Cysteine-108 is an acylation site in myelin proteolipid protein

Human myelin proteolipid protein structure and lipid bilayer stacking

The myelin sheath is an essential, multilayered membrane structure that insulates axons, enabling the rapid transmission of nerve impulses. The tetraspan myelin proteolipid protein (PLP) is the most abundant protein of compact myelin in the central nervous system (CNS). The integral membrane protein PLP adheres myelin membranes together and enhances the compaction of myelin, having a fundamental role in myelin stability and axonal support. PLP is linked to severe CNS neuropathies, including inherited Pelizaeus-Merzbacher disease and spastic paraplegia type 2, as well as multiple sclerosis. Nevertheless, the structure, lipid interaction properties, and membrane organization mechanisms of PLP have remained unidentified. We expressed, purified, and structurally characterized human PLP and its shorter isoform DM20. Synchrotron radiation circular dichroism spectroscopy and small-angle X-ray and neutron scattering revealed a dimeric, α-helical conformation for both PLP and DM20 in detergent complexes, and pinpoint structural variations between the isoforms and their influence on protein function. In phosphatidylcholine membranes, reconstituted PLP and DM20 spontaneously induced formation of multilamellar myelin-like membrane assemblies. Cholesterol and sphingomyelin enhanced the membrane organization but were not crucial for membrane stacking. Electron cryomicroscopy, atomic force microscopy, and X-ray diffraction experiments for membrane-embedded PLP/DM20 illustrated effective membrane stacking and ordered organization of membrane assemblies with a repeat distance in line with CNS myelin. Our results shed light on the 3D structure of myelin PLP and DM20, their structure-function differences, as well as fundamental protein-lipid interplay in CNS compact myelin.

Mass-spectrometric analysis of myelin proteolipids reveals new features of this family of palmitoylated membrane proteins

In this study, we have investigated the structure of the native myelin proteolipid protein (PLP), DM-20 protein and several low molecular mass proteolipids by mass spectrometry. The various proteolipid species were isolated from bovine spinal cord by size-exclusion and ion-exchange chromatography in organic solvents. Matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) of PLP and DM-20 revealed molecular masses of 31.6 and 27.2 kDa, respectively, which is consistent with the presence of six and four molecules of thioester-bound fatty acids. Electrospray ionization-MS analysis of the deacylated proteins in organic solvents produced the predicted molecular masses of the apoproteins (29.9 and 26.1 kDa), demonstrating that palmitoylation is the major post-translational modification of PLP, and that the majority of PLP and DM-20 molecules in the CNS are fully acylated. A series of myelin-associated, palmitoylated proteolipids with molecular masses raging between 12 kDa and 18 kDa were also isolated and subjected to amino acid analysis, fatty acid analysis, N- and C-terminal sequencing, tryptic digestion and peptide mapping by MALDI-TOF-MS. The results clearly showed that these polypeptides correspond to the N-terminal region (residues 1-105/112) and C-terminal region (residues 113/131-276) of the major PLP, and they appear to be produced by natural proteolytic cleavage within the 60 amino acid-long cytoplasmic domain. These proteolipids are not postmortem artifacts of PLP and DM-20, and are differentially distributed across the CNS.

Thiopalmitoylation of myelin proteolipid protein epitopes enhances immunogenicity and encephalitogenicity

Proteolipid protein (PLP) is the most abundant protein of CNS myelin, and is posttranslationally acylated by covalent attachment of long chain fatty acids to cysteine residues via a thioester linkage. Two of the acylation sites are within epitopes of PLP that are encephalitogenic in SJL/J mice (PLP(104-117) and PLP(139-151)) and against which increased immune responses have been detected in some multiple sclerosis patients. It is known that attachment of certain types of lipid side chains to peptides can result in their enhanced immunogenicity. The aim of this study was to determine whether thioacylated PLP peptides, as occur in the native protein, are more immunogenic than their nonacylated counterparts, and whether thioacylation influences the development of autoreactivity and experimental autoimmune encephalomyelitis. The results show that in comparison with nonacylated peptides, thioacylated PLP lipopeptides can induce greater T cell and Ab responses to both the acylated and nonacylated peptides. They also enhanced the development and chronicity of experimental autoimmune encephalomyelitis. Synthetic peptides in which the fatty acid was attached via an amide linkage at the N terminus were not encephalitogenic, and they induced greater proportions of CD8+ cells in initial in vitro stimulation. Therefore, the lability and the site of the linkage between the peptide and fatty acid may be important for induction of encephalitogenic CD4+ T cells. These results suggest that immune responses induced by endogenous thioacylated lipopeptides may contribute to the immunopathogenesis of chronic experimental demyelinating diseases and multiple sclerosis.

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.

Fatty acid composition of myelin proteolipid protein during vertebrate evolution

The hydrophobic myelin proteolipid protein (PLP) contains covalently bound long-chain fatty acids which are attached to intracellular cysteine residues via thioester linkages. To gain insight into the role of acylation in the structure and function of myelin PLP, the amount and pattern of acyl groups attached to the protein during vertebrate evolution was determined. PLP isolated from brain myelin of amphibians, reptiles, birds and several mammals was subjected to alkaline methanolysis and the released methyl esters were analyzed by gas-liquid chromatography. In all species studied, PLP contained approximately the same amount of covalently bound fatty acids (3% w/w), and palmitic, palmitoleic, oleic and stearic acids were always the major acyl groups. Although the relative proportions of these fatty acids changed during evolution, the changes did not necessarily follow the variations in the acyl chain composition of the myelin free fatty acid pool, suggesting fatty acid specificity. The phylogenetic conservation of acylation suggests that this post-translational modification is critical for PLP function.

Palmitoylation of proteolipid protein from rat brain myelin using endogenously generated 18O-fatty acids

Proteolipid protein (PLP), the major protein of central nervous system myelin, contains covalently bound fatty acids, predominantly palmitic acid. This study adapts a stable isotope technique (Kuwae, T., Schmid, P. C., Johnson, S. B., and Schmid, H. O. (1990) J. Biol. Chem. 265, 5002-5007) to quantitatively determine the minimal proportion of PLP molecules which undergo palmitoylation. In these experiments, brain white matter slices from 20-day-old rats were incubated for up to 6 h in a physiological buffer containing 50% H218O. The uptake of 18O into the carbonyl groups of fatty acids derived from PLP, phospholipids, and the free fatty acid pool was measured by gas-liquid chromatography/mass spectrometry of the respective methyl esters. Palmitic acid derived from PLP acquired increasing amounts of 18O, ending with 2.9% 18O enrichment after 6 h of incubation. 18O incorporation into myelin free palmitic acid also increased over the course of the incubation (67.2% 18O enrichment). After correcting for the specific activity of the 18O-enriched free palmitic acid pool, 7.6% of the PLP molecules were found to acquire palmitic acid in 6 h. This value is not only too large to be the result of the palmitoylation of newly synthesized PLP molecules, it was also unchanged upon the inhibition of protein synthesis with cycloheximide. 18O enrichment in less actively myelinating 60-day-old rats was significantly reduced. In conclusion, our experiments suggest that a substantial proportion of PLP molecules acquire palmitic acid via an acylation/deacylation cycle and that this profile changes during development.

Monoclonal antibody O10 defines a conformationally sensitive cell-surface epitope of proteolipid protein (PLP): evidence that PLP misfolding underlies dysmyelination in mutant mice

Mutations in the gene for proteolipid protein (PLP) have been associated with CNS dysmyelination and abnormal oligodendrocyte death in spontaneous mouse mutants and in Pelizaeus-Merzbacher disease; however, the effect of mutations on PLP structure and function are little understood. We have identified a monoclonal antibody directed against a novel cell surface epitope of PLP, termed O10. By immunofluorescence analysis, COS-7 cells transiently transfected to express PLP (or its isoform DM20) can be stained with antibody O10 and another antibody (A431) directed against the C terminus of PLP/DM20. The subcellular distribution of immunofluorescence labels for the two antibodies is not identical, suggesting that the O10 epitope is acquired post-translationally. When PLP/DM20 from jimpy, jimpymsd, and rumpshaker mutant mice is expressed in COS-7 cells and compared with wild-type PLP/DM20, none of the mutant isoforms displays the O10 epitope, whereas the C-terminal epitope is detected. Because the O10 but not the A431 epitope is also sensitive to SDS and reducing agents, this strongly suggests abnormal protein folding in the PLP mutants. PLP from jimpymsd mice is obviously misfolded, because the amino acid substitution (Ala242 --> Val) is located within a transmembrane domain to which the O10 antibody does not bind. We propose that the O10 epitope emerges as the full length protein reaches a functional tertiary structure and that the absence of this epitope marks a structural defect of PLP that leads to dysmyelination.

Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

The orientation of proteins within a cell membrane can often be difficult to determine. A number of models have been proposed for the orientation of the myelin protein, proteolipid protein (PLP), each of which includes exposed domains on the intracellular and extracellular membrane faces. Immunolabeling experiments have localized the C-terminus and the region spanning amino acids 103-116 to the cytoplasmic face of the membrane, but no well characterized antibodies have been available that label extracellular PLP domains. In this report, we describe the generation and characterization of mouse monoclonal antibodies (mAb) against putative extramembrane domains. Three of the mAb, specific for PLP peptides 40-59, 178-191, or 215-232, immunostain live oligodendrocytes, indicating that these regions of the molecule are exposed on the external surface of the cell. In addition, we have used these mAb to study the time-course of incorporation of PLP into the oligodendrocyte membrane. These studies increase our knowledge of the orientation of PLP in the lipid bilayer and are relevant for understanding myelin function.

Very long chain fatty acids in higher animals--a review

Fatty acids with greater than 22 carbon atoms (very long chain fatty acids, VLCFA) are present in small amounts in most animal tissues. Saturated and monoenoic VLCFA are major components of brain, while the polyenoic VLCFA occur in significant amounts in certain specialized animal tissues such as retina and spermatozoa. Biosynthesis of VLCFA occurs by carbon chain elongation of shorter chain fatty acid precursors while beta-oxidation takes place almost exclusively in peroxisomes. Mitochondria are unable to oxidize VLCFA because they lack a specific VLCFA coenzyme A synthetase, the first enzyme in the beta-oxidation pathway. VLCFA accumulate in the tissues of patients with inherited abnormalities in peroxisomal assembly, and also in individuals with defects in enzymes catalyzing individual reactions along the beta-oxidation pathway. It is believed that the accumulation of VLCFA in patient tissues contributes to the severe pathological changes which are a feature of these conditions. However, little is known of the role of VLCFA in normal cellular processes, and of the molecular basis for their contribution to the disease process. The present review provides an outline of the current knowledge of VLCFA including their biosynthesis, degradation, possible function and involvement in human disease.

Cysteine-containing peptide sequences exhibit facile uncatalyzed transacylation and acyl-CoA-dependent acylation at the lipid bilayer interface

A variety of simple cysteine-containing lipopeptides, with sequences modeled on those found in naturally occurring S-acylated proteins, undergo spontaneous S-acylation in phospholipid vesicles at physiological pH when either long-chain acyl-CoAs or other S-acylated peptides are added as acyl donors. Fluorescent or radiolabeled lipopeptides with the sequence myristoyl-GCX- (X = G, L, R, T, or V), a motif found to undergo S-acylation in several intracellular regulatory proteins, and the prenylated peptide -SCRC(farnesyl)-OMe, modeled on the carboxyl terminus of p21H-ras, were all found to be suitable acyl acceptors for such uncatalyzed S-acyl transfer reactions at physiological pH. Acylation of these cysteinyl-containing lipopeptides to high stoichiometry was observed, on time scales ranging from a few hours to a few tens of minutes, in vesicles containing relatively low concentrations (< or = mol %) and only a modest molar excess (2.5:1) of the acyl donor species. No evidence was obtained for acyl transfer to peptide serine or threonine hydroxyl groups under the same conditions. These observations may have significant implications both for the design of in vitro studies of the S-acylation of membrane-associated proteins and for our understanding of the mechanisms of S-acylation of these species in vivo.

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.

Immunolocalization of proteolipid protein peptide 103-116 in myelin

Determination of the topographic orientation of proteolipid protein (PLP) within myelin is part of an overall understanding of the functions of PLP and the roles of its multiple domains in diseases that primarily affect central nervous system (CNS) myelin. As part of an analysis of PLP orientation, two mouse monoclonal antibodies (mAb) and a rabbit antiserum against a synthetic peptide corresponding to PLP residues 103-116 (YKTTICGKGLSATV) were tested for their reactivity on compact CNS myelin. By ELISA, the antibodies react with intact PLP and PLP residues 103-116, but not with other PLP peptides. Ultrathin cryosections of adult rat optic nerve were immunostained and antibody binding was localized using appropriate second antibodies coupled to 1 nm gold particles that were visualized by silver enhancement. Localization of the particles on the major or intermediate dense lines was determined by three independent observers. Using the PLP peptide mAb and the polyclonal antibody, we demonstrated that > or = 71% of the particles were localized on the major dense line. At least 66% of particles directed against myelin basic protein, which is known to occur on the major dense line, were also found in that location. These semiquantitative morphologic observations suggest that PLP residues 103-116 occur on the cytoplasmic face of the myelin membrane.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Myelin proteolipid protein: minimum sequence requirements for active induction of autoimmune encephalomyelitis in SWR/J and SJL/J mice

Proteolipid protein (PLP) is the major protein constituent of mammalian central nervous system myelin. We have previously identified two different PLP encephalitogenic T cell epitopes in two mouse strains. Murine PLP peptides 103-116 YKTTICGKGLSATV and 139-151 HCLGKWLGHPDKF are encephalitogenic determinants in SWR/J (H-2q) and SJL/J (H-2s) mice, respectively. The purpose of the present study was to determine the minimum sequence requirements for each of these PLP encephalitogens. In SWR/J mice, at least two distinct overlapping peptides can induce experimental autoimmune encephalomyelitis (EAE). The eleven residue sequences PLP 105-115 TTICGKGLSAT and PLP 106-116 TICGKGLSATV are encephalitogenic in SWR/J mice, but PLP 106-115 TICGKGLSAT, the decapeptide indigenous to both sequences, is non-encephalitogenic. In contrast, the shortest PLP sequence capable of inducing EAE in SJL/J mice is the nonapeptide 141-149 LGKWLGHPD. These data indicate that encephalitogenic determinants of PLP are short contiguous peptide sequences similar in length and diversity to those of MBP.

Pelizaeus-Merzbacher disease: a valine to phenylalanine point mutation in a putative extracellular loop of myelin proteolipid

In the central nervous system, myelin proteolipid protein isoforms (PLP and DM20) play an essential structural role in myelination. It has been shown in several species that myelination is impaired by molecular defects resulting from single base mutations in the PLP gene. We have used DNA amplification by polymerase chain reaction to study the PLP gene coding regions from 17 patients in 15 unrelated families with similar Pelizaeus-Merzbacher phenotype. In one case amplification of peripheral nerve PLP/DM20 cDNAs revealed that a silent T----C transition was unrelated to the disease. In one family a nonsilent mutation was identified that leads to a phenylalanine substitution for valine-218 in PLP/DM20 proteins. We investigated the inheritance of the mutant allele in 19 subjects of this four-generation family and found a strict cosegregation of the Phe218 substitution with transmission and expression of the disease. The effect of the Val218----Phe mutation is discussed in the frame of a recently suggested topological model of PLP/DM20, according to which Val218 is part of an extracellular loop that connects the last two of four membrane-spanning alpha-helices.