The myristoyl moiety of myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein is embedded in the membrane

The myristoylated alanine-rich C-kinase substrates (MARCKS): A membrane-anchored mediator of the cell function

The myristoylated alanine-rich C-kinase substrate (MARCKS) and the MARCKS-related protein (MARCKSL1) are ubiquitous, highly conserved membrane-associated proteins involved in the structural modulation of the actin cytoskeleton, chemotaxis, motility, cell adhesion, phagocytosis, and exocytosis. MARCKS includes an N-terminal myristoylated domain for membrane binding, a highly conserved MARCKS Homology 2 (MH2) domain, and an effector domain (which is the phosphorylation site). MARCKS can sequester phosphatidylinositol-4, 5-diphosphate (PIP2) at lipid rafts in the plasma membrane of quiescent cells, an action reversed by protein kinase C (PKC), ultimately modulating the immune function. Being expressed mostly in innate immune cells, MARCKS promotes the inflammation-driven migration and adhesion of cells and the secretion of cytokines such as tumor necrosis factor (TNF). From a clinical point of view, MARCKS is overexpressed in patients with schizophrenia and bipolar disorders, while the brain level of MARCKS phosphorylation is associated with Alzheimer's disease. Furthermore, MARCKS is associated with the development and progression of numerous types of cancers. Data in autoimmune diseases are limited to rheumatoid arthritis models in which a connection between MARCKS and the JAK-STAT pathway is mediated by miRNAs. We provide a comprehensive overview of the structure of MARCKS, its molecular characteristics and functions from a biological and pathogenetic standpoint, and will discuss the clinical implications of this pathway.

Pathophysiological roles of myristoylated alanine-rich C-kinase substrate (MARCKS) in hematological malignancies

The myristoylated alanine-rich C-kinase substrate (MARCKS) protein has been at the crossroads of multiple signaling pathways that govern several critical operations in normal and malignant cellular physiology. Functioning as a target of protein kinase C, MARCKS shuttles between the phosphorylated cytosolic form and the unphosphorylated plasma membrane-bound states whilst regulating several molecular partners including, but not limited to calmodulin, actin, phosphatidylinositol-4,5-bisphosphate, and phosphoinositide-3-kinase. As a result of these interactions, MARCKS directly or indirectly modulates a host of cellular functions, primarily including cytoskeletal reorganization, membrane trafficking, cell secretion, inflammatory response, cell migration, and mitosis. Recent evidence indicates that dysregulated expression of MARCKS is associated with the development and progression of hematological cancers. While it is understood that MARCKS impacts the overall carcinogenesis as well as plays a part in determining the disease outcome in blood cancers, we are still at an early stage of interpreting the pathophysiological roles of MARCKS in neoplastic disease. The situation is further complicated by contradictory reports regarding the role of phosphorylated versus an unphosphorylated form of MARCKS as an oncogene versus tumor suppressor in blood cancers. In this review, we will investigate the current body of knowledge and evolving concepts of the physical properties, molecular network, functional attributes, and the likely pathogenic roles of MARCKS in hematological malignancies. Key emphasis will also be laid upon understanding the novel mechanisms by which MARCKS determines the overall disease prognosis by playing a vital role in the induction of therapeutic resistance. Additionally, we will highlight the importance of MARCKS as a valuable therapeutic target in blood cancers and will discuss the potential of existing strategies available to tackle MARCKS-driven blood cancers.

MARCKS and Lung Disease

MARCKS (myristoylated alanine-rich C kinase substrate) is a prominent PKC substrate expressed in all eukaryotic cells. It is known to bind to and cross-link actin filaments, to serve as a bridge between Ca²⁺/calmodulin and PKC signaling, and to sequester the signaling molecule phosphatidylinositol 4,5-bisphosphate in the plasma membrane. Since the mid-1980s, this evolutionarily conserved and ubiquitously expressed protein has been associated with regulating cellular events that require dynamic actin reorganization, including cellular adhesion, migration, and exocytosis. More recently, translational studies have implicated MARCKS in the pathophysiology of a number of airway diseases, including chronic obstructive pulmonary disease, asthma, lung cancer, and acute lung injury/acute respiratory distress syndrome. This article summarizes the structure and cellular function of MARCKS (also including MARCKS family proteins and MARCKSL1 [MARCKS-like protein 1]). Evidence for MARCKS's role in several lung diseases is discussed, as are the technological innovations that took MARCKS-targeting strategies from theoretical to therapeutic. Descriptions and updates derived from ongoing clinical trials that are investigating inhalation of a MARCKS-targeting peptide as therapy for patients with chronic bronchitis, lung cancer, and ARDS are provided.

A spectroscopic sensing platform for MARCKS protein monolayers

We developed a highly sensitive silicon platform, suitable to assess the molecular organization of protein samples. Prototype platforms were obtained using different electrochemical protocols for the electrodeposition of Ag-nanoparticles onto the hydrogenated silicon surface. A platform with high Surface Enhanced Raman Scattering efficiency was selected based on the surface coverage and the number density of particles size distribution. The performance of the platform was determined by studying the interaction of Myristoylated Alanine-Rich C Kinase Substrate (MARCKS) protein with the substrate according to its molecular organization. The chemical and structural characteristics of MARCKS molecules were examined under two configurations: i) a disordered distribution given by a MARCKS solution drop deposited onto the platform and, ii) a compact monolayer transferred to the platform by the Langmuir-Blodgett method. Raman spectra show vibrational bands of Phenylalanine and Lysine residues specific for the protein effector domain, and evidence the presence of alpha helix structure in both configurations. Moreover, we distinguished the supramolecular order between the compact monolayer and random molecular distribution. The platforms containing Ag-nanoparticles are suitable for studies of protein structure and interactions, advancing a methodological strategy for our long term goal, which is to explore the interaction of proteins with model membranes.

Alterations of the myristoylated, alanine-rich C kinase substrate (MARCKS) in prefrontal cortex in schizophrenia

Abnormal synaptic plasticity has been implicated in the cognitive deficits seen in schizophrenia, where alterations have been found in neurotransmission, signaling and dendritic dynamics. Rapid rearrangement of the actin cytoskeleton is critical for plasticity and abnormalities of molecular regulators of this process are candidates for understanding mechanisms underlying these changes in schizophrenia. The myristoylated, alanine-rich C-kinase substrate (MARCKS) is crucial for many roles associated with synaptic plasticity, including facilitation of neurotransmission, dendritic branching and in turn cognitive function. Accordingly, we hypothesized that this protein is abnormally expressed or regulated in schizophrenia. We measured protein expression of MARCKS by Western blot analysis in postmortem samples of dorsolateral prefrontal cortex (DLPFC) from elderly schizophrenia patients (N=16) and a comparison group (N=20). We also assayed phosphorylated-MARCKS (pMARCKS), given the role of phosphorylation in reversing membrane association by MARCKS. We found decreased expression of both MARCKS and pMARCKS in schizophrenia. Altered myristoylation may be a mechanism that explains this down-regulation of MARCKS, so we also assayed expression of the two isoforms of the key myristoylation enzyme, NMT, and an enzymatic inhibitor of this enzyme, NMT-inhibitor protein (NIP71) by Western blotting in these same subjects. Expression did not change between groups for these proteins, suggesting a mechanism other than myristoylation is responsible for decreased MARCKS expression in schizophrenia. These data suggest a potential mechanism underlying aspects of altered synaptic plasticity observed in schizophrenia.

The use of cardiolipin-containing liposomes as a model system to study the interaction between proteins and the inner mitochondrial membrane

The interaction of proteins with biological membranes is a key factor in their biogenesis and proper function. Hence, unraveling the properties of this interaction is very important and constitutes an essential step in deciphering the structural and functional characteristics of a membrane protein. Here we describe the use of cardiolipin-containing liposomes to analyze the interaction of the import protein Tim44 with the inner mitochondrial membrane. Using this system we showed that Tim44 is peripherally attached to the membrane and we detected the membrane binding site of the protein. The cardiolipin-containing liposomes serve as an excellent in vitro model system to the inner mitochondrial membrane and thus provide a good tool to analyze the interaction of various mitochondrial proteins with the inner membrane.

The rice ASR5 protein: a putative role in the response to aluminum photosynthesis disturbance

Under acidic soil conditions, aluminum (Al) becomes available to plants, which must cope with its toxicity by mechanisms involving both internal and external detoxification. Rice is the most Al-tolerant among the crop species, with Al detoxification being managed by both mechanisms. Recently, we focused on ASR (Abscisic acid, Stress and Ripening) gene expression analyses and observed increased ASR5 transcript levels in roots and shoots in response to Al. In addition, ASR5 RNAi knock down plants presented an Al-sensitive phenotype. A proteomic approach showed that ASR5 silencing affected several proteins related to photosynthesis in RNAi rice shoots. Furthermore, an ASR5-GFP fusion in rice protoplasts revealed for the first time a chloroplast localization of this protein. Because it is well known that Al induces photosynthetic dysfunction, here we discuss the hypothesis that ASR5 might be sequestered in the chloroplasts as an inactive transcription factor that could be released to the nucleus in response to Al to regulate genes related to photosynthesis.

MARCKS as a negative regulator of lipopolysaccharide signaling

Myristoylated alanine-rich C kinase substrate (MARCKS) is an intrinsically unfolded protein with a conserved cationic effector domain, which mediates the cross-talk between several signal transduction pathways. Transcription of MARCKS is increased by stimulation with bacterial LPS. We determined that MARCKS and MARCKS-related protein specifically bind to LPS and that the addition of the MARCKS effector peptide inhibited LPS-induced production of TNF-α in mononuclear cells. The LPS binding site within the effector domain of MARCKS was narrowed down to a heptapeptide that binds to LPS in an extended conformation as determined by nuclear magnetic resonance spectroscopy. After LPS stimulation, MARCKS moved from the plasma membrane to FYVE-positive endosomes, where it colocalized with LPS. MARCKS-deficient mouse embryonic fibroblasts (MEFs) responded to LPS with increased IL-6 production compared with the matched wild-type MEFs. Similarly, small interfering RNA knockdown of MARCKS also increased LPS signaling, whereas overexpression of MARCKS inhibited LPS signaling. TLR4 signaling was enhanced by the ablation of MARCKS, which had no effect on stimulation by TLR2, TLR3, and TLR5 agonists. These findings demonstrate that MARCKS contributes to the negative regulation of the cellular response to LPS.

COSMOmic: a mechanistic approach to the calculation of membrane-water partition coefficients and internal distributions within membranes and micelles

A new approach for the modeling of molecules in micellar systems and especially in biomembranes, COSMOmic, is presented, and its performance is validated on the example of the partitioning of molecules between water and biological membranes. Starting from quantum chemical calculations of the surfactant, solvent, and solute molecules, and being based on the COSMO-RS method for fluid-phase thermodynamic properties, COSMOmic is essentially free of additional adjustable parameters. The inclusion of an elastic energy correction into the COSMOmic model did not turn out to yield any significant improvement. The novel COSMOmic method allows for the efficient prediction of the distribution of molecules in micellar systems.

Membrane binding of the neuronal calcium sensor recoverin - modulatory role of the charged carboxy-terminus

BACKGROUND

The Ca2+-binding protein recoverin operates as a Ca2+-sensor in vertebrate photoreceptor cells. It undergoes a so-called Ca2+-myristoyl switch when cytoplasmic Ca2+-concentrations fluctuate in the cell. Its covalently attached myristoyl-group is exposed at high Ca2+-concentrations and enables recoverin to associate with lipid bilayers and to inhibit its target rhodopsin kinase. At low Ca2+-concentrations the myristoyl group is inserted into a hydrophobic pocket of recoverin thereby relieving inhibitory constraint on rhodopsin kinase. Hydrophobic and electrostatic interactions of recoverin with membranes have not been clearly determined, in particular the function of the positively charged carboxy-terminus in recoverin 191QKVKEKLKEKKL202 in this context is poorly understood.

RESULTS

Binding of myristoylated recoverin to lipid bilayer depends on the charge distribution in phospholipids. Binding was tested by equilibrium centrifugation and surface plasmon resonance (SPR) assays. It is enhanced to a certain degree by the inclusion of phosphatidylserine (up to 60%) in the lipid mixture. However, a recoverin mutant that lacked the charged carboxy-terminus displayed the same relative binding amplitudes as wildtype (WT) recoverin when bound to neutral or acidic lipids. Instead, the charged carboxy-terminus of recoverin has a significant impact on the biphasic dissociation of recoverin from membranes. On the other hand, the nonmyristoylated WT and truncated mutant form of recoverin did not bind to lipid bilayers to a substantial amount as binding amplitudes observed in SPR measurements are similar to bulk refractive index changes.

CONCLUSION

Our data indicate a small, but evident electrostatic contribution to the overall binding energy of recoverin association with lipid bilayer. Properties of the charged carboxy-terminus are consistent with a role of this region as an internal effector region that prolongs the time recoverin stays on the membrane by influencing its Ca2+-sensitivity.

The regulation of MacMARCKS expression by integrin β3

Integrin-mediated adhesion regulates multiple signaling pathways. Our group previously showed that ectopic expression of different integrin beta-subunits in the neuroepithelial cell line GE11, has distinct effects on cell morphology, actin cytoskeletal organization, and on focal contact distribution. In this report we have investigated changes in gene transcription levels resulting from overexpression of the integrin beta3 subunit. We found that beta3 overexpression leads to the transcriptional downregulation of MARCKS related protein (MRP) resulting in a decreased expression of the MRP protein. Furthermore, we show that the Ras/MAPK pathway controls the basal level of MRP expression but beta3 overexpression bypasses this pathway downstream of ERK to downregulate MRP. Further studies indicate that a region of the cytoplasmic tail of beta3 containing part of the NITY motif is responsible for increased cell spreading and MRP downregulation. However, MRP overexpression failed to inhibit the beta3-induced increase in cell spreading while the knock down of MRP expression in GE11 cells did not increase cell spreading. We suggest that the downregulation of MRP by beta3 is not required for increased cell spreading but instead that MRP downregulation is a secondary effect of increased cell spreading.

Thematic review series: Lipid Posttranslational Modifications. Lysosomal metabolism of lipid-modified proteins

Much is now understood concerning the synthesis of prenylated and palmitoylated proteins, but what is known of their metabolic fate? This review details metabolic pathways for the lysosomal degradation of S-fatty acylated and prenylated proteins. Central to these pathways are two lysosomal enzymes, palmitoyl-protein thioesterase (PPT1) and prenylcysteine lyase (PCL). PPT1 is a soluble lipase that cleaves fatty acids from cysteine residues in proteins during lysosomal protein degradation. Notably, deficiency in the enzyme causes a neurodegenerative lysosomal storage disorder, infantile neuronal ceroid lipofuscinosis. PCL is a membrane-associated flavin-containing lysosomal monooxygenase that metabolizes prenylcysteine to prenyl aldehyde through a completely novel mechanism. The eventual metabolic fates of other lipidated proteins (such as glycosylphosphatidylinositol-anchored and N-myristoylated proteins) are poorly understood, suggesting directions for future research.

G-protein-coupled receptor-associated A-kinase anchoring proteins: AKAP79 and AKAP250 (gravin)

A-kinase anchoring proteins (AKAPs) define an expanding group of scaffold proteins that display a signature binding site for the RI/RII subunit of protein kinase A. AKAPs are multivalent and a subset of these scaffold proteins also display the ability to associate with the prototypic member of G-protein-coupled receptors, the beta(2)-adrenergic receptor. Both AKAP79 (also known as AKAP5) and AKAP250 (also known as gravin or AKAP12) have been shown to associate with the beta(2)-adrenergic receptor, but each directs downstream signaling events in decidedly different manners. The primary structures, common and unique protein motifs are of interest. Both proteins display largely natively unfolded primary sequences that provide a necklace on which short, structured regions of sequence are found. Membrane association appears to involve both interactions with the lipid bilayer via docking to a G-protein-coupled receptor as well as interactions of short positively charged domains with the inner leaflet of the cell membrane. Gravin, unlike AKAP79, displays a canonical site at its N-terminus that is subject to N-myristoylation. AKAP79 appears to function in switching signaling pathways of the receptor from adenylylcyclase to activation of the mitogen-activated protein kinase cascade. Gravin, in contrast, is essential for the resensitization and recycling of the receptors following agonist-induced activation, desensitization, and internalization. Each AKAP provides a template that enables space-time continuum features to G-protein-coupled signaling pathways as well as a paradigm for explaining apparent compartmentalization of cell signaling.

Protein-membrane interaction and fatty acid transfer from intestinal fatty acid-binding protein to membranes. Support for a multistep process

Fatty acid transfer from intestinal fatty acid-binding protein (IFABP) to phospholipid membranes occurs during protein-membrane collisions. Electrostatic interactions involving the alpha-helical "portal" region of the protein have been shown to be of great importance. In the present study, the role of specific lysine residues in the alpha-helical region of IFABP was directly examined. A series of point mutants in rat IFABP was engineered in which the lysine positive charges in this domain were eliminated or reversed. Using a fluorescence resonance energy transfer assay, we analyzed the rates and mechanism of fatty acid transfer from wild type and mutant proteins to acceptor membranes. Most of the alpha-helical domain mutants showed slower absolute fatty acid transfer rates to zwitterionic membranes, with substitution of one of the lysines of the alpha2 helix, Lys27, resulting in a particularly dramatic decrease in the fatty acid transfer rate. Sensitivity to negatively charged phospholipid membranes was also reduced, with charge reversal mutants in the alpha2 helix the most affected. The results support the hypothesis that the portal region undergoes a conformational change during protein-membrane interaction, which leads to release of the bound fatty acid to the membrane and that the alpha2 segment is of particular importance in the establishment of charge-charge interactions between IFABP and membranes. Cross-linking experiments with a phospholipid-photoactivable reagent underscored the importance of charge-charge interactions, showing that the physical interaction between wild-type intestinal fatty acid-binding protein and phospholipid membranes is enhanced by electrostatic interactions. Protein-membrane interactions were also found to be enhanced by the presence of ligand, suggesting different collisional complex structures for holo- and apo-IFABP.

Expression of MARCKS Effector Domain Mutants Alters Phospholipase D Activity and Cytoskeletal Morphology of SK-N-MC Neuroblastoma Cells

Stable overexpression of myristoylated alanine-rich C-kinase substrate (MARCKS) is known to enhance phorbol ester stimulation of phospholipase D (PLD) activity and protein kinase Calpha (PKCalpha) levels in SK-N-MC neuroblastoma cells. In contrast, expression of MARCKS mutants (S152A or S156A) lacking key PKC phosphorylation sites within the central basic effector domain (ED) had no significant effect on PLD activity or PKCalpha levels relative to vector control cells. Like control cells, those expressing wild type MARCKS were elongated and possessed longitudinally oriented stress fibers, although these cells were more prone to detach from the substratum and undergo cell death upon phorbol ester treatment. However, cells expressing MARCKS ED mutants were irregularly shaped and stress fibers were either shorter or less abundant, and cell adhesion and viability were not affected. These results suggest that intact phosphorylation sites within the MARCKS ED are required for PLD activation and influence both membrane-cytoskeletal organization and cell viability.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

AKAPs (A-kinase anchoring proteins) and molecules that compose their G-protein-coupled receptor signalling complexes

Cell signalling mediated via GPCRs (G-protein-coupled receptors) is a major paradigm in biology, involving the assembly of receptors, G-proteins, effectors and downstream elements into complexes that approach in design 'solid-state' signalling devices. Scaffold molecules, such as the AKAPs (A-kinase anchoring proteins), were discovered more than a decade ago and represent dynamic platforms, enabling multivalent signalling. AKAP79 and AKAP250 were the first to be shown to bind to membrane-embedded GPCRs, orchestrating the interactions of various protein kinases (including tyrosine kinases), protein phosphatases (e.g. calcineurin) and cytoskeletal elements with at least one member of the superfamily of GPCRs, the prototypical beta2-adrenergic receptor. In this review, the multivalent interactions of AKAP250 with the cell membrane, receptor, cytoskeleton and constituent components are detailed, providing a working model for AKAP-based GPCR signalling complexes. Dynamic regulation of the AKAP-receptor complex is mediated by ordered protein phosphorylation.

Myristoylated Naked2 escorts transforming growth factor alpha to the basolateral plasma membrane of polarized epithelial cells

The epidermal growth factor receptor ligands transforming growth factor alpha (TGF alpha) and amphiregulin are delivered to the basolateral surface of polarized epithelial cells where they are cleaved by TACE/ADAM17. Basolateral sorting information resides in their cytoplasmic tail domains, but tail-interacting proteins required for basolateral trafficking have not been identified. Naked (NKD)1 and NKD2 are mammalian homologs of Drosophila Naked Cuticle, which negatively regulates canonical Wnt signaling by binding Dishevelled. We present evidence that NKD2, but not NKD1, binds to basolateral sorting motifs in the cytoplasmic tail of TGF alpha. Processing and cell-surface delivery of TGF alpha are accelerated in NKD2-overexpressing Madin-Darby canine kidney cells. NKD2 is myristoylated on glycine, the second residue. On expression of myristoylation-defective (G2A) NKD2, neither NKD2 nor TGF alpha appears at the basolateral plasma membrane of polarized Madin-Darby canine kidney cells; however, membrane staining for TGF alpha is restored on silencing expression of this mutant NKD2. Amphiregulin does not interact with NKD2 and retains its basolateral localization in G2A-NKD2-expressing cells, as do Na(+), K(+) ATPase alpha 1 and E-cadherin. These data identify an unexpected function for NKD2, i.e., myristoylation-dependent escort of TGF alpha to the basolateral plasma membrane of polarized epithelial cells.

The membrane-interactive tail of cytochrome b(5) can function as a stop-transfer sequence in concert with a signal sequence to give inversion of protein topology in the endoplasmic reticulum

Sequence analyses of the C-terminal membrane intercalative region of the rat cytochrome b(5) indicated that this domain has, in addition to a signal sequence, a combined element of the classic stop-transfer sequence typically found in a variety of transmembrane proteins. Such bitopic protein arrangements arise by tandem but topogenically displaced activities of cleavable/noncleavable signal and stop-transfer sequences. A fusion precursor comprising an N-terminally linked prokaryotic signal sequence and the full-length of mammalian cytochrome b(5), including its C-terminal membrane insertion sequence, was engineered to investigate the outcome of this combination of signals on the targeting and topology of the cytochrome b(5) in the endoplasmic reticulum membrane. Precytochrome b(5) was cotranslationally translocated across the endoplasmic reticulum membrane. The signal-processed cytochrome b(5) was integrally anchored in the membrane with the globular domain facing the lumen. Thus, the topology of the signal sequence-directed cytochrome b(5) in the microsomal vesicle was reversed with respect to that of the native form. Posttranslational incubation of the precytochrome b(5) with microsomes resulted in a "loose" incorporation of the unprocessed form onto the surface of the vesicle. Our findings suggest that the membrane-insertion sequence of cytochrome b(5) has a functional stop-transfer sequence. We discuss the implications of these findings with respect to selective targeting of cytochrome b(5) to the endoplasmic reticulum membrane in the view that signal and stop-transfer sequences are often interchangeable or combined for topogenic functions.

Cross-talk unfolded: MARCKS proteins

The proteins of the MARCKS (myristoylated alanine-rich C kinase substrate) family were first identified as prominent substrates of protein kinase C (PKC). Since then, these proteins have been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as endo-, exo- and phago-cytosis, and neurosecretion. The effector domain of MARCKS proteins is phosphorylated by PKC, binds to calmodulin and contributes to membrane binding. This multitude of mutually exclusive interactions allows cross-talk between the signal transduction pathways involving PKC and calmodulin. This review focuses on recent, mostly biophysical and biochemical results renewing interest in this protein family. MARCKS membrane binding is now understood at the molecular level. From a structural point of view, there is a consensus emerging that MARCKS proteins are "natively unfolded". Interestingly, domains similar to the effector domain have been discovered in other proteins. Furthermore, since the effector domain enhances the polymerization of actin in vitro, MARCKS proteins have been proposed to mediate regulation of the actin cytoskeleton. However, the recent observations that MARCKS might serve to sequester phosphatidylinositol 4,5-bisphosphate in the plasma membrane of unstimulated cells suggest an alternative model for the control of the actin cytoskeleton. While myristoylation is classically considered to be a co-translational, irreversible event, new reports on MARCKS proteins suggest a more dynamic picture of this protein modification. Finally, studies with mice lacking MARCKS proteins have investigated the functions of these proteins during embryonic development in the intact organism.

Multilamellar liposomes and solid-supported lipid membranes (TRANSIL): screening of lipid-water partitioning toward a high-throughput scale

PURPOSE

Lipid-water partitioning of 187 pharmaceuticals has been assessed with solid-supported lipid membranes (TRANSIL) in microwell plates and with multilamellar liposomes for a data comparison. The high-throughput potential of the new approach was evaluated.

METHODS

Drugs were incubated at pH 7.4 with egg yolk lecithin membranes either on a solid support (TRANSIL beads) or in the form of multilamellar liposomes. Phase separation of lipid and water phase was achieved by ultracentrifugation in case of liposomes or by a short filtration step in case of solid-supported lipid membranes.

RESULTS

Lipid-water partitioning data of both approaches correlate well without systematic deviations in the investigated lipophilicity range. The solid-supported lipid membrane approach provides high-precision data in an automated microwell-plate setup. The lipid composition of the solid-supported lipid membranes was varied to study the influence of membrane change on lipid-water partitioning. In addition, pH-dependent measurements have been performed with minimal experimental effort.

CONCLUSIONS

Solid-supported lipid membranes represent a valuable tool to determine physiologically relevant lipid-water partitioning data of pharmaceuticals in an automated setup and is well suited for high-throughput data generation in lead optimization programs.

Insertion of mitochondrial DNA-encoded F1F0-ATPase subunit 8 across the mitochondrial inner membrane in vitro

Cytochrome oxidase subunits I, II, and III, the mitochondrial DNA-encoded proteins, are inserted across the inner membrane by the Oxa1p-containing translocator in a membrane potential-dependent manner. Oxa1p is also involved in the insertion of the cytoplasmically synthesized precursor of Oxa1p itself into the inner membrane from the matrix via the conservative sorting pathway. The mechanism of insertion of the other mitochondrially synthesized proteins, however, is unexplored. The insertion of the mitochondrial DNA-encoded subunit 8 of F(1)F(0)-ATPase (Su8) across the inner membrane was analyzed in vitro using the inverted inner membrane vesicles and the Escherichia coli lysate-synthesized substrate. This assay revealed that the N-terminal segment of Su8 inserted across the membrane to the intermembrane space and assumed the correct trans-cis topology depending on the mitochondrial matrix fraction. This translocation reaction was similar to those of Sec-independent, direct insertion pathways of E. coli and chloroplast thylakoid membranes. (i) It required neither nucleotide triphosphates nor membrane potential, and hydrophobic forces drove the process. (ii) It did not require protease-sensitive membrane components facing the matrix space. (iii) It could be inserted across liposomes in the correct topology in a matrix fraction-dependent manner. Thus, a novel mechanism conserved in bacteria and chloroplasts also functions in the insertion of Su8 across the mitochondrial inner membrane.

Evidence for a central apolipoprotein A-I domain loosely bound to lipids in discoidal lipoproteins that is capable of penetrating the bilayer of phospholipid vesicles

Previous evidence indicated that discoidal reconstituted high density lipoproteins (rHDL) of apolipoprotein A-I (apoA-I) can interact with lipid membranes (Tricerri, M. A., Córsico, B., Toledo, J. D., Garda, H. A., and Brenner, R. R. (1998) Biochim. Biophys. Acta 1391, 67-78). With the aim of studying this interaction, photoactivable reagents and protein cleavage with CNBr and hydroxylamine were used. The generic hydrophobic reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine gave information on the apoA-I regions in contact with the lipid phase in the rHDL discs. Two protein regions loosely bound to lipids were detected: a C-terminal domain and a central one located between residues 87 and 112. They consist of class Y amphipathic alpha-helices that have a different distribution of the charged residues in their polar faces by comparison with class A helices, which predominate in the rest of the apoA-I molecule. The phospholipid analog 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4-(trifluoro-methyl-3-H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, which does not undergo significant exchange between membranes and lipoproteins, was used to identify the apoA-I domain directly involved in the interaction of rHDL discs with membranes. By incubating either rHDL or lipid-free apoA-I with lipid vesicles containing 125I-TID-PC, only the 87-112 apoA-I segment becomes labeled after photoactivation. These results indicate that the central domain formed by two type Y helices swings away from lipid contact in the discoidal lipoproteins and is able to insert into membrane bilayers, a process that may be of great importance for the mechanism of cholesterol exchange between high density lipoproteins and cell membranes.

Solid-supported lipid membranes as a tool for determination of membrane affinity: high-throughput screening of a physicochemical parameter

Quantification of membrane affinity is an important early screening step in modern drug design. However, current approaches using different lipid membrane models usually are time-consuming or show severe experimental drawbacks. In this paper we describe the use of solid-supported lipid membranes (TRANSIL) as a new tool for the determination of membrane affinity. Eighteen pharmaceuticals (neutrals, acids, and bases) have been analyzed for their lipophilicity at physiological pH in an automated setup; phase separation of lipid and aqueous phase can be achieved simply by a short low-speed centrifugation or filtration. The membrane affinity is then calculated by quantification of the total drug concentration and the amount of drug remaining in the aqueous phase after incubation with solid-supported lipid membranes. Lipophilicity parameters relying on solid-supported lipid membranes correlate well with octanol-water partition coefficients K(ow) for neutral organic compounds (range of log K(ow) = 1.5-5, n = 7, r = 0.93). Data acquisition with this lipid membrane model system is highly re-producible. Even in the case of ionizable drugs, where K(ow) tends to underestimate membrane affinity, the latter can be correctly quantified using solid-supported lipid membranes: data comparison shows good agreement of the presented approach with established but time-consuming standardized lipid/buffer systems. Solid-supported lipid membranes allow a fast and reliable quantification of membrane affinity, enabling high-throughput screening of this physicochemical parameter.

In vivo interaction between dynamitin and MacMARCKS detected by the fluorescent resonance energy transfer method

Dynamitin is a subunit of the dynactin complex regulating microtubule-dependent motor functions, and MacMARCKS (Macrophage-enriched myristoylated alanine-rich protein kinase C substrate) is a major protein kinase C substrate regulating integrin activation. The interaction between dynamitin and MacMARCKS has been implicated in integrin-dependent cell spreading. However, the in vivo interaction of these two proteins in living cells has not been demonstrated. Spatial and temporal information about the interaction is also lacking. In this study, we used the fluorescent resonance energy transfer method to demonstrate in vivo interaction between MacMARCKS and dynamitin with cyan fluorescent protein (CFP)-conjugated dynamitin as the donor fluorophore and yellow fluorescent protein (YFP)-conjugated MacMARCKS as the acceptor fluorophore. The interaction of these two fusion proteins was studied both in vitro and in vivo, and typical fluorescent resonance energy transfer was observed; the CFP emission peak increased while the YFP emission peak decreased when protein interaction was abolished. Spatial and temporal information was obtained in RAW macrophage cells. In resting macrophage cells, dynamitin-MacMARCKS interaction is concentrated at the cell periphery, although the majority of dynamitin is distributed at the perinuclear region of the cells. When cells were treated with phorbol 12-myristate 13-acetate, both proteins concentrated to perinuclear regions of the cells, and yet the interaction disappeared as the cell spread. Similar events were also observed in 293 cells. Thus, we conclude that dynamitin and MacMARCKS indeed interact in living cells.

The effector domain of myristoylated alanine-rich C kinase substrate binds strongly to phosphatidylinositol 4,5-bisphosphate

Both the myristoylated alanine-rich protein kinase C substrate protein (MARCKS) and a peptide corresponding to its basic effector domain, MARCKS-(151-175), inhibit phosphoinositide-specific phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in vesicles (Glaser, M., Wanaski, S., Buser, C. A., Boguslavsky, V., Rashidzada, W., Morris, A., Rebecchi, M., Scarlata, S. F., Runnels, L. W., Prestwich, G. D., Chen, J., Aderem, A., Ahn, J., and McLaughlin, S. (1996) J. Biol. Chem. 271, 26187-26193). We report here that adding 10-100 nm MARCKS-(151-175) to a subphase containing either PLC-delta or -beta inhibits hydrolysis of PIP(2) in a monolayer and that this inhibition is due to the strong binding of the peptide to PIP(2). Two direct binding measurements, based on centrifugation and fluorescence, show that approximately 10 nm PIP(2), in the form of vesicles containing 0.01%, 0.1%, or 1% PIP(2), binds 50% of MARCKS-(151-175). Both electrophoretic mobility measurements and competition experiments suggest that MARCKS-(151-175) forms an electroneutral complex with approximately 4 PIP(2). MARCKS-(151-175) binds equally well to PI(4,5)P(2) and PI(3,4)P(2). Local electrostatic interactions of PIP(2) with MARCKS-(151-175) contribute to the binding energy because increasing the salt concentration from 100 to 500 mm decreases the binding 100-fold. We hypothesize that the effector domain of MARCKS can bind a significant fraction of the PIP(2) in the plasma membrane, and release the bound PIP(2) upon interaction with Ca(2+)/calmodulin or phosphorylation by protein kinase C.

Myristoylation alters retinoic acid-induced down-regulation of MARCKS in immortalized hippocampal cells

The myristoylated alanine-rich C kinase substrate (MARCKS) is a prominent PKC-substrate in the brain, which has been implicated in brain development, cytoskeletal remodeling, calcium/calmodulin signaling, and neuroplasticity. The sequence of the Macs gene codes for a protein that has three highly conserved domains including a 5' myristoylation region and a 25-amino-acid phosphorylation site domain (PSD), which are involved in anchoring MARCKS to the cellular membrane. In this study, we examined the role of the myristoylation signal in the regulation of MARCKS in transfected rat hippocampal cells (H19-7) following retinoic acid (RA) treatment. A mutant MARCKS lacking the myristoylation signal was engineered by substitution of alanine for glycine at position 2 of the Macs gene and was found to be exclusively expressed in the cytosol fraction of transfected cells. Exposure of the wild-type MARCKS-transfected cells to RA resulted in an apparent shift of MARCKS from the membrane to the cytosol, while the total protein of wild-type MARCKS was not significantly changed. In contrast, RA-exposed cells transfected with the mutant MARCKS revealed a dramatic reduction of expression of MARCKS protein in both cytosol and total protein fractions. These data suggest that the absence of the myristoyl moiety may not only alter the anchoring of the protein to the membrane but also play a novel role in modulating cellular levels of MARCKS protein in response to RA.

MARCKS-related protein binds to actin without significantly affecting actin polymerization or network structure. Myristoylated alanine-rich C kinase substrate

Actinis a 42-kDa protein which, due to its ability to polymerize into filaments (F-actin), is one of the major constituents of the cytoskeleton. It has been proposed that MARCKS (an acronym for myristoylated alanine-rich C kinase substrate) proteins play an important role in regulating the structure and mechanical properties of the actin cytoskeleton by cross-linking actin filaments. We have recently reported that peptides corresponding to the effector domain of MARCKS proteins promote actin polymerization and cause massive bundling of actin filaments. We now investigate the effect of MARCKS-related protein, a 20-kDa member of the MARCKS family, on both filament structure and the kinetics of actin polymerization in vitro. Our experiments document that MRP binds to F-actin with micromolar affinity and that the myristoyl chain at the N-terminus of MRP is not required for this interaction. In marked contrast to the effector peptide, binding of MRP is not accompanied by an acceleration of actin polymerization kinetics, and we also could not reliably observe an actin cross-linking activity of MRP.

Membrane binding of MARCKS-related protein studied by tryptophan fluorescence spectroscopy

MARCKS-related protein (MRP) is a peripheral membrane protein whose binding to membranes is mediated by the N-terminal myristoyl moiety and a central, highly basic effector domain. MRP mediates cross-talk between protein kinase C and calmodulin and is thought to link the actin cytoskeleton to the plasma membrane. Since MRP contains no tryptophan residues, we mutated a phenylalanine in the effector domain to tryptophan (MRP F93W) and used fluorescence spectroscopy to monitor binding of the protein to phospholipid vesicles. We report in detail the evaluation procedure necessary to extract quantitative information from the raw data. The spectra of MRP F93W obtained in the presence of increasing amounts of lipid crossed at an isosbestic point, indicating a simple transition between two states: free and membrane-bound protein. The change in fluorescence toward values typical of a more hydrophobic environment was used to quantify membrane binding. The partition coefficient agreed well with values obtained previously by other methods. To study the interaction of the N-terminus of MRP with membranes, a tryptophan residue was also introduced at position 4 (MRP S4W). Our data suggest that only the myristoylated N-terminus interacted with liposomes. These results demonstrate the versatility of site-directed incorporation of tryptophan residues to study protein-membrane interactions.

Protein kinase C-regulated dynamitin-macrophage-enriched myristoylated alanine-rice C kinase substrate interaction is involved in macrophage cell spreading

Macrophage spreading requires the microtubule cytoskeleton and protein kinase C (PKC). The mechanism of involvement of the microtubules and PKC in this event is not fully understood. Dynamitin is a subunit of dynactin, which is important for linking the microtubule-dependent motor protein dynein to vesicle membranes. We report that dynamitin is a Ca(2+)/calmodulin-binding protein and that dynamitin binds directly to macrophage-enriched myristoylated alanine-rice C kinase substrate (MacMARCKS), a membrane-associated PKC substrate involved in macrophage spreading and integrin activation. Dynamitin was found to copurify with MacMARCKS both during MacMARCKS purification with conventional chromatography and during the immunoabsorption of MacMARCKS using anti-MacMARCKS antibody. Vice versa, MacMARCKS was also found to cosediment with the 20 S dynactin complex. We determined that the effector domain of MacMARCKS is required to interact with the N-terminal domain of dynamitin. MacMARCKS and dynamitin also partially colocalized at peripheral regions of macrophages and in the cell-cell border of 293 epithelial cells. Treatment with phorbol esters abolished this colocalization. Disrupting the interaction with a short peptide derived from the MacMARCKS-binding domain of dynamitin caused macrophages to spread and flatten. These data suggest that the dynamitin-MacMARCKS interaction is involved in cell spreading. Furthermore, the regulation of this interaction by PKC and Ca(2+)/calmodulin provides a possible regulatory mechanism for cell adhesion and spreading.

Myristoylation-dependent N-terminal cleavage of the myristoylated alanine-rich C kinase substrate (MARCKS) by cellular extracts

The myristoylated alanine-rich C kinase substrate (MARCKS) has been proposed to regulate the plasticity of the actin cytoskeleton at its site of attachment to membranes. In macrophages, MARCKS is implicated in various cellular events including motility, adhesion and phagocytosis. In this report we show that macrophage extracts contain a protease which specifically cleaves human MARCKS, expressed in a cell-free system or in E. coli, between Lys-6 and Thr-7. Cleavage of MARCKS decreases its affinity for macrophage membranes by ca. one order of magnitude, highlighting the contribution of the myristoyl moiety of MARCKS to membrane binding. Importantly, cleavage requires myristoylation of MARCKS. Furthermore, MARCKS-related protein (MRP), the second member of the MARCKS family, is not digested. Since Thr-7 is lacking in MRP this suggests that Thr-7 at the P1 position is important for the recognition of lipid-modified substrates. A different product is observed when MARCKS is incubated with a calf brain cytosolic extract. This product can be remyristoylated in the presence of myristoyl-CoA and N-myristoyl transferase, demonstrating that cycles of myristoylation/demyristoylation of MARCKS can be achieved in vitro. Although the physiological relevance of these enzymes still needs to be demonstrated, our results reveal the presence of a new class of cleaving enzymes recognizing lipid-modified protein substrates.

Interaction between actin and the effector peptide of MARCKS-related protein. Identification of functional amino acid segments

It is widely assumed that the members of the MARCKS protein family, MARCKS (an acronym for myristoylated alanine-rich C kinase substrate) and MARCKS-related protein (MRP), interact with actin via their effector domain, a highly basic segment composed of 24-25 amino acid residues. To clarify the mechanisms by which this interaction takes place, we have examined the effect of a peptide corresponding to the effector domain of MRP, the so-called effector peptide, on both the dynamic and the structural properties of actin. We show that in the absence of cations the effector peptide polymerizes monomeric actin and causes the alignment of the formed filaments into bundle-like structures. Moreover, we document that binding of calmodulin or phosphorylation by protein kinase C both inhibit the actin polymerizing activity of the MRP effector peptide. Finally, several effector peptides were synthesized in which positively charged or hydrophobic segments were deleted or replaced by alanines. Our data suggest that a group of six positively charged amino acid residues at the N-terminus of the peptide is crucial for its interaction with actin. While its actin polymerizing activity critically depends on the presence of all three positively charged segments of the peptide, hydrophobic amino acid residues rather modulate the polymerization velocity.

Regulation of the binding of myristoylated alanine-rich C kinase substrate (MARCKS) related protein to lipid bilayer membranes by calmodulin

The effector domain (ED) of MARCKS proteins can associate with calmodulin (CaM) as well as with phospholipids. It is not clear, however, whether a complex between MARCKS proteins and CaM can form at the surface of phospholipid membranes or whether CaM and membranes compete for ED binding. Using two-mode waveguide spectroscopy, we have investigated how CaM regulates the association of MARCKS-related protein (MRP) with planar supported phospholipid bilayer membranes. Bringing a solution containing CaM into contact with membranes on which MRP had previously been deposited results in low-affinity binding of CaM to MRP. A preformed, high-affinity CaM MRP complex in the aqueous phase binds much more slowly than pure MRP to membranes. Similar observations were made when a peptide corresponding to the ED of MRP was used instead of MRP. Hence CaM cannot form a stable complex with MRP once the latter is bound at the membrane surface. CaM can, however, strongly retard the association of MRP with lipid membranes. The most likely interpretation of these results is that CaM and the phospholipid membrane share the same binding region at the ED and that the ED is forced by membrane binding to adopt a conformation unfavorable for CaM binding.

Mapping the interface between calmodulin and MARCKS-related protein by fluorescence spectroscopy

MARCKS-related protein (MRP) is a myristoylated protein kinase C substrate that binds calmodulin (CaM) with nanomolar affinity. To obtain structural information on this protein, we have engineered 10 tryptophan residues between positions 89 and 104 in the effector domain, a 24-residue-long amphipathic segment that mediates binding of MRP to CaM. We show that the effector domain is in a polar environment in free MRP, suggesting exposure to water, in agreement with a rod-shaped structure of the protein. The effector domain participates in the binding of MRP to CaM, as judged by the dramatic changes observed in the fluorescent properties of the mutants on complex formation. Intermolecular quenching of the fluorescence emission of the tryptophan residues in MRP by selenomethionine residues engineered in CaM reveals that the N-terminal side of the effector domain contacts the C-terminal domain of CaM, whereas the C-terminal side of the effector domain contacts the N-terminal domain of CaM. Finally, a comparison of the fluorescent properties of the myristoylated and unmyristoylated forms of a construct in which a tryptophan residue was introduced at position 4 close to the myristoylated N terminus of MRP suggests that the lipid moiety is also involved in the interaction of MRP with CaM.

Expression of MacMARCKS restores cell adhesion to ICAM-1-coated surface

To evaluate the role of MacMARCKS, a major substrate of protein kinase C, in cell adhesion, we selected a macrophage cell line, Wehi 274.1.7. Although surface expression of beta2-integrins can be detected on these cells, they lack the phorbol ester- or chemokine-induced adhesion to ICAM-1-coated surface, an event mediated by beta2-integrins. Concomitantly, these cells lack expression of both MacMARCKS and its homologue, MARCKS. When wild type MacMARCKS was expressed in these cells, the phorbol ester-induced adhesion to ICAM-1-coated surface increased approximately 5-fold compared to vector transfected control cells. To further investigate the potential physiological role of MacMARCKS in this adhesion event, we also tested the effect of monocyte chemotactic protein-1, and a 3-fold increase in the adhesion to ICAM-1-coated surface was observed with MacMARCKS-transfected cells. Therefore, these data suggest that MacMARCKS is an essential component in regulating cell adhesion.

Nonelectrostatic contributions to the binding of MARCKS-related protein to lipid bilayers

The association of various protein constructs of MARCKS-related protein (MRP) lacking the myristoyl moiety or the basic effector domain (ED) or both to neutral and acidic supported planar phospholipid bilayer membranes has been monitored using two-mode optical waveguide spectroscopy. The importance of the myristoyl moiety for interaction with both neutral and acidic membranes is demonstrated but unmyristoylated MRP still binds appreciably to neutral membranes, albeit less than to acidic membranes. Only when both the myristoyl moiety and the ED are excised does the interaction fall to zero in the case of the acidic membranes, with very small residual binding still detectable in the presence of neutral membranes. These results point to the importance of hydrophobic interactions apart from those associated with the myristoyl moiety in the association of MRP with membranes. The ED is well endowed with hydrophobic as well as with basic residues, and the former are chiefly responsible for binding unmyristoylated MRP to neutral membranes: The very small residual attraction between MRP lacking both the myristoyl moiety and the ED is completely outweighed by electrostatic repulsion between the net acidic MRP and the acidic lipid head groups.

Phosphorylated form of MacMARCKS is essential to LFA-1-dependent cell-cell adhesion of U937 monocytic cells

MacMARCKS (MRP, F52), a protein kinase C (PKC) substrate, is involved in the activation of beta2-integrin. To determine the role of the PKC-mediated phosphorylation of MacMARCKS in this process, human U937 monocytic cells were transfected with cDNAs encoding wild type or mutant MacMARCKS. We observed that the expression of the exogenous wild type MacMARCKS greatly enhanced LFA-1-mediated cell-cell adhesion in U937 cells treated with phorbol 12-myristate 13-acetate (PMA). This MacMARCKS-stimulated adhesion depended on the phosphorylation status of MacMARCKS: whereas phosphorylated MacMARCKS enhanced adhesion, unphosphorylated MacMARCKS inhibited it. However, phosphorylated MacMARCKS alone could not induce LFA-1-mediated cell-cell adhesion unless phorbol esters were added, suggesting that the phosphorylation of other proteins might also be involved. Okadaic acid, a phosphatase inhibitor, induced LFA-1-mediated cell-cell adhesion only in the cells expressing wild type or phosphorylated MacMARCKS and not in the cells expressing unphosphorylated MacMARCKS. Therefore, we conclude that the phosphorylated form of MacMARCKS is essential to LFA-1-mediated cell-cell adhesion.

MARCKS-related protein (MRP) is a substrate for the Leishmania major surface protease leishmanolysin (gp63)

Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP; MacMARCKS) are protein kinase C substrates in diverse cell types. Activation of murine macrophages by cytokines increases MRP expression, but infection with Leishmania promastigotes during activation results in MRP depletion. We therefore examined the effect of Leishmania major LV39 on recombinant MRP. Both live promastigotes and a soluble fraction of LV39 lysates degraded MRP to yield lower molecular weight fragments. Degradation was independent of MRP myristoylation and was inhibited by protein kinase C-dependent phosphorylation of MRP. MRP was similarly degraded by purified leishmanolysin (gp63), a Leishmania surface metalloprotease. Degradation was evident at low enzyme/substrate ratios, over a broad pH range, and was inhibited by 1,10-phenanthroline and by a hydroxamate dipeptide inhibitor of leishmanolysin. Using mass spectrometric analysis, cleavage was shown to occur within the effector domain of MRP between Ser(92) and Phe(93), in accordance with the substrate specificity of leishmanolysin. Moreover, an MRP construct in which the effector domain had been deleted was resistant to cleavage. Thus, Leishmania infection may result in leishmanolysin-dependent hydrolysis of MRP, a major protein kinase C substrate in macrophages.

Domain structure and lipid interaction of recombinant yeast Tim44

Tim44 is an essential component of the machinery that mediates the translocation of nuclear-encoded proteins across the mitochondrial inner membrane. It functions as a membrane anchor for the ATP-driven protein import motor whose other subunits are the mitochondrial 70-kDa heat-shock protein (mhsp70) and its nucleotide exchange factor, mGrpE. To understand how this motor is anchored to the inner membrane, we have overexpressed Tim44 in Escherichia coli and studied the properties of the pure protein and its interaction with model lipid membranes. Limited proteolysis and analytical ultracentrifugation indicate that Tim44 is an elongated monomer with a stably folded C-terminal domain. The protein binds strongly to liposomes composed of phosphatidylcholine and cardiolipin but only weakly to liposomes containing phosphatidylcholine alone. Studies with phospholipid monolayers suggest that Tim44 binds to phospholipids of the mitochondrial inner membrane both by electrostatic interactions and by penetrating the polar head group region.

Amphitropic proteins: regulation by reversible membrane interactions (review)

What do Src kinase, Ras-guanine nucleotide exchange factor, cytidylyltransferase, protein kinase C, phospholipase C, vinculin, and DnaA protein have in common? These proteins are amphitropic, that is, they bind weakly (reversibly) to membrane lipids, and this process regulates their function. Proteins functioning in transduction of signals generated in cell membranes are commonly regulated by amphitropism. In this review, the strategies utilized by amphitropic proteins to bind to membranes and to regulate their membrane affinity are described. The recently solved structures of binding pockets for specific lipids are described, as well as the amphipathic alpha-helix motif. Regulatory switches that control membrane affinity include modulation of the membrane lipid composition, and modification of the protein itself by ligand binding, phosphorylation, or acylation. How does membrane binding modulate the protein's function? Two mechanisms are discussed: (1) localization with the substrate, activator, or downstream target, and (2) activation of the protein by a conformational switch. This paper also addresses the issue of specificity in the cell membrane targetted for binding.

Down-regulation of MARCKS-related protein (MRP) in macrophages infected with Leishmania

Leishmania, a protozoan parasite of macrophages, has been shown to interfere with host cell signal transduction pathways including protein kinase C (PKC)-dependent signaling. Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP, MacMARCKS) are PKC substrates in diverse cell types. MARCKS and MRP are thought to regulate the actin network and thereby participate in cellular responses involving cytoskeletal rearrangement. Because MRP is a major PKC substrate in macrophages, we examined its expression in response to infection by Leishmania. Activation of murine macrophages by cytokines increased MRP expression as determined by Western blot analysis. Infection with Leishmania promastigotes at the time of activation or up to 48 h postactivation strongly decreased MRP levels. Leishmania-dependent MRP depletion was confirmed by [3H]myristate labeling and by immunofluorescence microscopy. All species or strains of Leishmania parasites tested, including lipophosphoglycan-deficient Leishmania major L119, decreased MRP levels. MRP depletion was not obtained with other phagocytic stimuli including zymosan, latex beads, or heat-killed Streptococcus mitis, a Gram-positive bacterium. Experiments with [3H]myristate labeled proteins revealed the appearance of lower molecular weight fragments in Leishmania-infected cells suggesting that MRP depletion may be due to proteolytic degradation.

Interactions of myristoylated alanine-rich C kinase substrate (MARCKS)-related protein with a novel solid-supported lipid membrane system (TRANSIL)

The determination of partition coefficients is crucial for the biochemical analysis of membrane-based processes, but requires tedious procedures. We have facilitated this analysis using a silica gel coated with a single phospholipid bilayer (TRANSIL) as the membranous phase. We demonstrate the validity of this method using MARCKS-related protein, a 20-kDa member of the MARCKS family (an acronym for myristoylated alanine-rich C kinase substrate). The partition coefficients describing the association of unmyristoylated and myristoylated MARCKS-related protein with membranes of different phospholipid composition are in agreement with previous work with vesicles and show that both the myristoyl moiety and the basic effector domain of MARCKS-related protein mediate the binding. However, no significant cooperativity is observed between these two domains. Interestingly, MARCKS-related protein binds to TRANSIL membranes more strongly at temperatures below their phase-transition temperature. Taking advantage of this property, MARCKS-related protein was purified by phase-transition chromatography, loading Escherichia coli lysates on a TRANSIL column at 4 degrees C and eluting MRP at room temperature. In conclusion, TRANSIL is a versatile tool to determine the affinity of compounds for phospholipid membranes and to purify membrane-bound proteins. TRANSIL should also enable functional studies of protein-ligand and protein-protein interactions at the surface of membranes.

Myristoylated alanine-rich C-kinase substrate is phosphorylated and translocated by a phorbol ester-insensitive and calcium-independent protein kinase C isoform in C6 glioma cell membranes

Myristoylated alanine-rich C-kinase substrate (MARCKS), a prominent substrate for conventional and novel protein kinase C (PKC) isoforms, is involved in the regulation of membrane-cytoskeletal interactions. Addition of [gamma-32P]ATP to the membrane fraction of digitonin-permeabilized C6 glioma cells resulted in phosphorylation and release of MARCKS, indicating involvement of an active membrane-bound kinase. Pretreatment of cells with 2 microM 4 beta-12-O-tetradecanoyl-phorbol-13-acetate (beta-TPA) for 18 h downregulated conventional (PKC alpha) and novel (PKC delta) isoforms of PKC by > 90% in both membrane and soluble fractions, but did not inhibit the rate of ATP-dependent phosphorylation or release of MARCKS, or decrease levels of membrane-bound PKC zeta or PKC mu. MARCKS phosphorylation was inhibited by staurosporine, bis-indolylmaleimide (a PKC-specific inhibitor), Gö6983 (inhibits all isoforms except PKC mu), and a peptide from the calmodulin-binding domain of MARCKS, but was unaffected by EGTA or Gö6976 (inhibits cPKCs and PKC mu). Peptide mapping indicated similar in vivo and in vitro phosphorylation at serine residue(s) known to be phosphorylated by PKC. These findings support a novel mechanism by which MARCKS may be regulated by an atypical PKC isoform in phorbol ester-downregulated cells.

Dynamics and ordering in mixed model membranes of dimyristoylphosphatidylcholine and dimyristoylphosphatidylserine: a 250-GHz electron spin resonance study using cholestane

We report here on a 250-GHz electron spin resonance (ESR) study of macroscopically aligned model membranes composed of mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS), utilizing the nixtroxide-labeled cholesterol analog cholestane (CSL). Two clearly resolved spectral components, distinct in both their ordering and dynamics, are resolved. The major component in membranes composed mostly of DMPC shows typical characteristics, with the long axis of CSL parallel to the bilayer normal with slow (10(6) </= R </= 10(7) s-1) rotational diffusion rates, as expected for cholesterol. The second component grows in as the mole fraction of DMPS increases. A detailed analysis shows that CSL senses a local, strongly biaxial environment. Our results imply that the inefficient packing between cholesterol and DMPS occurs probably because of the strong interactions between the PS headgroups, which provide the local biaxiality. Such a packing of the headgroups has been predicted by molecular dynamics simulations but had not been observed experimentally. The analysis of these spectral components was greatly aided by the excellent orientational resolution provided by the 250-GHz spectra. This enabled the key qualitative features of this interpretation to be "read" off the spectra before the detailed analysis.