically active under the conditions of the assay. We conclude that the model of essential coactivation best describes the activation of PDE by T(*) in a reconstituted vertebrate visual cascade using recombinant heterotrimeric transducin.
Collapse
Affiliation(s)
- K C Min
- Laboratory of Molecular Biology and Biochemistry, Howard Hughes Medical Institute, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
| | | | | |
Collapse
|
31
|
Abstract
Freshly prepared proteolyzed (deprenylated) T beta gamma and material isolated from retina are inert with respect to activating T alpha in the presence of R* in detergent and in disk membranes. In addition, proteolyzed T beta gamma is also incapable of supporting the pertussis toxin-catalyzed ADP ribosylation of T alpha-GDP. These experiments show that isoprenylation/methylation is essential for the fruitful interactions between T alpha and T beta gamma at the membrane. When tested for its ability to support GTP-for-GDP exchange catalyzed by R*, demethylated T beta gamma proved to be approximately 50% as active as methylated T beta gamma in photoreceptor disk membranes (Fig. 3) and in reconstituted liposomes containing rhodopsin. In detergent, no difference was observed between methylated and demethylated T beta gamma, suggesting no role at all for the methyl group in functional interactions between T alpha, T beta gamma, and R*. The twofold activity difference observed in membranes can be accounted for by the twofold lessened affinity of the demethylated T beta gamma, compared with its methylated counterpart, for membranes in the presence of R* and T alpha. It is interesting to note that a substantially larger difference (> 10-fold) in the relative binding of methylated versus demethylated T beta gamma to membranes is observed in the absence of R* and T alpha. However, R* has a substantial affinity for T alpha beta gamma, and the influence of R* and T alpha greatly reduces any differences resulting from the presence or absence of a methyl group on T beta gamma. The results from studies of demethylated T beta gamma demonstrate that specific lipid-receptor interactions are unlikely to play a critical role in the rhodopsin-transducin system, and further show that the effect of methylation is probably due to the increased hydrophobicity of methylated T beta gamma versus its unmethylated counterpart. These studies are, of course, relevant to heterotrimeric G proteins, and specifically to the interactions of receptor (R*) with T alpha and T beta gamma. If a hydrophobic lipid-lipid mechanism is operative, the state of methylation would be expected to have a more profound effect on the membrane-associative properties of farnesylated proteins, but not on those of geranylgeranylated proteins. The increased hydrophobicity of the C20 geranylgeranyl group relative to the C15 farnesyl group will compensate for the loss of the methyl substituent. The results obtained in the transducin-rhodopsin system can be contrasted with the effect of gamma-subunit methylation on effector enzyme activation. In the case of the geranylgeranylated beta 1 gamma 2, methylation proved to have only a small effect on PIPLC beta activation (Fig. 4B). An approximately 25% diminution in efficacy, but not potency, was observed for the demethylated geranylgeranylated beta 1 gamma 2 versus its methylated counterpart. This again shows that specific lipid-protein interactions are unimportant. The effect of methylation on membrane binding would be expected to be small, given that beta 1 gamma 2 is geranylgeranylated. It is of interest to compare these results with those found with methylated and unmethylated T beta gamma as activators of PIPLC beta. In this instance there was a large effect noted, with methylated T beta gamma being at least 10-fold more potent than its unmethylated counterpart with respect to activating either enzyme (Fig. 4A). This result is readily understandable in light of the role of methylation in selectively enhancing hydrophobicity of farnesylated proteins as opposed to geranyl-geranylated proteins. Similar results were obtained for the activation of PI3K, further strengthening the conclusion that it is lipid-lipid interactions that direct beta gamma subunit membrane association. (ABSTRACT TRUNCATED)
Collapse
Affiliation(s)
- C A Parish
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA
| | | |
Collapse
|
32
|
Affiliation(s)
- T Matsuda
- Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Japan
| | | |
Collapse
|
33
|
Abstract
Domains rich in sphingolipids and cholesterol, or rafts, may organize signal transduction complexes at the plasma membrane. Raft lipids are believed to exist in a state similar to the liquid-ordered phase. It has been proposed that proteins with a high affinity for an ordered lipid environment will preferentially partition into rafts (Melkonian, K. A., Ostermeyer, A. G., Chen, J. Z., Roth, M. G., and Brown, D. A. (1999) J. Biol. Chem. 274, 3910-3917). We investigated the possibility that lipid-lipid interactions between lipid-modified proteins and raft lipids mediate targeting of proteins to these domains. G protein monomers or trimers were reconstituted in liposomes, engineered to mimic raft domains. Assay for partitioning of G proteins into rafts was based on Triton X-100 insolubility. Myristoylation and palmitoylation of Galpha(i) were necessary and sufficient for association with liposomes and partitioning into rafts. Strikingly, the amount of fatty-acylated Galpha(i) in rafts was significantly reduced when myristoylated Galpha(i) was thioacylated with cis-unsaturated fatty acids instead of saturated fatty acids such as palmitate. Prenylated betagamma subunits were excluded from rafts, whether reconstituted alone or with fatty-acylated alpha subunits. These results suggest that the structural difference between lipids that modify proteins is one basis for the selectivity of protein targeting to rafts.
Collapse
Affiliation(s)
- S Moffett
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
| | | | | |
Collapse
|
34
|
Pugh E, Lamb T. Chapter 5 Phototransduction in vertebrate rods and cones: Molecular mechanisms of amplification, recovery and light adaptation. HANDBOOK OF BIOLOGICAL PHYSICS 2000. [DOI: 10.1016/s1383-8121(00)80008-1] [Citation(s) in RCA: 112] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
|
35
|
Hofmann KP. Signalling states of photoactivated rhodopsin. NOVARTIS FOUNDATION SYMPOSIUM 1999; 224:158-75; discussion 175-80. [PMID: 10614051 DOI: 10.1002/9780470515693.ch10] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
In microseconds after photoexcitation, rhodopsin forms the Meta I intermediate from lumirhodopsin. In this conversion, contacts between retinal and the apoprotein are formed, which result in a defined arrangement of donor and acceptor groups for proton translocations. A system of protonation-dependent coupled equilibria is now adopted, comprising Meta intermediates I, II and III, and their isospectral subforms. Some Meta states were identified as signalling states, in which the receptor interacts with transducin (Gt), rhodopsin kinase (RK) and arrestin. The binding of Gt or arrestin shifts the equilibrium to Meta II, while RK does not, indicating exposure of the RK binding site(s) before Meta II is formed. On contact with the activated receptor, each signalling protein responds with a conformational change, which transforms it into a functionally active state. The bell-shaped pH/rate profiles which are seen for the activation of both the G protein and the receptor kinase, indicate the necessary protonation and deprotonation of groups with different pKa. The right wing of the profile reflects the formation of the protonated subconformation (termed MIIb) of Meta II. For the interaction with Gt, recent work suggests a 'sequential fit' mechanism, involving the recognition of the C-terminal peptide of the Gt alpha subunit and of the farnesylated C-terminus of the gamma subunit. Isolated peptides derived from these portions of the G protein mimic the left wing of the pH/rate profile. We discuss the sequential fit as a time-ordered sequence of microscopic recognition and conformational interlocking in the interaction with the G protein.
Collapse
Affiliation(s)
- K P Hofmann
- Institut für Medizinische Physik und Biophysik, Medizinische Fakultät Charité, Humboldt-Universität zu Berlin, Germany
| |
Collapse
|
36
|
Abstract
RTX toxins are important virulence factors produced by a wide range of Gram-negative bacteria. They fall into two categories: the hemolysins, which affect a variety of cell types, and the leukotoxins, which are cell-type- and species-specific. These toxins offer interesting models for targeting, insertion and translocation of aqueous proteins into lipid membranes.
Collapse
Affiliation(s)
- E T Lally
- Leon Levy Research Center for Oral Biology, School of medicine, University of Pennsylvania, Philadelphia, PA 19104-6002, USA.
| | | | | | | |
Collapse
|
37
|
Anis Y, Nürnberg B, Visochek L, Reiss N, Naor Z, Cohen-Armon M. Activation of Go-proteins by membrane depolarization traced by in situ photoaffinity labeling of galphao-proteins with [alpha32P]GTP-azidoanilide. J Biol Chem 1999; 274:7431-40. [PMID: 10066808 DOI: 10.1074/jbc.274.11.7431] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Evidence for depolarization-induced activation of G-proteins in membranes of rat brain synaptoneurosomes has been previously reported (Cohen-Armon, M., and Sokolovsky, M. (1991) J. Biol. Chem. 266, 2595-2605; Cohen-Armon, M., and Sokolovsky, M. (1993) J. Biol. Chem. 268, 9824-9838). In the present work we identify the activated G-proteins as Go-proteins by tracing their depolarization-induced in situ photoaffinity labeling with [alpha32P]GTP-azidoanilide (GTPAA). Labeled GTPAA was introduced into transiently permeabilized rat brain-stem synaptoneurosomes. The resealed synaptoneurosomes, while being UV-irradiated, were depolarized. Relative to synaptoneurosomes at resting potential, the covalent binding of [alpha32P]GTPAA to Galphao1- and Galphao3-proteins, but not to Galphao2- isoforms, was enhanced by 5- to 7-fold in depolarized synaptoneurosomes, thereby implying an accelerated exchange of GDP for [alpha32P]GTPAA. Their depolarization-induced photoaffinity labeling was independent of stimulation of Go-protein-coupled receptors and could be reversed by membrane repolarization, thus excluding induction by transmitters release. It was, however, dependent on depolarization-induced activation of the voltage-gated sodium channels (VGSC), regardless of Na+ current. The alpha subunit of VGSC was cross-linked and co-immunoprecipitated with Galphao-proteins in depolarized brain-stem and cortical synaptoneurosomes. VGSC alpha subunit most efficiently cross-linked with guanosine 5'-O-2-thiodiphosphate-bound rather than to guanosine 5'-O-(3-thiotriphosphate)-bound Galphao-proteins in isolated synaptoneurosomal membranes. These findings support a possible involvement of VGSC in depolarization-induced activation of Go-proteins.
Collapse
Affiliation(s)
- Y Anis
- Department of Physiology and The Cardiac Research Institute, Sackler School of Medicine, Tel-Aviv University, 69978 Tel-Aviv, Israel
| | | | | | | | | | | |
Collapse
|
38
|
Abstract
The non-covalent assembly of proteins that fold separately is central to many biological processes, and differs from the permanent macromolecular assembly of protein subunits in oligomeric proteins. We performed an analysis of the atomic structure of the recognition sites seen in 75 protein-protein complexes of known three-dimensional structure: 24 protease-inhibitor, 19 antibody-antigen and 32 other complexes, including nine enzyme-inhibitor and 11 that are involved in signal transduction.The size of the recognition site is related to the conformational changes that occur upon association. Of the 75 complexes, 52 have "standard-size" interfaces in which the total area buried by the components in the recognition site is 1600 (+/-400) A2. In these complexes, association involves only small changes of conformation. Twenty complexes have "large" interfaces burying 2000 to 4660 A2, and large conformational changes are seen to occur in those cases where we can compare the structure of complexed and free components. The average interface has approximately the same non-polar character as the protein surface as a whole, and carries somewhat fewer charged groups. However, some interfaces are significantly more polar and others more non-polar than the average. Of the atoms that lose accessibility upon association, half make contacts across the interface and one-third become fully inaccessible to the solvent. In the latter case, the Voronoi volume was calculated and compared with that of atoms buried inside proteins. The ratio of the two volumes was 1.01 (+/-0.03) in all but 11 complexes, which shows that atoms buried at protein-protein interfaces are close-packed like the protein interior. This conclusion could be extended to the majority of interface atoms by including solvent positions determined in high-resolution X-ray structures in the calculation of Voronoi volumes. Thus, water molecules contribute to the close-packing of atoms that insure complementarity between the two protein surfaces, as well as providing polar interactions between the two proteins.
Collapse
Affiliation(s)
- L Lo Conte
- MRC Laboratory of Molecular Biology, Hills Road, Cambridge, CB1 1JX, England
| | | | | |
Collapse
|
39
|
DeMar JC, Rundle DR, Wensel TG, Anderson RE. Heterogeneous N-terminal acylation of retinal proteins. Prog Lipid Res 1999; 38:49-90. [PMID: 10396602 DOI: 10.1016/s0163-7827(98)00020-4] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Affiliation(s)
- J C DeMar
- Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA
| | | | | | | |
Collapse
|
40
|
Abstract
Covalent lipid modifications anchor numerous signalling proteins to the cytoplasmic face of the plasma membrane. These modifications mediate protein-membrane and protein-protein interactions and are often essential for function. Protein palmitoylation, due to its reversible nature, may be particularly important for modulating protein function during cycles of activation and deactivation. Despite intense investigation, the exact functions of protein palmitoylation are not well understood. However, it is clear that palmitoylation can affect a protein's affinity for membranes, subcellular localization, and interactions with other proteins. In this review, recent advances in understanding the functions and mechanisms of protein palmitoylation are discussed, with particular emphasis on how this lipid affects the biochemistry and cell biology of signalling proteins.
Collapse
Affiliation(s)
- J T Dunphy
- Department of Cell Biology and Physiology, Washington University School of Medicine, 660 S. Euclid, St. Louis, MO 63110, USA
| | | |
Collapse
|
41
|
Vestal DJ, Buss JE, McKercher SR, Jenkins NA, Copeland NG, Kelner GS, Asundi VK, Maki RA. Murine GBP-2: a new IFN-gamma-induced member of the GBP family of GTPases isolated from macrophages. J Interferon Cytokine Res 1998; 18:977-85. [PMID: 9858320 DOI: 10.1089/jir.1998.18.977] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
We have cloned a new member of the interferon (IFN)-induced guanylate-binding protein (GBP) family of GTPases, murine GBP-2 (mGBP-2), from bone marrow-derived macrophages. mGBP-2 is located on murine chromosome 3, where it is linked to mGBP-1. With the identification of mGBP-2 there are now two human and two murine GBPs. Like other GBPs, mGBP-2 RNA and protein are induced by IFN-gamma. In addition, mGBP-2 shares with the other GBPs important structural features that distinguish this family from other GTPases. First, mGBP-2 contains only two of the three consensus sequences for nucleotide binding found within the classic GTP binding regions of other GTPases. A second amino acid motif found in mGBP-2 is a potential C-terminal site for isoprenoid modification, called a CaaX sequence. mGBP-2 is prenylated, as detected by [3H]mevalonate incorporation, when expressed in COS cells and preferentially incorporates the C-20 isoprenoid geranylgeraniol. Surprisingly, despite having a functional CaaX sequence, mGBP-2 is primarily cytosolic. GBP proteins are very abundant in IFN-exposed cells, but little is known about their function. mGBP-2 is expressed by IFN-gamma-treated cells from C57Bl/6 mice, whereas mGBP-1 is not. Thus, the identification of mGBP-2 makes possible the study of GBP function in the absence of a second family member.
Collapse
Affiliation(s)
- D J Vestal
- The Burnham Institute, La Jolla, CA 92037, USA.
| | | | | | | | | | | | | | | |
Collapse
|
42
|
Stanley P, Koronakis V, Hughes C. Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function. Microbiol Mol Biol Rev 1998; 62:309-33. [PMID: 9618444 PMCID: PMC98917 DOI: 10.1128/mmbr.62.2.309-333.1998] [Citation(s) in RCA: 156] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.
Collapse
Affiliation(s)
- P Stanley
- Department of Pathology, University of Cambridge, Cambridge CB2 1QP, United Kingdom.
| | | | | |
Collapse
|
43
|
Gudi S, Nolan JP, Frangos JA. Modulation of GTPase activity of G proteins by fluid shear stress and phospholipid composition. Proc Natl Acad Sci U S A 1998; 95:2515-9. [PMID: 9482917 PMCID: PMC19396 DOI: 10.1073/pnas.95.5.2515] [Citation(s) in RCA: 232] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
Mechanical forces arising from strain, pressure, and fluid shear stress are sensed by cells through an unidentified mechanoreceptor(s) coupled to intracellular signaling pathways. In vascular endothelial cells, fluid shear stress is transduced via pathway(s) involving heterotrimeric guanine nucleotide-binding proteins (G proteins) by molecular mechanisms that are unknown. In the present study, we investigated the activation of purified G proteins reconstituted into phospholipid vesicles. Vesicles containing G proteins were loaded with [gamma-32P]GTP and subjected to physiological levels of fluid shear stress in a cone-and-plate viscometer. Steady-state GTP hydrolysis was measured as an index of G protein function. Shear stress (0-30 dynes/cm2) activated G proteins in dose-dependent manner (0.48-4.6 pmol/min per microg of protein). Liposomes containing lysophosphatidylcholine (30 mol %) or treated with benzyl alcohol (40 mM), conditions that increase bilayer fluidity, exhibited 3- to 5-fold enhancement of basal GTPase activity. Conversely, incorporation of cholesterol (24 mol %) into liposomes reduced the activation of G proteins by shear. These results demonstrate the ability of the phospholipid bilayer to mediate the shear stress-induced activation of membrane-bound G proteins in the absence of protein receptors and that bilayer physical properties modulate this response.
Collapse
Affiliation(s)
- S Gudi
- Department of Bioengineering, University of California at San Diego, La Jolla, CA 92093-0412, USA
| | | | | |
Collapse
|
44
|
Gelb MH, Scholten JD, Sebolt-Leopold JS. Protein prenylation: from discovery to prospects for cancer treatment. Curr Opin Chem Biol 1998; 2:40-8. [PMID: 9667914 DOI: 10.1016/s1367-5931(98)80034-3] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
A specific set of proteins in eukaryotic cells contain covalently attached carboxy-terminal prenyl groups (15-carbon farnesyl and 20-carbon geranylgeranyl). Many of them are signaling proteins including Ras, heterotrimeric G proteins and Rab proteins. The protein prenyltransferases which attach prenyl groups to proteins have been well characterized, and an X-ray structure is available for protein farnesyltransferase. Inhibitors of protein farnesyltransferase are showing sufficient promise in preclinical trials as anti-cancer drugs to warrant widespread interest in the pharmaceutical industry.
Collapse
Affiliation(s)
- M H Gelb
- Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195, USA.
| | | | | |
Collapse
|
45
|
Morales J, Fishburn CS, Wilson PT, Bourne HR. Plasma membrane localization of G alpha z requires two signals. Mol Biol Cell 1998; 9:1-14. [PMID: 9436987 PMCID: PMC25209 DOI: 10.1091/mbc.9.1.1] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Three covalent attachments anchor heterotrimeric G proteins to cellular membranes: the alpha subunits are myristoylated and/or palmitoylated, whereas the gamma chain is prenylated. Despite the essential role of these modifications in membrane attachment, it is not clear how they cooperate to specify G protein localization at the plasma membrane, where the G protein relays signals from cell surface receptors to intracellular effector molecules. To explore this question, we studied the effects of mutations that prevent myristoylation and/or palmitoylation of an epitope-labeled alpha subunit, alpha z. Wild-type alpha z (alpha z-WT) localizes specifically at the plasma membrane. A mutant that incorporates only myristate is mistargeted to intracellular membranes, in addition to the plasma membrane, but transduces hormonal signals as well as does alpha z-WT. Removal of the myristoylation site produced a mutant alpha z that is located in the cytosol, is not efficiently palmitoylated, and does not relay the hormonal signal. Coexpression of beta gamma with this myristoylation defective mutant transfers it to the plasma membrane, promotes its palmitoylation, and enables it to transmit hormonal signals. Pulse-chase experiments show that the palmitate attached to this myristoylation-defective mutant turns over much more rapidly than does palmitate on alpha z-WT, and that the rate of turnover is further accelerated by receptor activation. In contrast, receptor activation does not increase the slow rate of palmitate turnover on alpha z-WT. Together these results suggest that myristate and beta gamma promote stable association with membranes not only by providing hydrophobicity, but also by stabilizing attachment of palmitate. Moreover, palmitoylation confers on alpha z specific localization at the plasma membrane.
Collapse
Affiliation(s)
- J Morales
- Department of Cellular and Molecular Pharmacology, University of California, San Francisco, California 94143, USA
| | | | | | | |
Collapse
|
46
|
Ong OC, Hu K, Rong H, Lee RH, Fung BK. Gene structure and chromosome localization of the G gamma c subunit of human cone G-protein (GNGT2). Genomics 1997; 44:101-9. [PMID: 9286705 DOI: 10.1006/geno.1997.4814] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Phototransduction in the vertebrate rod and cone photoreceptors is regulated by structurally homologous and yet distinct groups of signaling proteins. We have previously identified in bovine retinas a cone-specific G-protein gamma subunit (G gamma c, previously named G gamma b), which may play a key role in coupling the cone visual pigment to phosphodiesterase (O. C. Ong et al., 1995, J. Biol. Chem. 270:8495-8500). We report here the characterization of human G gamma c and its gene structure. Human G gamma c subunit shares a high degree of sequence identity with the corresponding bovine G gamma c isoform (85%) and human rod G gamma 1 (63%). The protein is specifically localized in cones, as indicated by immunohistochemical staining using anti-G gamma c antibodies. Nucleotide sequence analysis of the G gamma c gene (GNGT2) reveals a structure consisting of three exons and two introns, with the intron splice sites similar to that of the rod G gamma 1 gene (GNGT1). By using fluorescence in situ hybridization, we have further localized the human GNGT2 gene to chromosome 17q21. The elucidation of the G gamma c gene structure would facilitate the identification of genetic defects associated with cone degeneration.
Collapse
MESH Headings
- Amino Acid Sequence
- Animals
- Blotting, Northern
- Blotting, Western
- Cattle
- Chromosome Mapping
- Chromosomes, Human, Pair 17/genetics
- Cloning, Molecular
- GTP-Binding Proteins/chemistry
- GTP-Binding Proteins/genetics
- Humans
- Immunohistochemistry
- In Situ Hybridization, Fluorescence
- Molecular Sequence Data
- RNA, Messenger/genetics
- RNA, Messenger/metabolism
- Restriction Mapping
- Retinal Cone Photoreceptor Cells/chemistry
- Sequence Analysis, DNA
- Sequence Homology, Amino Acid
Collapse
Affiliation(s)
- O C Ong
- Jules Stein Eye Institute, Department of Molecular and Medical Pharmacology, University of California at Los Angeles School of Medicine 90095, USA
| | | | | | | | | |
Collapse
|
47
|
Soneoka Y, Kingsman SM, Kingsman AJ. Mutagenesis analysis of the murine leukemia virus matrix protein: identification of regions important for membrane localization and intracellular transport. J Virol 1997; 71:5549-59. [PMID: 9188629 PMCID: PMC191797 DOI: 10.1128/jvi.71.7.5549-5559.1997] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
We have created two sets of substitution mutations in the Moloney murine leukemia virus (Mo-MuLV) matrix protein in order to identify domains involved in association with the plasma membrane and in incorporation of the viral envelope glycoproteins into virus particles. The first set of mutations was targeted at putative membrane-associating regions similar to those of the human immunodeficiency virus type 1 matrix protein, which include a polybasic region at the N terminus of the Mo-MuLV matrix protein and two regions predicted to form beta strands. The second set of mutations was created within hydrophobic residues to test for the production of virus particles lacking envelope proteins, with the speculation of an involvement of the membrane-spanning region of the envelope protein in incorporation into virus particles. We have found that mutation of the N-terminal polybasic region redirected virus assembly to the cytoplasm, and we show that tryptophan residues may also play a significant role in the intracellular transport of the matrix protein. In total, 21 mutants of the Mo-MuLV matrix protein were produced, but we did not observe any mutant virus particles lacking the envelope glycoproteins, suggesting that a direct interaction between the Mo-MuLV matrix protein and envelope proteins either may not exist or may occur through multiple redundant interactions.
Collapse
Affiliation(s)
- Y Soneoka
- Department of Biochemistry, University of Oxford, United Kingdom
| | | | | |
Collapse
|
48
|
Mazzoni MR, Hamm HE. Interaction of transducin with light-activated rhodopsin protects It from proteolytic digestion by trypsin. J Biol Chem 1996; 271:30034-40. [PMID: 8939950 DOI: 10.1074/jbc.271.47.30034] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
The tryptic cleavage pattern of transducin (Gt) in solution was compared with that in the presence of phospholipid vesicles, rod outer segment (ROS) membranes kept in the dark, or ROS membranes containing light-activated rhodopsin, metarhodopsin II (Rh*). When Gt was in the high affinity complex with Rh*, the alphat subunit was almost completely protected from proteolysis. The protection of alphat at Arg310 was complete, while Arg204 was substantially protected. The cleavage of alphat at Lys18 was protected in the presence of phospholipid vesicles, ROS membranes kept in the dark, or ROS membranes containing Rh*. The cleavage of betat was slower in the presence of ROS membranes or phospholipid vesicles. When the Rh*. Gt complex was incubated with guanyl-5'-yl thiophosphate, a guanine nucleotide analog known to release the high affinity interaction between Gt and Rh*, the protection at Arg310 and Arg204 was diminished. From our results, we propose that Rh* either physically blocks access of trypsin to Arg204 and Arg310 or maintains the heterotrimer in such a conformation that these cleavage sites are not available. Since Arg204 is involved in the switch interface with betagammat (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319), it may be that betagammat is implicated in protecting this cleavage site in the receptor-bound, stabilized heterotrimer. Arg310 is not near the betagammat subunit, thus we believe that the high affinity binding of Gt to Rh* physically or sterically blocks access of trypsin to this site. Thus, Arg310, only a few angstroms away from the carboxyl terminus of alphat, which is known to directly bind to Rh*, is likely to also be a part of the Rh* binding site. This is in agreement with other studies and has implications for the mechanism by which receptors catalyze GDP release from G proteins. The protection of Lys18 in the presence of phospholipid vesicles suggests that the amino-terminal region is in contact with the membrane, consistent with the crystal structure of the heterotrimer (Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319).
Collapse
Affiliation(s)
- M R Mazzoni
- Istituto Policattedra di Discipline Biologiche, University of Pisa, 56126 Pisa, Italy
| | | |
Collapse
|
49
|
Helmreich EJ, Hofmann KP. Structure and function of proteins in G-protein-coupled signal transfer. BIOCHIMICA ET BIOPHYSICA ACTA 1996; 1286:285-322. [PMID: 8982287 DOI: 10.1016/s0304-4157(96)00013-5] [Citation(s) in RCA: 97] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
- E J Helmreich
- Department of Clinical Biochemistry and Pathobiochemistry, University of Würzburg, Germany
| | | |
Collapse
|
50
|
Abstract
Heterotrimeric guanine nucleotide-binding regulatory proteins (G-proteins) are vital components of numerous signal transduction pathways, including sensory and hormonal response systems. G-proteins transduce signals from heptahelical transmembrane receptors to downstream effectors. The localization of a G-protein to the plasma membrane, as well as its interaction with the appropriate receptor and effector, are essential for its function. In addition, the association of a G-protein's subunits to form its trimer is required for interaction with its receptor. The G-protein gamma subunits (G gamma) are subject to a set of carboxyl-terminal processing events that include prenylation of a cysteine, proteolysis, and methylation. Recent advances which elucidate the contributions that the post-translational modifications of the G gamma subunit have on the assembly, membrane association, and function of the G-protein trimer reveal that these modifications are required for important protein-protein, in addition to membrane-protein, interactions.
Collapse
Affiliation(s)
- J B Higgins
- Department of Molecular Cancer Biology, Duke University Medical Center, Durham, NC 27710-3686, USA
| | | |
Collapse
|