Identification of the palmitoylation site in rat myelin P0 glycoprotein

The cytoplasmic tail of myelin protein zero induces morphological changes in lipid membranes

The major myelin protein expressed by the peripheral nervous system Schwann cells is protein zero (P0), which represents 50% of the total protein content in myelin. This 30-kDa integral membrane protein consists of an immunoglobulin (Ig)-like domain, a transmembrane helix, and a 69-residue C-terminal cytoplasmic tail (P0ct). The basic residues in P0ct contribute to the tight packing of myelin lipid bilayers, and alterations in the tail affect how P0 functions as an adhesion molecule necessary for the stability of compact myelin. Several neurodegenerative neuropathies are related to P0, including the more common Charcot-Marie-Tooth disease (CMT) and Dejerine-Sottas syndrome (DSS) as well as rare cases of motor and sensory polyneuropathy. We found that high P0ct concentrations affected the membrane properties of bicelles and induced a lamellar-to-inverted hexagonal phase transition, which caused bicelles to fuse into long, protein-containing filament-like structures. These structures likely reflect the formation of semicrystalline lipid domains with potential relevance for myelination. Not only is P0ct important for stacking lipid membranes, but time-lapse fluorescence microscopy also shows that it might affect membrane properties during myelination. We further describe recombinant production and low-resolution structural characterization of full-length human P0. Our findings shed light on P0ct effects on membrane properties, and with the successful purification of full-length P0, we have new tools to study the role of P0 in myelin formation and maintenance in vitro.

How Does Protein Zero Assemble Compact Myelin?

Myelin protein zero (P0), a type I transmembrane protein, is the most abundant protein in peripheral nervous system (PNS) myelin-the lipid-rich, periodic structure of membrane pairs that concentrically encloses long axonal segments. Schwann cells, the myelinating glia of the PNS, express P0 throughout their development until the formation of mature myelin. In the intramyelinic compartment, the immunoglobulin-like domain of P0 bridges apposing membranes via homophilic adhesion, forming, as revealed by electron microscopy, the electron-dense, double "intraperiod line" that is split by a narrow, electron-lucent space corresponding to the extracellular space between membrane pairs. The C-terminal tail of P0 adheres apposing membranes together in the narrow cytoplasmic compartment of compact myelin, much like myelin basic protein (MBP). In mouse models, the absence of P0, unlike that of MBP or P2, severely disturbs myelination. Therefore, P0 is the executive molecule of PNS myelin maturation. How and when P0 is trafficked and modified to enable myelin compaction, and how mutations that give rise to incurable peripheral neuropathies alter the function of P0, are currently open questions. The potential mechanisms of P0 function in myelination are discussed, providing a foundation for the understanding of mature myelin development and how it derails in peripheral neuropathies.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Lipid membrane association of myelin proteins and peptide segments studied by oriented and synchrotron radiation circular dichroism spectroscopy

Myelin-specific proteins are either integral or peripheral membrane proteins that, in complex with lipids, constitute a multilayered proteolipid membrane system, the myelin sheath. The myelin sheath surrounds the axons of nerves and enables rapid conduction of axonal impulses. Myelin proteins interact intimately with the lipid bilayer and play crucial roles in the assembly, function, and stability of the myelin sheath. Although myelin proteins have been investigated for decades, their structural properties upon membrane surface binding are still largely unknown. In this study, we have used simplified model systems consisting of synthetic peptides and membrane mimics, such as detergent micelles and/or lipid vesicles, to probe the conformation of peptides using synchrotron radiation circular dichroism spectroscopy (SRCD). Additionally, oriented circular dichroism spectroscopy (OCD) was employed to examine the orientation of myelin peptides in macroscopically aligned lipid bilayers. Various representative peptides from the myelin basic protein (MBP), P0, myelin/oligodencrocyte glycoprotein, and connexin32 (cx32) were studied. A helical peptide from the central immunodominant epitope of MBP showed a highly tilted orientation with respect to the membrane surface, whereas the N-terminal cytoplasmic segment of cx32 folded into a helical structure that was only slightly tilted. The folding of full-length myelin basic protein was, furthermore, studied in a bicelle environment. Our results provide information on the conformation and membrane alignment of important membrane-binding peptides in a membrane-mimicking environment, giving novel insights into the mechanisms of membrane binding and stacking by myelin proteins.

The palmitoylation state of PMP22 modulates epithelial cell morphology and migration

PMP22 (peripheral myelin protein 22), also known as GAS 3 (growth-arrest-specific protein 3), is a disease-linked tetraspan glycoprotein of peripheral nerve myelin and constituent of intercellular junctions in epithelia. To date, our knowledge of the post-translational modification of PMP22 is limited. Using the CSS-Palm 2.0 software we predicted that C85 (cysteine 85), a highly conserved amino acid located between the second and third transmembrane domains, is a potential site for palmitoylation. To test this, we mutated C85S (C85 to serine) and established stable cells lines expressing the WT (wild-type) or the C85S-PMP22. In Schwann and MDCK (Madin–Darby canine kidney) cells mutating C85 blocked the palmitoylation of PMP22, which we monitored using 17-ODYA (17-octadecynoic acid). While palmitoylation was not necessary for processing the newly synthesized PMP22 through the secretory pathway, overexpression of C85S-PMP22 led to pronounced cell spreading and uneven monolayer thinning. To further investigate the functional significance of palmitoylated PMP22, we evaluated MDCK cell migration in a wound-healing assay. While WT-PMP22 expressing cells were resistant to migration, C85S cells displayed lamellipodial protrusions and migrated at a similar rate to vector control. These findings indicate that palmitoylation of PMP22 at C85 is critical for the role of the protein in modulating epithelial cell shape and motility.

Conformations of peptides derived from myelin-specific proteins in membrane-mimetic conditions probed by synchrotron radiation CD spectroscopy

Myelin is a tightly packed membrane multilayer in the nervous system, which harbours a specific set of quantitatively major proteins. All these proteins interact with the lipid bilayer, being either peripheral or integral membrane proteins. In this study, we examined the conformational properties of peptides from the myelin proteins P0, CNPase, MOBP, P2 and MOG, using trifluoroethanol and micelles of different detergents as membrane-like mimics. The peptides showed significant differences in their folding under the employed conditions, as evidenced by synchrotron radiation circular dichroism spectroscopy. Our experiments provide new structural information on the interactions between myelin proteins and membranes, using a simplified model system of synthetic peptides and micelles.

Thiopalmitoylated peptides from the peripheral nervous system myelin p0 protein: synthesis, characterization, and neuritogenic properties

Thiopalmitoylation (i.e., the covalent attachment of palmitic acid via a thioester linkage to cysteine residues in the polypeptide backbone) is a common post-translational modification of proteins. Several proteins that have been identified as putative autoantigens in a variety of T-cell mediated autoimmune diseases are thiopalmitoylated, and thus, we have hypothesized that endogenous thiopalmitoylated peptides released during tissue breakdown may play a role in the development and chronicity of autoimmune diseases. To investigate this, we have studied the effect of thiopalmitoylation on the immunogenic and neuritogenic properties of P0, the major peripheral nervous system (PNS) myelin protein, which is thiopalmitoylated at cysteine 153, and described as a candidate autoantigen in Guillain-Barre syndrome (GBS), a human inflammatory demyelinating disease of the PNS. This paper describes the synthesis of palmitoylated peptide P0(180-199) and P0(152-171) by on-resin acylation using specific cysteine side-chain protecting groups: Mmt (labile in diluted acid) and StBu (labile in the presence of tributylphosphine). Our results show that the thiol protecting group had to be adjusted to the peptide sequence: Mmt was efficiently used for P0(180-199) thioacylation, but it was not suitable for thiopalmitoylation of P0(152-171) because of a premature deprotection of the Boc protecting group on the epsilon-NH(2) Lys in the presence of 2% TFA, leading to dipalmitoylation. Palmitoylated P0(152-171) was successfully obtained by using StBu as the thiol protecting group. We could show by circular dichroism that palmitoylation has no influence on the structuration of the peptide in solution but palmitoylation increased the stability of the peptide in the presence of serum. Using EAN (experimental autoimmune neuritis), the rat model of GBS, we have compared the immunological properties of palm and non-palm P0 peptides and showed that thiopalmitoylation has indeed a great influence on their neuritogenic and immunogenic properties. This study provides further support for our hypothesis concerning the role of thiopalmitoylation in the development and chronicity of inflammatory demyelinating diseases and confirms that thiopalmitoylation of peptides may provide a simple means to increase MHC class II restricted responses.

Myelin sheaths are formed with proteins that originated in vertebrate lineages

All vertebrate nervous systems, except those of agnathans, make extensive use of the myelinated fiber, a structure formed by coordinated interplay between neuronal axons and glial cells. Myelinated fibers, by enhancing the speed and efficiency of nerve cell communication allowed gnathostomes to evolve extensively, forming a broad range of diverse lifestyles in most habitable environments. The axon-covering myelin sheaths are structurally and biochemically novel as they contain high portions of lipid and a few prominent low molecular weight proteins often considered unique to myelin. Here we searched genome and EST databases to identify orthologs and paralogs of the following myelin-related proteins: (1) myelin basic protein (MBP), (2) myelin protein zero (MPZ, formerly P0), (3) proteolipid protein (PLP1, formerly PLP), (4) peripheral myelin protein-2 (PMP2, formerly P2), (5) peripheral myelin protein-22 (PMP22) and (6) stathmin-1 (STMN1). Although widely distributed in gnathostome/vertebrate genomes, neither MBP nor MPZ are present in any of nine invertebrate genomes examined. PLP1, which replaced MPZ in tetrapod CNS myelin sheaths, includes a novel 'tetrapod-specific' exon (see also Möbius et al., 2009). Like PLP1, PMP2 first appears in tetrapods and like PLP1 its origins can be traced to invertebrate paralogs. PMP22, with origins in agnathans, and STMN1 with origins in protostomes, existed well before the evolution of gnathostomes. The coordinated appearance of MBP and MPZ with myelin sheaths and of PLP1 with tetrapod CNS myelin suggests interdependence - new proteins giving rise to novel vertebrate structures.

Molecular Characterization of Myelin Protein Zero in Xenopus laevis Peripheral Nerve: Equilibrium between Non-covalently Associated Dimer and Monomer

Myelin protein zero (P0), a glycosylated single-pass transmembrane protein, is essential in the formation and maintenance of peripheral nervous system (PNS) compact myelin. P0 in Xenopus (xP0) exists primarily as a dimeric form that remains stable after various physical and chemical treatments. In exploring the nature of the interactions underlying the dimer stability, we found that xP0 dimer dissociated into monomer during continuous elution gel electrophoresis and conventional SDS-PAGE, indicating that the dimer is stabilized by non-covalent interactions. Furthermore, as some of the gel-purified monomer re-associated into dimer on SDS-PAGE gels, there is likely a dynamic equilibrium between xP0 dimer and monomer in vivo. Because the carbohydrate and fatty acyl moieties may be crucial for the adhesion role of P0, we used sensitive mass spectrometry approaches to elucidate the detailed N-glycosylation and S-acylation profiles of xP0. Asn92 was determined to be the single, fully-occupied glycosylation site of xP0, and a total of 12 glycans was detected that exhibited new structural features compared with those observed from P0 in other species: (1) the neutral glycans were composed mainly of high mannose and hybrid types; (2) five of twelve were acidic glycans, among which three were sialylated and the other two were sulfated; (3) none of the glycans had core fucosylation; and (4) no glucuronic acid, hence no HNK-1 epitope, was detected. The drastically different carbohydrate structures observed here support the concept of the species-specific variation in N-glycosylation of P0. Cys152 was found to be acylated with stearoyl (C18:0), whereas palmitoyl (C16:0) is the corresponding predominant fatty acyl group on P0 from higher vertebrates. We propose that the unique glycosylation and acylation patterns of Xenopus P0 may underlie its unusual dimerization behaviour. Our results should shed light on the understanding of the phylogenetic development of P0's adhesion role in PNS compact myelin.

Peripheral myelin of Xenopus laevis: role of electrostatic and hydrophobic interactions in membrane compaction

P0 glycoprotein is the major structural protein of peripheral nerve myelin where it is thought to modulate inter-membrane adhesion at both the extracellular apposition, which is labile upon changes in pH and ionic strength, and the cytoplasmic apposition, which is resistant to such changes. Most studies on P0 have focused on structure-function correlates in higher vertebrates. Here, we focused on its role in the structure and interactions of frog (Xenopus laevis) myelin, where it exists primarily in a dimeric form. As part of our study, we deduced the full sequence of X. laevis P0 (xP0) from its cDNA. The xP0 sequence was found to be similar to P0 sequences of higher vertebrates, suggesting that a common mechanism of PNS myelin compaction via P0 interaction might have emerged through evolution. As previously reported for mouse PNS myelin, a similar change of extracellular apposition in frog PNS myelin as a function of pH and ionic strength was observed, which can be explained by a conformational change of P0 due to protonation-deprotonation of His52 at P0's putative adhesive interface. On the other hand, the cytoplasmic apposition in frog PNS myelin, like that in the mouse, remained unchanged at different pH and ionic strength. The contribution of hydrophobic interactions to stabilizing the cytoplasmic apposition was tested by incubating sciatic nerves with detergents. Dramatic expansion at the cytoplasmic apposition was observed for both frog and mouse, indicating a common hydrophobic nature at this apposition. Urea also expanded the cytoplasmic apposition of frog myelin likely owing to denaturation of P0. Removal of the fatty acids that attached to the single Cys residue in the cytoplasmic domain of P0 did not change PNS myelin structure of either frog or mouse, suggesting that the P0-attached fatty acyl chain does not play a significant role in PNS myelin compaction and stability. These results help clarify the present understanding of P0's adhesion role and the role of its acylation in compact PNS myelin.

Cytoplasmic domain of zebrafish myelin protein zero: adhesive role depends on beta-conformation

Solution spectroscopy studies on the cytoplasmic domain of human myelin protein zero (P0) (hP0-cyt) suggest that H-bonding between beta-strands from apposed molecules is likely responsible for the tight cytoplasmic apposition in compact myelin. As a follow-up to these findings, in the current study we used circular dichroism and x-ray diffraction to analyze the same type of model membranes previously used for hP0-cyt to investigate the molecular mechanism underlying the zebrafish cytoplasmic apposition. This space is significantly narrower in teleosts compared with that in higher vertebrates, and can be accounted for in part by the much shorter cytoplasmic domain in the zebrafish protein (zP0-cyt). Circular dichroism measurements on zP0-cyt showed similar structural characteristics to those of hP0-cyt, i.e., the protein underwent a beta-->alpha structural transition at lipid/protein (L/P) molar ratios >50, and adopted a beta-conformation at lower L/P molar ratios. X-ray diffraction was carried out on lipid vesicle solutions with zP0-cyt before and after dehydration to study the effect of protein on membrane lipid packing. Solution diffraction revealed the electron-density profile of a single membrane bilayer. Diffraction patterns of dried samples suggested a multilamellar structure with the beta-folded P0-cyt located at the intermembrane space. Our findings support the idea that the adhesive role of P0 at the cytoplasmic apposition in compact myelin depends on the cytoplasmic domain of P0 being in the beta-conformation.

Cytoplasmic domain of human myelin protein zero likely folded as beta-structure in compact myelin

Myelin protein zero (P0 or P0 glycoprotein), the major integral membrane protein in peripheral nervous system myelin, plays a key role in myelin membrane compaction and stability. While the structure of P0 extracellular domain was determined by crystallography, the paucity of any structural data on the highly positive-charged P0 cytoplasmic domain (P0-cyt) has greatly limited our understanding of the mechanism of P0 function. Here, using circular dichroism and intrinsic fluorescence spectroscopy, we attempted to elucidate the structure of human P0-cyt (hP0-cyt) in membrane mimetic environments composed of detergents or lipid vesicles. We found that the secondary structure of P0-cyt was polymorphic-at the lipid/protein ratio corresponding to that of mature peripheral myelin ( approximately 50:1), hP0-cyt mainly adopted a beta-conformation, whereas when the proportion of lipid increased, the structure underwent a beta-->alpha transition. By contrast, the secondary structure of the major isoform of myelin basic protein, another myelin protein with a very large positive charge, remained unchanged across a wide range of lipid/protein ratios. We propose that when hP0-cyt is bound at sufficient concentration to lamellar lipid bilayers such as myelin, it folds into a beta-conformation; before this threshold lipid/protein ratio is reached, the domain is alpha-helical. We suggest that the cytoplasmic apposition (major dense line) in compact myelin may be stabilized via the hydrogen-bonding of beta-strands formed as a result of local P0-P0 aggregation.

Myelin protein zero: mutations in the cytoplasmic domain interfere with its cellular trafficking

The cytoplasmic domain of myelin protein zero (MPZ), the principal protein of peripheral myelin, undergoes phosphorylation on several serine residues and a tyrosine group that is maximal during peak nerve myelination. Mutations that could affect MPZ phosphorylation cause the inherited neuropathy, Charcot-Marie-Tooth disease Type 1B. To investigate a possible role for phosphorylation in regulation of MPZ trafficking within the cell, we expressed wild-type and mutated MPZ-enhanced green fluorescent protein (GFP) fusion proteins in cultured Schwann-like cells. Whereas wild-type protein is present almost entirely at the cell surface, mutation of serine 204 to alanine or at a nearby presumed PKC substrate motif (198RSTK201) causes 40-60% of protein to be retained in the cytoplasm. Mutation of S204 to aspartate, which introduces a permanent negative charge, also impairs MPZ movement to the plasma membrane. In contrast, tyrosine 191 mutation has no effect on MPZ cellular distribution. Simultaneous alteration of S204 and Y191 produces much less perturbation of MPZ trafficking than mutation of S204 alone. Colocalization studies showed that mutated MPZ-EGFP trapped in the cytoplasm associates with all organelles in the secretory pathway. Previous studies have shown that cytoplasmic mutations at serine, but not tyrosine phosphorylation sites, abolish MPZ adhesive properties. Our results suggest that this loss of adhesion may be due, at least in part, to a failure of sufficient MPZ to reach the cell surface and that this impaired trafficking is associated with deficient serine phosphorylation in the cytoplasmic domain.

Palmitoylation is a sorting determinant for transport to the myelin membrane

Myelin is a specialized membrane enriched in glycosphingolipids and cholesterol that contains a restricted set of proteins. The mechanisms by which oligodendrocytes target myelin components to myelin are not known. To identify the sorting determinants for protein transport to myelin, we used a primary oligodendrocyte culture system in which terminal differentiation is synchronized and there is excessive deposition of myelin-like membranes (MLMs). Because several myelin proteins are palmitoylated, we explored the role of acylation in protein transport to MLMs. We found that palmitoylation-deficient mutants of a major myelin protein, proteolipid protein (PLP/DM20), were less efficiently targeted to MLMs. The N-terminal 13 amino acids of PLP/DM20, which are palmitoylated at three sites, were sufficient to direct a fluorescent fusion protein to MLMs. Mutagenesis of the N-terminal palmitoylation motif abolished the transport of the fusion protein to MLMs, indicating that palmitoylation is required for sorting to myelin. Similar results were obtained in myelinating co-cultures of oligodendrocytes and neurons. Furthermore, the combined farnesylation/palmitoylation signals from c-Ha-Ras and the N-terminal consensus sequence for dual palmitoylation from neuromodulin were sufficient for the transport of fluorescent fusion proteins to MLMs. Thus, we conclude that palmitoylation is a sorting determinant for transport to the myelin membrane.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Myelin P0: new knowledge and new roles

Protein zero (P0) is an integral transmembrane glycoprotein that serves as the major protein component of peripheral nerve myelin and is a member of the immunoglobulin (IgG) gene superfamily. As a cell adhesion molecule, P0 mediates homophilic adhesive interactions between Schwann cell plasma membranes and is a key structural constituent of both the major dense line and intraperiod line of compact myelin. Both the extracellular and cytoplasmic domains contribute to these interactions and evidence indicates that the post-translational modifications of the molecule, including glycosylation, acylation and phosphorylation, play an important modulatory role in adhesion and likely in the proper trafficking of P0 from the endoplasmic reticulum to the plasma membrane as well. Structural and genetic studies indicate that mutations in P0 producing human demyelinating diseases probably do so by perturbing or preventing homophilic interactions during myelination, or by producing cellular toxicity or an unstable myelin sheath. A variety of transcription factors, growth factors and neurosteroids both directly and indirectly influence P0 gene expression during maturation of the myelinating Schwann cell. Besides its structural function in myelin, P0 may have roles in the delivery of other Schwann cell proteins to their proper location, especially at or near nodes of Ranvier, and in neuronal-glial interactions.

Chemical deacylation reduces the adhesive properties of proteolipid protein and leads to decompaction of the myelin sheath

Myelin proteolipid protein (PLP) contains thioester-bound, long-chain fatty acids which are known to influence the structure of the molecule. To gain further insights into the role of this post-translational modification, we studied the effect that chemical deacylation of PLP had on the morphology of myelin and on the protein's ability to mediate the clustering of lipid vesicles. Incubation of rat optic nerves in isoosmotic solutions containing 100 mM hydroxylamine (HA) pH 7.4 led to deacylation of PLP and decompaction of myelin lamellae at the level of the intraperiod line. Incubation of nerves with milder nucleophilic agents (Tris and methylamine) or diluted HA, conditions that do not remove protein-bound fatty acids, caused no alterations in myelin structure. Other possible effects of HA which could have affected myelin compaction indirectly were ruled out. Incubation of optic nerves with 50 mM dithioerythritol (DTE) also led to the splitting of the myelin intraperiod line and this change again coincided with the removal of fatty acids. In addition, the apparently compacted CNS myelin in the PLP-less myelin-deficient rat, like that in tissue containing deacylated PLP, was readily decompacted upon incubation in isoosmotic buffers, suggesting that the function of PLP as a stabilizer of the interlamellar attachment is, at least in part, mediated by fatty acylation. Furthermore, in contrast to the native protein, PLP deacylated with either HA or DTE failed to induce the clustering of phosphatidylcholine/cholesterol vesicles in vitro. This phenomenon is not due to side-effects of the deacylation procedure since, upon partial repalmitoylation, the protein recovered most of its original vesicle-clustering activity. Collectively, these findings suggest that palmitoylation, by influencing the adhesive properties of PLP, is important for stabilizing the multilamellar structure of myelin.

Identification and analysis of vaccinia virus palmitylproteins

Vaccinia virus encodes at least eight proteins that incorporate label from tritiated palmitic acid when it is added to infected cell cultures. Three of these palmitylproteins are encoded by the A33R, B5R, and F13L open reading frames and migrate by gel electrophoresis with relative molecular masses of 23-28, 42, and 37 kDa, respectively. In this report we provide evidence that the A22R and A36R open reading frames also encode palmitylproteins with apparent molecular masses of 22 and 50-55 kDa, respectively. Furthermore, the hemagglutinin protein (A56R) from the Copenhagen strain is shown to be palmitylated while the hemagglutinin protein from the WR and IHD-J strains is not. A 94-kDa VV palmitylprotein appears to be a multimeric complex composed of the B5R protein and possibly others. All vaccinia-encoded palmitylproteins are present in the membranous fraction of cells and are specific for the trans-Golgi network membrane-enveloped forms of the virus, suggesting that these proteins play a role in the envelopment and egress of virions or the infectivity of released virus.

Acylation of myelin Po protein is required for adhesion

The extracellular domains of myelin Po protein interact homophilically and hence hold myelin compact at the intraperiod line. The cytoplasmic domain of Po, however, can also affect the interactions of its extracellular sequences. Po is acylated, mostly with palmitic acid, at Cys 153, just at the transmembrane:cytoplasmic domain interface. Here we show that Po mutated at Cys 153 to alanine (C153A), is not acylated and is not adhesive. Like wild-type Po, C153A Po clusters within the membrane and seems to interact with the cytoskeleton. On the other hand, the rate of turnover of C153A Po in transfected Chinese hamster ovary cells is almost 4 times faster than wild-type Po. The increased instability of C153A Po compared to wild-type Po may account for its loss of adhesion.

Tetrameric assembly of full-sequence protein zero myelin glycoprotein by synchrotron x-ray scattering

Highly purified myelin P0 glycoprotein was solubilized to 1-8 mg/ml in 0.1% sodium dodecyl sulfate (SDS), and the solution structure of the P0 assembly was studied using synchrotron x-ray scattering. The full-length P0, which was isolated from bovine intradural roots, included both the extracellular and cytoplasmic domains of the molecule. At the higher concentrations (4, 6, and 8 mg/ml, respectively), an x-ray intensity maximum was observed at 316 A, 245 A, and 240 A Bragg spacing. Because the position of this intensity depended on P0 concentration, it is most likely due to interparticle interference. By contrast, the position of a second intensity maximum, which was at approximately 40 A Bragg spacing, was invariant with P0 concentration. This latter intensity was accounted for by monodispersed, 80 A-diameter particles that are composed of eight, approximately 30 A-diameter spheres. Chemical parameters suggest that the 80 A particles correspond to the size of a tetramer of P0 molecules. Therefore, the approximately 30 A spheres would correspond to the sizes of the extracellular and cytoplasmic domains for each of the P0 monomers. The invariance of the second intensity maximum with P0 concentration indicates that the structure of the 80 A-diameter, tetrameric particles is unaltered. According to the liquid model for interparticle interference from charged spheres, the 80 A-diameter particle has 10 negative surface charges which likely arise from negatively charged SDS molecules bound to the transmembrane domain of P0. This binding, however, apparently does not alter the tetrameric assembly of P0, suggesting that intermolecular interactions involving extracellular domains and cytoplasmic domains likely stabilize this assembly. Some of our results have been published in abstract form (Inouye, H., H. Tsuruta, D. A. Kirschner, J. Sedzik, and K. Uyemura. Abstracts of the 4th International School and Symposium on Synchrotron Radiation in Natural Science, June 15-20, 1998. Ustron-Jaszowiec, Poland. p. 31).

Posttranslational modification of tubulin by palmitoylation: II. Identification of sites of palmitoylation

As shown in the companion article, tubulin is posttranslationally modified in vivo by palmitoylation. Our goal in this study was to identify the palmitoylation sites by protein structure analysis. To obtain quantities of palmitoylated tubulin required for this analysis, a cell-free system for enzymatic [3H]palmitoylation was developed and characterized in our companion article. We then developed a methodology to examine directly the palmitoylation of all 451 amino acids of alpha-tubulin. 3H-labeled palmitoylated alpha-tubulin was cleaved with cyanogen bromide (CNBr). The CNBr digest was resolved according to peptide size by gel filtration on Sephadex LH60 in formic acid:ethanol. The position of 3H-labeled palmitoylated amino acids in peptides could not be identified by analysis of the Edman degradation sequencer product because the palmitoylated sequencer products were lost during the final derivatization step to phenylthiohydantoin derivatives. Modification of the gas/liquid-phase sequencer to deliver the intermediate anilinothiozolinone derivative, rather than the phenylthiohydantoin derivative, identified the cycle containing the 3H-labeled palmitoylated residue. Therefore, structure analysis of peptides obtained from gel filtration necessitated dual sequencer runs of radioactive peptides, one for sequence analysis and one to identify 3H-labeled palmitoylated amino acids. Further cleavage of the CNBr peptides by trypsin and Lys-C protease, followed by gel filtration on Sephadex LH60 and dual sequencer runs, positioned the 3H-labeled palmitoylated amino acid residues in peptides. Integration of all the available structural information led to the assignment of the palmitoyl moiety to specific residues in alpha-tubulin. The palmitoylated residues in alpha-tubulin were confined to cysteine residues only. The major site for palmitoylation was cysteine residue 376.

The variant-specific surface protein of Giardia, VSP4A1, is a glycosylated and palmitoylated protein

The variant-specific surface proteins (VSPs) of the ancient protist Giardia duodenalis (syn.: Giardia intestinalis, Giardia lamblia) are cysteine- and threonine-rich polypeptides that can vary considerably in sequence and size. In the present study, we have purified a VSP (VSP4A1, formerly called CR1SP-90) from a cloned Giardia isolate, derived from a sheep, by Triton X-114 phase partitioning and anion-exchange chromatography. Analysis of the purified VSP4A1 showed that this protein is posttranslationally modified with both glycans and lipid. The glycans of VSP4A1 were detected and partially characterized by (1) compositional analysis, which indicated the presence of GlcNAc and Glc (0.5 and 1.0 mol/mol of protein respectively), and (2) the specific labelling of VSP4A1 with galactosyltransferase/UDP-[3H]Gal. The glycans were released by beta-elimination, suggesting that they are O-linked to the protein. Bio-Gel P4 chromatography of the released galactosylated glycans and further compositional analysis suggested that the major glycan on the VSP is a trisaccharide with Glc at the reducing terminus. These and other results indicate the absence of any N-linked glycans on the VSP and suggest instead that it is elaborated with a novel type of short O-linked glycan. Compositional analysis and radiolabelling experiments also indicated that VSP4A1 is modified with covalently linked palmitate (1 mol/mol of protein). Hydroxylamine treatment at neutral pH of[3H]palmitate-labelled VSP4A1 indicated that the acyl chain may be attached by a thioester linkage. A likely location for the lipid modification appears to be in the region of the C-terminal domain where it may facilitate association of the protein with the plasma membrane.

Post-translational modifications of apolipoprotein A-I and Po proteins in the avian peripheral nerve

Apolipoprotein A-I (apo A-I), a soluble lipid transporter, and Po, the major glycoprotein of myelin, are actively synthesized during myelination. To explore the status of post-translational modifications of these proteins in the avian PNS during rapid myelination, endoneurial slices from one day old chick sciatic nerves were incubated with various radioactive precursors that could serve as indicators of such processes. The proteins were isolated from the incubation medium (secreted fraction), the 1% Triton-X-100-soluble intracellular-endoneurial (intracellular) fraction, and myelin-related and purified compact myelin fractions by immunoprecipitation with monospecific anti-apo A-I and or anti-Po antisera. Our results demonstrated that secreted apo A-I is fatty acylated, but not phosphorylated or sulfated. Avian Po protein was phosphorylated by a phorbol ester sensitive protein kinase. Sulfation, as well as fatty acylation, of avian Po protein was observed in organ culture using highly sensitive methods of detection. These results indicate that fatty acylation of secreted apo A-I and phosphorylation, sulfation and fatty acylation of Po have been conserved during evolution, and that these post-translational modifications may play a common function in various species.

The serine-rich Entamoeba histolytica protein is a phosphorylated membrane protein containing O-linked terminal N-acetylglucosamine residues

Previously, we described the isolation of a cDNA clone and the gene encoding a protective antigen of the protozoan parasite Entamoeba histolytica, the serine-rich Entamoeba histolytica protein (SREHP). The derived amino acid sequence of the SREHP cDNA clone was remarkable for a high serine content (52/233 amino acids), a putative signal sequence, multiple hydrophilic dodecapeptide and octapeptide tandem repeats, and a hydrophobic C-terminal putative membrane-spanning region. Here, we show that SREHP is modified by the addition of phosphate at serine residues, O-linked terminal N-acetylglucosamine residues, and by acylation. When the SREHP gene is expressed in baculovirus transformed Sf-9 cells, the product is also phosphorylated and glycosylated and is localized to the plasma membrane of the insect cells. The native SREHP molecule also serves as a potent chemoattractant for amebic trophozoites. The data presented here suggest that SREHP is a unique membrane protein with phosphorylation and glycosylation patterns usually associated with nuclear or cytoplasmic proteins.

Overview: protein palmitoylation in the nervous system: current views and unsolved problems

Palmitoylation refers to a dynamic post-translational modification of proteins involving the covalent attachment of long-chain fatty acids to the side chains of cysteine, threonine or serine residues. In recent years, palmitoylation has been identified as a widespread modification of both viral and cellular proteins. Because of its dynamic nature, protein palmitoylation, like phosphorylation, appears to have a crucial role in the functioning of the nervous system. Several important questions regarding the post-translational acylation of cysteine residues in proteins are briefly discussed: (a) What are the molecular mechanisms involved in dynamic acylation? (b) What are the determinants of the fatty acid specificity and the structural requirements of the acceptor proteins? (c) What are the physiological signals regulating this type of protein modification, and (d) What is the biological role(s) of this reaction with respect to the functioning of specific nervous system proteins? We also present the current experimental obstacles that have to be overcome to fully understand the biology of this dynamic modification.