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Abstract
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Peptide amphiphiles (PAs) are small molecules
that contain hydrophobic
components covalently conjugated to peptides. In this Account, we
describe recent advances involving PAs that consist of a short peptide sequence linked to an aliphatic tail. The peptide sequence
can be designed to form β-sheets among the amino acids near
the alkyl tail, while the residues farthest from the tail are charged
to promote solubility and in some cases contain a bioactive sequence.
In water, β-sheet formation and hydrophobic collapse of the
aliphatic tails induce assembly of the molecules into supramolecular
one-dimensional nanostructures, commonly high-aspect-ratio cylindrical
or ribbonlike nanofibers. These nanostructures hold significant promise
for biomedical functions due to their ability to display a high density of biological signals on their surface for targeting or to activate pathways,
as well as for biocompatibility and biodegradable nature. Recent
studies have shown that supramolecular systems, such as
PAs, often become kinetically trapped in local minima along their
self-assembly reaction coordinate, not unlike the pathways associated
with protein folding. Furthermore, the assembly pathway can influence
the shape, internal structure, and dimension of nanostructures and
thereby affect their bioactivity. We discuss methods to map the energy
landscape of a PA structure as a function of thermal energy and ionic
strength and vary these parameters to convert between kinetically
trapped and thermodynamically favorable states. We also demonstrate
that the pathway-dependent morphology of the PA assembly can determine
biological cell adhesion and survival rates. The dynamics associated
with the nanostructures are also critical
to their function, and techniques are now available to probe the internal
dynamics of these nanostructures. For example, by conjugating radical
electron spin labels to PAs, electron paramagnetic resonance spectroscopy can be
used to study the rotational diffusion rates within the fiber, showing
a liquidlike to solidlike transition through the cross section of
the nanofiber. PAs can also be labeled with fluorescent dyes, allowing
the use of super-resolution microscopy techniques to study the molecular
exchange dynamics between PA fibers. For a weak hydrogen-bonding PA,
individual PA molecules or clusters exchange between fibers in time
scales as short as minutes. The amount of hydrogen bonding within
PAs that dictates the dynamics also plays an important role in biological
function. In one case, weak hydrogen bonding within a PA resulted
in cell death through disruption of lipid membranes, while in another
example reduced hydrogen bonding enhanced growth factor signaling
by increasing lipid raft mobility. PAs are a promising platform
for designing advanced hybrid materials.
We discuss a covalent polymer with a rigid aromatic imine backbone
and alkylated peptide side chains that simultaneously polymerizes
and interacts with a supramolecular PA structure with identical chemistry
to that of the side chains. The covalent polymerization can be “catalyzed”
by noncovalent polymerization of supramolecular monomers, taking advantage
of the dynamic nature of supramolecular assemblies. These novel hybrid
structures have potential in self-repairing materials and as reusable
scaffolds for delivery of drugs or other chemicals. Finally, we highlight
recent biomedical applications of PAs and related structures, ranging
from bone regeneration to decreasing blood loss during internal bleeding.
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Affiliation(s)
- Mark P. Hendricks
- Simpson Querrey Institute for BioNanotechnology, Northwestern University, Chicago, Illinois 60611, United States
| | - Kohei Sato
- Simpson Querrey Institute for BioNanotechnology, Northwestern University, Chicago, Illinois 60611, United States
| | - Liam C. Palmer
- Simpson Querrey Institute for BioNanotechnology, Northwestern University, Chicago, Illinois 60611, United States
- Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
| | - Samuel I. Stupp
- Simpson Querrey Institute for BioNanotechnology, Northwestern University, Chicago, Illinois 60611, United States
- Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
- Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
- Department of Medicine, Northwestern University, Chicago, Illinois 60611, United States
- Department of Biomedical Engineering, Northwestern University, Evanston, Illinois 60208, United States
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Aittaleb M, Chen PJ, Akaaboune M. Failure of lysosome clustering and positioning in the juxtanuclear region in cells deficient in rapsyn. J Cell Sci 2015; 128:3744-56. [PMID: 26330529 DOI: 10.1242/jcs.172536] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2015] [Accepted: 08/23/2015] [Indexed: 01/06/2023] Open
Abstract
Rapsyn, a scaffold protein, is required for the clustering of acetylcholine receptors (AChRs) at contacts between motor neurons and differentiating muscle cells. Rapsyn is also expressed in cells that do not express AChRs. However, its function in these cells remains unknown. Here, we show that rapsyn plays an AChR-independent role in organizing the distribution and mobility of lysosomes. In cells devoid of AChRs, rapsyn selectively induces the clustering of lysosomes at high density in the juxtanuclear region without affecting the distribution of other intracellular organelles. However, when the same cells overexpress AChRs, rapsyn is recruited away from lysosomes to colocalize with AChR clusters on the cell surface. In rapsyn-deficient (Rapsn(-/-)) myoblasts or cells overexpressing rapsyn mutants, lysosomes are scattered within the cell and highly dynamic. The increased mobility of lysosomes in Rapsn(-/-) cells is associated with a significant increase in lysosomal exocytosis, as evidenced by increased release of lysosomal enzymes and plasma membrane damage when cells were challenged with the bacterial pore-forming toxin streptolysin-O. These findings uncover a new link between rapsyn, lysosome positioning, exocytosis and plasma membrane integrity.
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Affiliation(s)
- Mohamed Aittaleb
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI 48109, USA
| | - Po-Ju Chen
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI 48109, USA
| | - Mohammed Akaaboune
- Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI 48109, USA Program in Neuroscience, University of Michigan, Ann Arbor, MI 48109, USA
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Antolik C, Catino DH, O’Neill AM, Resneck WG, Ursitti JA, Bloch RJ. The actin binding domain of ACF7 binds directly to the tetratricopeptide repeat domains of rapsyn. Neuroscience 2007; 145:56-65. [PMID: 17222516 PMCID: PMC1868462 DOI: 10.1016/j.neuroscience.2006.11.047] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2006] [Revised: 09/01/2006] [Accepted: 11/12/2006] [Indexed: 11/16/2022]
Abstract
Formation of the neuromuscular junction requires the release of agrin from the presynaptic terminal of motor neurons. Clustering of acetylcholine receptors (AChRs) on the postsynaptic sarcolemma is initiated by agrin-dependent activation of the muscle-specific kinase. While the postsynaptic scaffolding protein rapsyn is vital for high density AChR aggregation, little is known about the mechanism through which AChRs are immobilized on the postsynaptic membrane. Ultrastructural and immunohistochemical studies of rat skeletal muscle have suggested that AChRs are anchored to a membrane-associated cytoskeleton that contains spectrin-like proteins and is thus similar to that of the human erythrocyte [Bloch RJ, Bezakova G, Ursitti JA, Zhou D, Pumplin DW (1997) A membrane skeleton that clusters nicotinic acetylcholine receptors in muscle. Soc Gen Physiol Ser 52:177-195]. We are studying a protein of the spectrin superfamily, ACF7 (also known as MACF), as a postsynaptic cytoskeletal component of the neuromuscular junction. ACF7 has multiple cytoskeleton-binding domains, including an N-terminal actin-binding domain that, we postulate, may interact with rapsyn, the scaffolding protein that binds directly to AChRs. To test this hypothesis, we co-expressed fragments of these molecules in cultured fibroblasts and assessed their co-distribution and interaction using confocal microscopy and co-immunoprecipitation. We demonstrate that the actin-binding domain of ACF7 specifically interacts with the tetratricopeptide repeat domains of rapsyn. Furthermore, we show using surface plasmon resonance and blot overlay that the actin-binding domain of ACF7 binds directly to rapsyn. These results suggest that, in mammalian skeletal muscle, AChRs are immobilized in the membrane through rapsyn-mediated anchoring to an ACF7-containing network that in turn is linked to the actin cytoskeleton.
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Affiliation(s)
- Christian Antolik
- Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
| | - Dawn H. Catino
- Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
| | - Andrea M. O’Neill
- Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
| | - Wendy G. Resneck
- Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
| | - Jeanine A. Ursitti
- University of Maryland Biotechnology Institute, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
| | - Robert J. Bloch
- Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA
- Corresponding author. 655 W. Baltimore St., Baltimore, MD 21201, USA; Tel: +1-410-706-3020; Fax: +1-410-706-8341. E-mail address: (R.J. Bloch)
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Hoffman MD, Kast J. Mass spectrometric characterization of lipid-modified peptides for the analysis of acylated proteins. JOURNAL OF MASS SPECTROMETRY : JMS 2006; 41:229-41. [PMID: 16421873 DOI: 10.1002/jms.981] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/06/2023]
Abstract
The analysis of acylated proteins by mass spectrometry (MS) has largely been overshadowed in proteomics by the analysis of glycosylated and phosphorylated proteins; however, lipid modifications on proteins are proving to be of increasing importance in biomedical research. In order to identify the marker ions and/or neutral loss fragments that are produced upon collision-induced dissociation, providing a means to identify the common lipid modifications on proteins, peptides containing an N-terminally myristoylated glycine, a palmitoylated cysteine and a farnesylated cysteine were chemically synthesized. Matrix-assisted laser desorption/ionization time-of-flight time-of-flight (MALDI-TOF-TOF), electrospray ionization quadrupole time-of-flight (ESI Q-TOF), and electrospray ionization hybrid triple-quadrupole/linear ion trap (ESI QqQ(LIT)) mass spectrometers were used for the analysis. The peptide containing the N-terminally myristoylated glycine, upon CID, produced the characteristic fragments a1 (240.4 Th) and b1 (268.4 Th) ions as well as a low-intensity neutral loss of 210 Da (C14H26O). The peptides containing a farnesylated cysteine residue fragmented to produce a marker ion at a m/z of 205 Th (C15H25) as well as other intense farnesyl fragment ions, and a neutral loss of 204 Da (C15H24). The peptides containing a palmitoylated cysteine moiety generated neutral losses of 238 Da (C16H30O) and 272 Da (C16H32OS); however, no marker ions were produced. The neutral losses were more prominent in the MALDI-TOF-TOF spectra, whereas the marker ions were more abundant in the ESI QqQ(LIT) and Q-TOF mass spectra.
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Affiliation(s)
- Michael D Hoffman
- The Biomedical Research Centre, University of British Columbia, Vancouver, BC, Canada
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Chen TF, Yoder JD, Hruby DE. Mass spectrometry analysis of synthetically myristoylated peptides. EUROPEAN JOURNAL OF MASS SPECTROMETRY (CHICHESTER, ENGLAND) 2004; 10:501-508. [PMID: 15302975 DOI: 10.1255/ejms.652] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Myristoylpeptides were synthesized in order to determine if a neutral loss of 210 Da, C14H26O (the mass of the myristoyl moiety), was universal and observable by both liquid chromatography electrospray ionization quadrupole ion trap (LC-ESI-QIT) and matrix-assisted laser desorption/ionization time-of-flight time-of-flight (MALDI-ToF/ToF) mass spectrometry. Myristoylation was successfully introduced on the N-terminus, internally on the amino group of lysine and arginine. Larger peptides and the arginine compounds needed elevated temperatures for myristoylation. To our knowledge, this is the first report of a chemically-synthesized myristoylated arginine in a peptide. Collision energy studies for the LC-ESI-QIT instrument showed that modified peptides and a loss of 210 Da could be detected under commonly used conditions (energy level between 30 and 42%) with picomole amounts of sample. The loss of myristoyl moiety is observed on the MALDI-Tof/Tof mass spectrometer as well. Due to the hydrophobic properties of the myristoyl moiety, it is not surprising that the modified peptides all formed at least dimers, and in some cases trimers. We were also able to distinguish a mixture of two mono-myristoylated peptides. MS3 data from the LC-ESI-QIT instrument on a di-myristoylated peptide indicates the loss of 210 Da at either the N-terminus or lysine. We were also able to analyze a mixture of modified and unmodified peptides on the MALDI-ToF/ToF instrument. The data presented in this paper demonstrates the constant neutral loss of the 210 Da, C14H26O, from both N-terminally and internally myristoylated peptides can be identified unambiguously using LC-ESI-QIT or MALDI-ToF/ToF mass spectrometers. This will be a useful tool in determining the myristoylation status of candidate proteins after enzyme digestion, and in elucidating the modification sites of internal myristoyl proteins.
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Affiliation(s)
- Tsefang F Chen
- Department of Microbiology, Oregon State University, 220 Nash Hall, Corvallis, OR 97331-3804, USA
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Lukas RJ, Tubbs KA, Krivoshein AV, Bieber AL, Nelson RW. Mass spectrometry of nicotinic acetylcholine receptors and associated proteins as models for complex transmembrane proteins. Anal Biochem 2002; 301:175-88. [PMID: 11814288 DOI: 10.1006/abio.2001.5491] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Studies were conducted to optimize matrix-assisted laser desorption/ionization, time-of-flight mass spectrometry (MALDI TOF MS) in analyzing the composition of nicotinic acetylcholine receptors (nAChR) from Torpedo californica electric tissue in their membrane-bound, detergent-solubilized, and affinity-purified states. Mass spectra obtained from nAChR-rich membrane fractions gave reasonably good representations of protein compositions indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of those same samples. Efficiency of extraction of nAChR from membranes was not markedly different for most detergents, but quality and signal size of mass spectra were clearly influenced by detergent composition and concentration, protein concentration, and MALDI matrix composition. The best spectra, allowing detection and accurate size determinations for samples containing as little as 10 fmol of pure nAChR, were obtained for samples solubilized in Triton X-100 and assayed by use of a sinapinic acid matrix. Although informative spectra could be obtained for nAChR affinity purified on alpha-cobratoxin (Naja naja siamensis) columns and extracted using sinapinic acid, superior spectra with much higher signal:noise were obtained if extraction media contained Triton X-100 or sodium dodecyl sulfate. nAChR subunit masses determined were similar regardless of the membrane-associated, detergent-solubilized, or affinity-purified state of the preparation. These studies illustrate how masses can be determined for nAChR subunits and for other protein components in Torpedo membrane preparations, such as RAPsyn and Na(+)-K(+)-ATPase alpha and beta subunits. They also provide an underpinning for streamlined analysis of the composition of complex transmembrane proteins using MALDI TOF MS.
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Affiliation(s)
- Ronald J Lukas
- Division of Neurobiology, Barrow Neurological Institute, 350 West Thomas Road, Phoenix, Arizona 85013, USA.
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Ramarao MK, Bianchetta MJ, Lanken J, Cohen JB. Role of rapsyn tetratricopeptide repeat and coiled-coil domains in self-association and nicotinic acetylcholine receptor clustering. J Biol Chem 2001; 276:7475-83. [PMID: 11087759 DOI: 10.1074/jbc.m009888200] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Rapsyn, a 43-kDa peripheral membrane protein of skeletal muscle, is essential for clustering nicotinic acetylcholine receptors (nAChR) in the postsynaptic membrane. Previous studies with rapsyn NH(2)-terminal fragments fused to green fluorescent protein, expressed in 293T cells along with nAChRs, establish the following: Rapsyn-(1-90), containing the myristoylated amino terminus and two tetratricopeptide repeats (TPRs), was sufficient for self-association at the plasma membrane; rapsyn-(1-287), containing seven TPRs, did not cluster nAChRs; whereas rapsyn-(1-360)(,) containing a coiled-coil domain (rapsyn-(298-331)), clustered nAChRs. To further analyze the role of rapsyn structural domains in self-association and nAChR clustering, we have characterized the clustering properties of additional rapsyn mutants containing deletions and substitutions within the TPR and coiled-coil domains. A mutant lacking the coiled-coil domain alone (rapsyn-(black triangle288-348)), failed to cluster nAChRs. Within the coiled-coil domain neutralization of the charged side chains was tolerated, while alanine substitutions of large hydrophobic residues resulted in the loss of nAChR clustering. Rapsyn self-association requires at least two TPRs, as a single TPR (TPR1 or TPR2 alone) was not sufficient. While TPRs 1 and 2 are sufficient for self-association, they are not necessary, as TPRs 3-7 also formed clusters similar to wild-type rapsyn. Fragments containing TPRs co-localized with full-length rapsyn, while the expressed coiled-coil or RING-H2 domain did not. These results are discussed in terms of a homology model of rapsyn, based on the three-dimensional structure of the TPR domain of protein phosphatase 5.
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Affiliation(s)
- M K Ramarao
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA
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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
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9
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Bignami F, Camus G, Marchand S, Bailly L, Stetzkowski-Marden F, Cartaud J. Targeting of acetylcholine receptor and 43 kDa rapsyn to the postsynaptic membrane in Torpedo marmorata electrocyte. JOURNAL OF PHYSIOLOGY, PARIS 1998; 92:177-81. [PMID: 9789804 DOI: 10.1016/s0928-4257(98)80006-5] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
In this study we have investigated the intracellular routing of two major components of the postsynaptic membrane in Torpedo electrocytes, the nicotinic acetylcholine receptor and the extrinsic 43 kDa protein rapsyn, and of a protein from the non-innervated membrane, the Na+,K+ ATPase. We isolated subpopulations of post-Golgi vesicles (PGVs) enriched either in AChR or in Na+,K+ ATPase. Rapsyn was associated to AChR-containing PGVs suggesting that both AChR and rapsyn are targeted to intracellular organelles in the secretory pathway before delivery to the postsynaptic membrane. In vitro assays further show that rapsyn-containing PVGs do bind more efficiently to microtubules compared to Na+,K+ ATPase-enriched PVGs. These data provide evidence in favor of the contribution of the secretory pathway to the delivery of synaptic components.
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Affiliation(s)
- F Bignami
- Institut Jacques-Monod, CNRS, Universités Paris 6, France
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Ramarao MK, Cohen JB. Mechanism of nicotinic acetylcholine receptor cluster formation by rapsyn. Proc Natl Acad Sci U S A 1998; 95:4007-12. [PMID: 9520483 PMCID: PMC19953 DOI: 10.1073/pnas.95.7.4007] [Citation(s) in RCA: 121] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
Rapsyn, a peripheral membrane protein of skeletal muscle, clusters nicotinic acetylcholine receptors (nAChRs) at high density in the postsynaptic membrane. The mechanism of nAChR clustering by rapsyn was analyzed by expressing nAChRs in HEK293T cells with various fragments of mouse rapsyn fused to green fluorescent protein. Membrane targeting of rapsyn is conferred solely by its acylated N terminus, as the myristoylated N-terminal 15 amino acids of rapsyn are sufficient to target green fluorescent protein to the plasma membrane. However, neither N-terminal myristoylation nor the conserved N-terminal amino acid sequence is essential. Membrane targeting, self-association, and nAChR clustering are preserved when the first 10 amino acids of rapsyn were replaced by those of src, which also contains a consensus sequence for N-myristoylation, or by those of GAP43, which contains a palmitoylation sequence. Rapsyn1-90, containing two tetratrichopeptide repeats is sufficient for self-association. Rapsyn1-360, lacking the cysteine rich domain, clusters nAChRs, while rapsyn1-287, containing seven tetratrichopeptide repeats, does not cluster nAChRs. We identified rapsyn298-331 as a potential coiled-coil domain, and established that mutations disrupting coiled-coil propensity prevent nAChR clustering. Thus the structural domains of rapsyn necessary for membrane targeting, self-association, and nAChR clustering are distinct, with nAChR-rapsyn interaction mediated by a previously unrecognized coiled-coil motif.
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Affiliation(s)
- M K Ramarao
- Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115, USA
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Scotland PB, Colledge M, Melnikova I, Dai Z, Froehner SC. Clustering of the acetylcholine receptor by the 43-kD protein: involvement of the zinc finger domain. J Cell Biol 1993; 123:719-28. [PMID: 8227134 PMCID: PMC2200117 DOI: 10.1083/jcb.123.3.719] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
A postsynaptic membrane-associated protein of M(r) 43,000 (43-kD protein) is involved in clustering of the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Previous studies have shown that recombinant mouse 43-kD protein forms membrane-associated clusters when expressed in Xenopus oocytes. Coexpression with the AChR results in colocalization of the receptor with the 43-kD protein clusters (Froehner, S. C., C. W. Luetje, P. B. Scotland, and J. Patrick, 1990. Neuron. 5:403-410). To understand the mechanism of this clustering, we have studied the role of the carboxy-terminal region of the 43-kD protein. The amino acid sequence of this region predicts two tandem zinc finger structures followed by a serine phosphorylation site. Both Torpedo 43-kD protein and the carboxy-terminal region of the mouse 43-kD protein bind radioisotopic zinc. Mutation of two histidine residues in this predicted domain greatly attenuates zinc binding, lending support to the proposal that this region forms zinc fingers. When expressed in oocytes, the ability of this mutant 43-kD protein to form clusters is greatly reduced. Its ability to interact with AChR, however, is retained. In contrast, a mutation that eliminates the potential serine phosphorylation site has no effect on clustering of the 43-kD protein or on interaction with the AChR. These findings suggest that protein interactions via the zinc finger domain of the 43-kD protein may be important for AChR clustering at the synapse.
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Affiliation(s)
- P B Scotland
- Department of Physiology, University of North Carolina, Chapel Hill 27599
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Phillips WD, Maimone MM, Merlie JP. Mutagenesis of the 43-kD postsynaptic protein defines domains involved in plasma membrane targeting and AChR clustering. J Cell Biol 1991; 115:1713-23. [PMID: 1757470 PMCID: PMC2289204 DOI: 10.1083/jcb.115.6.1713] [Citation(s) in RCA: 79] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
The postsynaptic membrane of the neuromuscular junction contains a myristoylated 43-kD protein (43k) that is closely associated with the cytoplasmic face of the nicotinic acetylcholine receptor (AChR)-rich plasma membrane. Previously, we described fibroblast cell lines expressing recombinant AChRs. Transfection of these cell lines with 43k was necessary and sufficient for reorganization of AChR into discrete 43k-rich plasma membrane domains (Phillips, W. D., C. Kopta, P. Blount, P. D. Gardner, J. H. Steinbach, and J. P. Merlie. 1991. Science (Wash. DC). 251:568-570). Here we demonstrate the utility of this expression system for the study of 43k function by site-directed mutagenesis. Substitution of a termination codon for Asp254 produced a truncated (28-kD) protein that associated poorly with the cell membrane. The conversion of Gly2 to Ala2, to preclude NH2-terminal myristoylation, reduced the frequency with which 43k formed plasma membrane domains by threefold, but did not eliminate the aggregation of AChRs at these domains. Since both NH2 and COOH-termini seemed important for association of 43k with the plasma membrane, a deletion mutant was constructed in which the codon Gln15 was fused in-frame to Ile255 to create a 19-kD protein. This mutated protein formed 43k-rich plasma membrane domains at wild-type frequency, but the domains failed to aggregate AChRs, suggesting that the central part of the 43k polypeptide may be involved in AChR aggregation. Our results suggest that membrane association and AChR interactions are separable functions of the 43k molecule.
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Affiliation(s)
- W D Phillips
- Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110
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Hartman DS, Millar NS, Claudio T. Extracellular synaptic factors induce clustering of acetylcholine receptors stably expressed in fibroblasts. J Cell Biol 1991; 115:165-77. [PMID: 1918134 PMCID: PMC2289913 DOI: 10.1083/jcb.115.1.165] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
The clustering of nicotinic acetylcholine receptors (AChRs) is one of the first events observed during formation of the neuromuscular junction. To determine the mechanism involved in AChR clustering, we established a nonmuscle cell line (mouse fibroblast L cells) that stably expresses just one muscle-specific gene product, the AChR. We have shown that when Torpedo californica AChRs are expressed in fibroblasts, their immunological, biochemical, and electrophysiological properties all indicate that fully functional cell surface AChRs are produced. In the present study, the cell surface distribution and stability of Torpedo AChRs expressed in fibroblasts (AChR-fibroblasts) were analyzed and shown to be similar to nonclustered AChRs expressed in muscle cells. AChR-fibroblasts incubated with antibodies directed against the AChR induced the formation of small AChR microclusters (less than 0.5 micron 2) and caused an increase in the internalization rate and degradation of surface AChRs (antigenic modulation) in a manner similar to that observed in muscle cells. Two disparate sources of AChR clustering factors, extracellular matrix isolated from Torpedo electric organ and conditioned media from a rodent neuroblastoma-glioma hybrid cell line, each induced large (1-3 microns 2), stable AChR clusters with no change in the level of surface AChR expression. By exploiting the temperature-sensitive nature of Torpedo AChR assembly, we were able to demonstrate that factor-induced clusters were produced by mobilization of preexisting surface AChRs, not by directed insertion of newly synthesized AChRs. AChR clusters were never observed in the absence of extracellular synaptic factors. Our results suggest that these factors can interact directly with the AChR.
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Affiliation(s)
- D S Hartman
- Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510
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14
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Affiliation(s)
- S C Froehner
- Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03756
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15
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Cockcroft VB, Osguthorpe DJ, Barnard EA, Friday AE, Lunt GG. Ligand-gated ion channels. Homology and diversity. Mol Neurobiol 1990; 4:129-69. [PMID: 1725701 DOI: 10.1007/bf02780338] [Citation(s) in RCA: 53] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
MESH Headings
- Amino Acid Sequence
- Animals
- Binding Sites
- Biological Evolution
- Consensus Sequence
- GABA-A Receptor Antagonists
- Humans
- Ion Channel Gating/drug effects
- Ion Channel Gating/physiology
- Ion Channels/drug effects
- Ion Channels/genetics
- Ion Channels/physiology
- Ligands
- Models, Molecular
- Molecular Sequence Data
- Multigene Family
- Nicotinic Antagonists
- Protein Conformation
- Receptors, GABA-A/drug effects
- Receptors, GABA-A/genetics
- Receptors, GABA-A/physiology
- Receptors, Glycine
- Receptors, Neurotransmitter/antagonists & inhibitors
- Receptors, Neurotransmitter/drug effects
- Receptors, Neurotransmitter/genetics
- Receptors, Neurotransmitter/physiology
- Receptors, Nicotinic/drug effects
- Receptors, Nicotinic/genetics
- Receptors, Nicotinic/physiology
- Sequence Alignment
- Sequence Homology, Nucleic Acid
- Signal Transduction
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16
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Tsui HC, Cohen JB, Fischbach GD. Variation in the ratio of acetylcholine receptors and the Mr 43,000 receptor-associated protein in embryonic chick myotubes and myoblasts. Dev Biol 1990; 140:437-46. [PMID: 2373261 DOI: 10.1016/0012-1606(90)90092-w] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- H C Tsui
- Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110
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17
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Froehner SC. Expression of RNA transcripts for the postsynaptic 43 kDa protein in innervated and denervated rat skeletal muscle. FEBS Lett 1989; 249:229-33. [PMID: 2737281 DOI: 10.1016/0014-5793(89)80629-5] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
A cDNA clone encoding the mouse muscle postsynaptic 43 kDa protein was isolated and sequenced. The amino acid sequence of this protein, which is closely associated with nicotinic acetylcholine receptors at Torpedo electrocyte and vertebrate skeletal muscle synapses, is very similar in different species. A cysteine-rich region homologous to part of the regulatory domain of protein kinase C may be important in interactions of this protein with the lipid bilayer. RNA transcripts for the 43 kDa protein increase only 2-3 fold after denervation of rat skeletal muscle, in sharp contrast to the alpha-subunit of the muscle nicotinic receptor which increases more than 30-fold. Thus, the expression of these two proteins is regulated by different mechanisms.
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Affiliation(s)
- S C Froehner
- Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03756
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