Non-covalent cross-linking of lipid bilayers by myelin basic protein: a possible role in myelin formation

Intrinsic Disorder as a Natural Preservative: High Levels of Intrinsic Disorder in Proteins Found in the 2600-Year-Old Human Brain

Proteomic analysis revealed the preservation of many proteins in the Heslington brain (which is at least 2600-year-old brain tissue uncovered within the skull excavated in 2008 from a pit in Heslington, Yorkshire, England). Five of these proteins-"main proteins": heavy, medium, and light neurofilament proteins (NFH, NFM, and NFL), glial fibrillary acidic protein (GFAP), and myelin basic (MBP) protein-are engaged in the formation of non-amyloid protein aggregates, such as intermediate filaments and myelin sheath. We used a wide spectrum of bioinformatics tools to evaluate the prevalence of functional disorder in several related sets of proteins, such as the main proteins and their 44 interactors, all other proteins identified in the Heslington brain, as well as the entire human proteome (20,317 manually curated proteins), and 10,611 brain proteins. These analyses revealed that all five main proteins, half of their interactors and almost one third of the Heslington brain proteins are expected to be mostly disordered. Furthermore, most of the remaining Heslington brain proteins are expected to contain sizable levels of disorder. This is contrary to the expected substantial (if not complete) elimination of the disordered proteins from the Heslington brain. Therefore, it seems that the intrinsic disorder of NFH, NFM, NFL, GFAP, and MBP, their interactors, and many other proteins might play a crucial role in preserving the Heslington brain by forming tightly folded brain protein aggregates, in which different parts are glued together via the disorder-to-order transitions.

Human cellular models of medium spiny neuron development and Huntington disease

The loss of gamma-aminobutyric acid (GABA)-ergic medium spiny neurons (MSNs) in the striatum is the hallmark of Huntington disease (HD), an incurable neurodegenerative disorder characterized by progressive motor, psychiatric, and cognitive symptoms. Transplantation of MSNs or their precursors represents a promising treatment strategy for HD. In initial clinical trials in which HD patients received fetal neurografts directly into the striatum without a pretransplant cell-differentiation step, some patients exhibited temporary benefits. Meanwhile, major challenges related to graft overgrowth, insufficient survival of grafted cells, and limited availability of donated fetal tissue remain. Thus, the development of approaches that allow modeling of MSN differentiation and HD development in cell culture platforms may improve our understanding of HD and translate, ultimately, into HD treatment options. Here, recent advances in the in vitro differentiation of MSNs derived from fetal neural stem cells/progenitor cells (NSCs/NPCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and induced NSCs (iNSCs) as well as advances in direct transdifferentiation are reviewed. Progress in non-allele specific and allele specific gene editing of HTT is presented as well. Cell characterization approaches involving phenotyping as well as in vitro and in vivo functional assays are also discussed.

MyelStones: the executive roles of myelin basic protein in myelin assembly and destabilization in multiple sclerosis

The classic isoforms of myelin basic protein (MBP, 14–21.5 kDa) are essential to formation of the multilamellar myelin sheath of the mammalian central nervous system (CNS). The predominant 18.5-kDa isoform links together the cytosolic surfaces of oligodendrocytes, but additionally participates in cytoskeletal turnover and membrane extension, Fyn-mediated signalling pathways, sequestration of phosphoinositides and maintenance of calcium homoeostasis. All MBP isoforms are intrinsically disordered proteins (IDPs) that interact via molecular recognition fragments (MoRFs), which thereby undergo local disorder-to-order transitions. Their conformations and associations are modulated by environment and by a dynamic barcode of post-translational modifications, particularly phosphorylation by mitogen-activated and other protein kinases and deimination [a hallmark of demyelination in multiple sclerosis (MS)]. The MBPs are thus to myelin what basic histones are to chromatin. Originally thought to be merely structural proteins forming an inert spool, histones are now known to be dynamic entities involved in epigenetic regulation and diseases such as cancer. Analogously, the MBPs are not mere adhesives of compact myelin, but active participants in oligodendrocyte proliferation and in membrane process extension and stabilization during myelinogenesis. A central segment of these proteins is pivotal in membrane-anchoring and SH3 domain (Src homology 3) interaction. We discuss in the present review advances in our understanding of conformational conversions of this classic basic protein upon membrane association, including new thermodynamic analyses of transitions into different structural ensembles and how a shift in the pattern of its post-translational modifications is associated with the pathogenesis and potentially onset of demyelination in MS.

Myelin-specific proteins: a structurally diverse group of membrane-interacting molecules

The myelin sheath is a multilayered membrane in the nervous system, which has unique biochemical properties. Myelin carries a set of specific high-abundance proteins, the structure and function of which are still poorly understood. The proteins of the myelin sheath are involved in a number of neurological diseases, including autoimmune diseases and inherited neuropathies. In this review, we briefly discuss the structural properties and functions of selected myelin-specific proteins (P0, myelin oligodendrocyte glycoprotein, myelin-associated glycoprotein, myelin basic protein, myelin-associated oligodendrocytic basic protein, P2, proteolipid protein, peripheral myelin protein of 22 kDa, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and periaxin); such properties include, for example, interactions with lipid bilayers and the presence of large intrinsically disordered regions in some myelin proteins. A detailed understanding of myelin protein structure and function at the molecular level will be required to fully grasp their physiological roles in the myelin sheath.

Cholesterol, regulated exocytosis and the physiological fusion machine

Exocytosis is a highly conserved and essential process. Although numerous proteins are involved throughout the exocytotic process, the defining membrane fusion step appears to occur through a lipid-dominated mechanism. Here we review and integrate the current literature on protein and lipid roles in exocytosis, with emphasis on the multiple roles of cholesterol in exocytosis and membrane fusion, in an effort to promote a more molecular systems-level view of the as yet poorly understood process of Ca2+-triggered membrane mergers.

Phosphatidylinositol 4,5-bisphosphate-dependent interaction of myelin basic protein with the plasma membrane in oligodendroglial cells and its rapid perturbation by elevated calcium

Myelin basic protein (MBP) is an essential structural component of CNS myelin. The electrostatic association of this positively charged protein with myelin-forming membranes is a crucial step in myelination, but the mechanism that regulates myelin membrane targeting is not known. Here, we demonstrate that phosphatidylinositol 4,5-bisphosphate (PIP2) is important for the stable association of MBP with cellular membranes. In oligodendrocytes, overexpression of synaptojanin 1-derived phosphoinositide 5-phosphatase, which selectively hydrolyzes membrane PIP2, causes the detachment of MBP from the plasma membrane. In addition, constitutively active Arf6/Q67L induces the formation of PIP2-enriched endosomal vacuoles, leading to the redistribution of MBP to intracellular vesicles. Fluorescence resonance energy transfer imaging revealed an interaction of the PIP2 sensing probe PH-PLCdelta1 with wild-type MBP, but not with a mutant MBP isoform that fails to associate with the plasma membrane. Moreover, increasing intracellular Ca(2+), followed by phospholipase C-mediated PIP2 hydrolysis, as well as reduction of the membrane charge by ATP depletion, resulted in the dissociation of MBP from the glial plasma membrane. When the corpus callosum of mice was analyzed in acute brain slices by electron microscopy, the reduction of membrane surface charge led to the loss of myelin compaction and rapid vesiculation. Together, these results establish that PIP2 is an essential determinant for stable membrane binding of MBP and provide a novel link between glial phosphoinositol metabolism and MBP function in development and disease.

Spatiotemporal patterns of SSeCKS expression after rat spinal cord injury

Src suppressed C kinase substrate (SSeCKS) was identified as a PKC substrate/PKC-binding protein, which plays a role in mitogenic regulatory activity and has a function in the control of cell signaling and cytoskeletal arrangement. However its distribution and function in the central nervous system (CNS) lesion remain unclear. In this study, we mainly investigated the mRNA and protein expression and cellular localization of SSeCKS during spinal cord injury (SCI). Real-time PCR and Western blot analysis revealed that SSeCKS was present in normal whole spinal cord. It gradually increased, reached a peak at 3 days for its mRNA level and 5 days for its protein level after SCI, and then declined during the following days. In ventral horn, the expression of SSeCKS underwent a temporal pattern that was similar with the whole spinal cord in both mRNA and protein level. However, in dorsal horn, the mRNA and protein for SSeCKS expression were significantly increased at 1 day for its mRNA level and 3 days for its protein level, and then gradually declined to the baseline level, ultimately up-regulated again from 7 to 14 days. The protein expression of SSeCKS was further analysed by immunohistochemistry. The positively stained areas for SSeCKS changed with the similar pattern to that of protein expression detected by immunoblotting analysis. Double immunofluorescence staining showed that SSeCKS immunoreactivity (IR) was found in neurons, astrocytes, oligodendrocytes of spinal cord tissues within 5 mm from the lesion site. Importantly, injury-induced expression of SSeCKS was co-labeled by active caspase-3 (apoptotic marker), Tau-1 (the marker for pathological oligodendrocyte) and beta-1,4-galactosyltransferase 1 (GalT). All the results suggested that SSeCKS might play important roles in spinal cord pathophysiology and further research is needed to have a good understanding of its function and mechanism.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Carp ovarian cystatin binds and agglutinates spermatozoa via electrostatic interaction

A sperm-agglutinating factor was purified from ovulated carp eggs and the conditioned medium (CM) of cortical-reacted eggs. It was identified to be the carp ovarian cystatin. Three cystatin isoforms were found. The cystatin isolated from the CM had a higher sperm-agglutinating activity than that isolated from eggs, although the cystatins have identical N-terminal amino acid sequences, masses, and positive charges. Differences in sperm-agglutinating activity between the cystatins of the CM and eggs may be caused by the different conformations because they differed in circular dichroism spectrum and tryptic map. Cystatin was discharged from cortical granules to the perivitelline space after fertilization and is abundant in the perivitelline fluid (PVF) of early stage embryos. Cystatin rapidly agglutinated spermatozoa via an electrostatic interaction. Other basic proteins also agglutinated carp spermatozoa. Their activities were inhibited by salt and high pH. Cystatin bound to the entire surface of carp spermatozoa. The PVF of early embryos agglutinated carp spermatozoa. The activity was related to the cystatin content and influenced by ionic strength and pH. Therefore, cystatin is the major sperm-agglutinating factor of PVF. Owing to the rapid action of cystatin on spermatozoa agglutination and the presence of a high concentration of cystatin in PVF, cystatin is considered important for preventing polyspermy in carp eggs.

Myelin basic protein component C1 in increasing concentrations can elicit fusion, aggregation, and fragmentation of myelin-like membranes

Myelin basic protein (MBP) is considered to have a primary role in the formation and maintenance of the myelin sheath. Many studies using artificial vesicle systems of simple lipid composition, and generally small size, have shown that MBP can elicit vesicle fusion, aggregation, or even fragmentation under different conditions. Here, we have studied the effects of increasing concentrations of bovine MBP charge isomer C1 (MBP/C1) on large unilamellar vesicles (LUVs) composed of phosphatidylcholine and phosphatidylserine (92:8 molar ratio), or with a lipid composition similar to that of the myelin membrane in vivo (Cyt-LUVs). Using absorbance spectrophotometry, fluorescence resonance energy transfer, dynamic light scattering and transmission electron microscopy, we have shown that vesicle aggregation and some vesicle fusion occurred upon addition of MBP/C1, and as the molar protein-lipid ratio increased. Fragmentation of Cyt-LUVs was observed at very high protein concentrations. These results showed that the phenomena of vesicle fusion, aggregation, and fragmentation can all be observed in one in vitro system, but were dependent on lipid composition and on the relative proportions of protein and lipid.

Multilamellar packing of myelin modeled by lipid-bound MBP

Membrane compaction and adhesion at the major dense line (cytoplasmic apposition) of myelin, particularly in the central nervous system (CNS), is typically attributed to myelin basic protein (MBP). To explore the role of MBP in myelin membrane adhesion, we attempted to reconstitute the major dense line of myelin from purified lipid-bound MBP, which is a detergent-soluble form of MBP that retains the binding of all the myelin lipids. Removal of detergent by long-term dialysis yielded a precipitate, which, when analyzed by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and thin-layer chromatography, contained MBP that was still associated with myelin lipids, but in different proportions than in the native membrane. Comparison of lipid composition among isolated myelin, MBP-free myelin lipids, and lipid-bound MBP aggregates showed that the lipid-bound form of the protein was specifically enriched in phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, phosphatidylinositol, and phosphatidylserine. Electron microscopy and x-ray diffraction demonstrated that the lipid-MBP complexes formed multilayers having periods of 70-85 A, which correspond in width to individual myelin membranes. By contrast, the lipids alone assembled as multilayers having a period of approximately 40 A. Thus, the detergent-soluble form of MBP, which is bound to lipids, might serve as a simple model for the cytoplasmic apposition of myelin.

Adhesion of acidic lipid vesicles by 21.5 kDa (recombinant) and 18.5 kDa isoforms of myelin basic protein

Myelin basic protein (MBP) is thought to be responsible for adhesion of the intracellular surfaces of compact myelin to give the major dense line. The 17 and 21.5 kDa isoforms containing exon II have been reported by others to localize to the cytoplasm and nucleus of murine oligodendrocytes and HeLa cells while the 14 and 18.5 kDa isoforms lacking exon II are confined to the plasma membrane. However, we show that the exon II(-) 18.5 kDa form and a recombinant exon II(+) 21.5 kDa isoform both caused similar aggregation of acidic lipid vesicles, indicating that they should have similar abilities to bind to the intracellular lipid surface of the plasma membrane and to cause adhesion of those surfaces to each other. The circular dichroism spectra of the two isoforms indicated that both had a similar secondary structure. Thus, both isoforms should be able to bind to and cause adhesion of the cytosolic surfaces of compact myelin. The fact that they do not could be due to differences in post-translational modification in vivo, trafficking through the cell and/or subcellular location of synthesis, but it is not due to differences in their lipid binding.

Analysis of the membrane-interacting domains of myelin basic protein by hydrophobic photolabeling

Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.

CNP overexpression induces aberrant oligodendrocyte membranes and inhibits MBP accumulation and myelin compaction

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) is highly enriched in myelin-forming cells where it is concentrated at the cytoplasmic side of all surface membranes except those of compact myelin. Previous studies have provided evidence that CNP is functionally involved in migration or expansion of membranes during myelination. This hypothesis is supported, in part, by the production of aberrant myelin membranes in transgenic mice that have a 6-fold increase in CNP expression. In addition, many myelin lamellae in these CNP-overexpressing mice lacked major dense lines (MDLs). The purpose of the present study was to determine if CNP overexpression altered: (1) oligodendrocyte and myelin membrane production during early stages of myelination, and (2) the ultrastructural distribution of CNP and myelin basic protein (MBP) in myelin membranes. We identified aberrant membrane expanses that extended from premyelinating oligodendrocyte processes, the periaxonal membrane, and the contact point between oligodendrocyte processes and myelin internodes. Myelin membranes without MDLs were deficient in MBP and enriched in CNP. These data support a functional role for CNP during oligodendrocyte membrane expansion and indicate, for the first time, that CNP may help target MBP to compact myelin.

Myelin basic protein is a zinc-binding protein in brain: possible role in myelin compaction

The zinc-binding proteins (ZnBPs) in porcine brain were characterized by the radioactive zinc-blot technique. Three ZnBPs of molecular weights about 53 kDa, 42 kDa, and 21 kDa were identified. The 53 kDa and 42 kDa ZnBPs were found in all subcellular fractions while the 21 kDa ZnBP was mainly associated with particulate fractions. This 21 kDa ZnBP was identified by internal protein sequence data as the myelin basic protein. Further characterization of its electrophoretic properties and cyanogen bromide cleavage pattern with the authentic protein confirmed its identity. The zinc binding properties of myelin basic protein are metal specific, concentration dependent and pH dependent. The zinc binding property is conferred by the histidine residues since modification of these residues by diethyl-pyrocarbonate would abolish this activity. Furthermore, zinc ion was found to potentiate myelin basic protein-induced phospholipid vesicle aggregation. It is likely that zinc plays an important role in myelin compaction by interacting with myelin basic protein.

Membrane adhesion and other functions for the myelin basic proteins

The myelin basic proteins are a set of peripheral membrane polypeptides which play an essential role in myelination. Their most well-documented property is the unique ability to 'seal' the cytoplasmic aspects of the myelin membrane, but this is probably not the only function for these highly charged molecules. Despite extensive homology, the individual myelin basic proteins (MBPs) exhibit different expression patterns and biochemical properties, and so it is now believed that the various isoforms are not functionally equivalent in myelinating cells. We now think that while the major MBPs are intracellular adhesion molecules, some of the quantitatively less abundant isoforms that are expressed very early in development may have regulatory effects on the myelination program.

Interaction of myelin basic protein with artificial membranes. Parameters governing binding, aggregation and dissociation

The interaction of myelin basic protein (MBP) with large unilamellar vesicles, composed of phosphatidylserine (PtdSer), phosphatidylserine/phosphatidylcholine (PtdSer/Ole2GroPCho) and phosphatidylcholine/cholesterol (Ole2GroPCho/cholesterol) was examined. Binding of MBP to the bilayers as well as the kinetics of this process were determined by a resonance energy transfer procedure. The ability of the protein to aggregate the vesicles subsequently was monitored continuously by absorbance measurements. The interaction was further characterized by determining the ability of MBP to induce membrane perturbations, as reflected by release of aqueous vesicle contents, and lipid mixing. The results demonstrate that Ole2GroPCho inhibits, while PtdSer and cholesterol strongly facilitate MBP-induced membrane aggregation. Furthermore, binding of MBP to vesicles and the subsequent aggregation event are separate processes, i.e. the extent of binding does not necessarily reflect the aggregation susceptibility. Overall, aggregation appears to be the rate-limiting step. Interaction of MBP with PtdSer bilayers results in a limited degree of lipid mixing, which is accompanied by extensive release of vesicle contents. For all other compositions, no lipid mixing occurs, while cholesterol effectively prevents release of vesicle contents. pH-dependent experiments indicate distinct mechanisms to be operative in MBP-induced aggregation of PtdSer and Ole2GroPCho/cholesterol bilayers. At neutral pH, protein-protein interactions appear relevant, while at acidic pH intervesicular bridges, established by monomers that may cause aggregation of PtdSer vesicles, but not of Ole2GroPCho/cholesterol vesicles. The observation that divalent cations reverse MBP-induced vesicle aggregation may have physiological relevance.

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.

Modulation by glycosphingolipids of membrane-membrane interactions induced by myelin basic protein and melittin

The effect of glycosphingolipids (GSLs) with oligosaccharide chains of different length and charge on membrane-membrane interactions induced by myelin basic protein (MBP) or melittin (Mel) was comparatively investigated with small unilamellar vesicles. MBP induces a fast vesicle aggregation and close membrane apposition. Merging of lipid bilayers and vesicle fusion induced by MBP are slower and less extensive processes compared to membrane apposition. The changes of membrane permeability concomitant to these phenomena are small. The Trp region of MBP remains in a rather polar environment when interacting with vesicles; its accessibility to NO3- or acrylamide quenching depends on the type of GSLs in the membrane. The Trp region of Mel is inserted more deeply into the lipid bilayer and its accessibility to the aqueous quenchers is less dependent on variations of the oligosaccharide chain of the GSLs. Mel induces a faster and more extensive membrane apposition and bilayer merging than does MBP. Extensive vesicle disruption occurs in the presence of Mel. Negatively charged GSLs facilitate membrane proximity and vesicle aggregation but an increase of the oligosaccharide chain length of either neutral or acidic GSLs decreases the interaction among vesicles that are induced by either protein. This effect is independent of the different mode of insertion of MBP and Mel into the membrane. Our results suggest that the modulation by the oligosaccharide chain on the protein-induced interactions between bilayers containing GSLs is probably exerted beyond the level of local molecular interactions between the basic proteins and the lipids.

The basic protein of CNS myelin: its structure and ligand binding

Consideration of the evidence presented in this review leads to the following conclusions: (a) Isolated MBP in aqueous solution has little ordered secondary or tertiary structure. (b) In this state, the protein can associate with a wide range of hydrophobic and amphiphilic compounds, these interactions involving limited sections of the protein. (c) The strength of binding to bilayers and the accompanying conformational changes in the protein are greatest for systems containing acidic lipids, presumably because of the involvement of ionic interactions. (d) When bound to bilayers of acidic lipids, MBP will have substantially more ordered secondary structure than it manifests in aqueous solution, and it is likely to be oligomeric (possibly hexameric). (e) MBP does affect the organization of lipid aggregates. It influences strongly the separation of bilayers in multilayers of purified lipids, and at present this must be viewed as its prime role within myelin. The greatest impediment to our understanding of MBP is the lack of an assayable biological activity. In contrast to the situation with enzymes, for example, we have no functional test for changes in protein structure or changes accompanying interactions with other molecules. Current evidence suggests that the protein has a structural role within myelin and that its own three-dimensional structure is strongly dependent on the molecules with which it is associated. If this picture is correct, studies of the isolated protein or of the protein in reconstituted lipid systems may yield, at best, a rough guide to the structure within its biological environment. Further clarification of the structure and function of MBP may have to await development of more powerful techniques for studying proteins bound to large molecular aggregates, such as lipid bilayers. The paucity of generally applicable methods is reflected in the fact that even low resolution structures are known for only a handful of intrinsic membrane proteins, and even more limited information exists for proteins associated with membrane surfaces. However, the increasing use of a combination of electron microscopy and diffraction on two-dimensional arrays of proteins formed on lipid bilayers (Henderson et al., 1990) offers the hope that it may not be too long before it will be possible to study at moderate resolution the three-dimensional structure of MBP bound to a lipid membrane.

The ectopic expression of myelin basic protein isoforms in Shiverer oligodendrocytes: implications for myelinogenesis

The myelin basic proteins (MBPs) are a set of peripheral membrane polypeptides that are required for the compaction of the major dense line of central nervous system myelin. We have used primary cultures of oligodendrocytes from MBP-deficient shiverer mice as host cells for the expression by cDNA transfection of each of the four major MBP isoforms. The distributions of the encoded polypeptides were studied by immunofluorescence and confocal microscopy and compared with patterns of MBP expression in normal mouse oligodendrocytes in situ and in culture. The exon II-containing 21.5- or 17-kD MBPs were distributed diffusely in the cytoplasm and in the nucleus of the transfectants, closely resembling the patterns obtained in myelinating oligodendrocytes in 9-d-old normal mouse brains. By contrast, the distribution of the 14- and 18.5-kD MBPs in the transfectants was confined to the plasma membrane and mimicked the distribution of MBP in cultures of normal adult oligodendrocytes. Our results strongly suggest that the exon II-containing MBPs are expressed first and exclusively during oligodendrocyte maturation, where they may play a role in the early phase of implementation of the myelination program. In contrast, the 14- and 18.5-kD MBPs that possess strong affinity for the plasma membrane are likely to be the principle inducers of myelin compaction at the major dense line.

Central nervous system myelin: structure, function, and pathology

Multiple sclerosis (MS) and a number of related distinctive diseases are characterized by the active degradation of central nervous system (CNS) myelin, an axonal sheath comprised essentially of proteins and lipids. These demyelinating diseases appear to arise from complex interactions of genetic, immunological, infective, and biochemical mechanisms. While circumstances of MS etiology remain hypothetical, one persistent theme involves recognition by the immune system of myelin-specific antigens derived from myelin basic protein (MBP), the most abundant extrinsic myelin membrane protein, and/or another equally susceptible myelin protein or lipid component. 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 the understanding of how and why these antigens become selected during the development of MS. This review focuses on current understanding of the molecular basis underlying demyelinating disease as it may relate to the impact of the various protein and lipid components on myelin morphology; the precise molecular architecture of this membrane as dictated by 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 basic protein stimulates insulin and glucagon secretion from rat pancreatic islets in vitro and in vivo

The effect of myelin basic protein on insulin and glucagon secretion from rat pancreatic islets was studied in vivo and in vitro. The myelin basic proteins isolated from bovine, human and rat brains all stimulated insulin secretion in a similar fashion. In a static incubation of isolated pancreatic islets, myelin basic protein at doses of 15.6-250 micrograms in a 0.5-ml reaction volume (1.7 X 10(-6) to 2.7 X 10(-5) M) significantly stimulated hormone release. Maximal stimulation, obtained at the 250-micrograms dose, was 6.5-fold greater than control for insulin secretion and 6.7-fold greater than control for glucagon secretion. In the case of glucagon no saturation was observed, but saturation was obvious for insulin release at doses of myelin basic protein of 62.5-250 micrograms, larger doses causing permeabilization of the islet membranes as indicated by leakage of acid phosphatase. At a 100-micrograms dose the time course of insulin secretion induced by myelin basic protein indicated a fast initial release, and after the first 2 h only a little more insulin was released. At the lower doses of myelin basic protein (11 and 33 micrograms) the secretion rate was nearly constant after the first hour. Significant stimulation of glucagon release by myelin basic protein was seen after 60 min, the rate of release being roughly constant at 33- and 100-micrograms doses thereafter. At the 11-micrograms dose significant stimulation of hormone release was observed only after a 4-h incubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of myelin basic protein isoforms in nonglial cells

The myelin basic proteins (MBPs) mediate the cytoplasmic apposition of the oligodendrocyte plasma membrane to form the major dense line of central nervous system myelin. Four major isoforms of murine MBP, obtained by alternative splicing of seven exons from a single primary transcript, display distinct developmental profiles. We expressed these major MBPs individually in HeLa cells and mapped their distributions by immunofluorescence and confocal microscopy. The 14- and 18.5-kD MBPs that are the predominant forms in compact myelin distributed primarily in the perinuclear regions of the cell in configurations highly suggestive of close association with membranes. We infer that these MBP isoforms possess strong, nonspecific membrane-binding properties that have been adapted by the oligodendrocyte to mediate compaction of the sheaths of plasma membrane that form myelin. In contrast, the 17- and 21.5-kD isoforms distributed diffusely in both the cytoplasm and the nucleoplasm and often accumulated within the nucleus. This distribution can be correlated with the presence of the peptide segment encoded by exon II, which is unique to these isoforms. The physiological significance of the nuclear targeting displayed by the 17- and 21.5-kD MBP isoforms in HeLa cells remains to be determined.

Interaction and fusion of unilamellar vesicles containing cerebrosides and sulfatides induced by myelin basic protein

The effects of myelin basic protein on the aggregation, lipid bilayer merging, intercommunication of aqueous compartments and leakage of small unilamellar vesicles of egg phosphatidylcholine containing different proportions of galactocerebroside and sulfatide were investigated. This was performed employing light scattering, absorbance changes and fluorescence assays (resonance energy transfer, Terbium/dipicolinic acid assay and carboxyfluorescein release). The apposition of membranes rapidly induced by myelin basic protein is enhanced by sulfatide but reduced by galactocerebroside compared to vesicles of egg phosphatidylcholine alone. On the other hand, the presence of either glycosphingolipid in the membrane interferes with the induction by myelin basic protein of lipid bilayer merging, subsequent fusion and changes of the membrane permeability. Our results support an important modulation by sulfatide and galactocerebroside on the interactions among membranes induced by myelin basic protein, depending on the relative proportions of the glycosphingolipids and phosphatidylcholine.

The thermodynamically stable state of myelin basic protein in aqueous solution is a flexible coil

Conformational studies of myelin basic protein (MBP) in solution generally have used protein purified in organic solvents and acid. The use of such conditions raises the possibility that the secondary structure reported for the basic protein represents a denatured state. Therefore we have purified this protein by using a procedure that avoids denaturants. Bovine myelin was extracted with 0.2 M-CaCl2 and the protein was purified from the supernatant by chromatography on Sephadex G-75. The conformation of the basic protein was characterized by using c.d. and 1H-n.m.r. spectroscopy. In solution, it appeared to be predominantly randomly coiled, with only small segments of persistent structure. However, in the presence of myristoyl lysophosphatidylcholine the secondary structure of MBP became more ordered, and sedimentation-velocity experiments showed that MBP aggregated. Comparison of our results with published data indicates that Ca2+-extracted basic protein behaves similarly to the protein purified by traditional methods with respect to its ordered conformation in solution in the absence and in the presence of lipid and with respect to its self-association. Thus its thermodynamically stable structure in aqueous solution appears to be a highly flexible coil.

Myelin basic protein is an endogenous inhibitor of the high-affinity cannabinoid binding site in brain

Radioligand binding studies with the water-soluble cannabinoid [3H]5'-trimethylammonium delta 8-tetrahydrocannabinol ([3H]TMA) have revealed a saturable high-affinity site in brain that is specific for cannabinoids. To determine whether endogenous compounds of brain might act upon the site physiologically, we sought inhibitors in extracts of brain. An endogenous inhibitor has been purified to homogeneity by acid extraction of rat brain followed by adsorption to a reverse-phase matrix and gel filtration chromatography. The purified inhibitor has a subunit molecular mass of 14,500 daltons by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Inhibition of [3H]TMA binding by the purified inhibitor occurs with a Ki of about 4 nM in a noncompetitive manner. The molecular weight, abundance, and extraction properties are the same as a species of myelin basic protein (MBP). The MBPs of rat, rabbit, pig, and cow also inhibit [3H]TMA binding noncompetitively with similar potencies. The purified inhibitor comigrates with rat MBP-small form on SDS-PAGE, has a similar amino acid composition, and is recognized by antibody directed against MBP. Studies of fragments of rabbit MBP suggest that the determinants of affinity for the [3H]TMA site are contained primarily within the C-terminal half of the rabbit MBP. Synthetic polycationic peptides such as polylysine and polyarginine mimic the effects of MBP, suggesting that the high-affinity cannabinoid binding site recognizes large polycations. The identification of the endogenous inhibitor of [3H]TMA binding as MBP suggests that MBP interacts physiologically with the high-affinity cannabinoid site.

Lipid composition of myelin and protein--lipid complex in a neurological rabbit mutant

The neurological mutant of the paralytic tremor "pt" rabbit is characterized by a reduced amount of myelin to 25-30% of control with no change in specific activity for 2',3'-cyclic nucleotide phosphodiesterase. The ratio of total lipids to protein was higher in "pt" rabbit myelin than in control by about 20%. Analysis of the lipid composition of "pt" rabbit myelin indicated a significantly lower level of galactolipids (by about 30%) and a higher level of gangliosides compared to control. The percentage composition of phospholipids in "pt" myelin was characterized by a lower proportion of acidic phospholipids (polyphosphoinositides, phosphatidic acid, phosphatidylserine). The protein-lipid complex in "pt" rabbit myelin was decreased by about 10-15% compared to control. The ratio by weight of protein to lipid in myelin was 0.24 and 0.20 in control and "pt" rabbit, respectively, but in protein-lipid complex isolated from myelin it was 1.73 in control and 0.72 in "pt" rabbit. Protein-lipid complex isolated from myelin of "pt" rabbit brain contained about 34% less protein and about 16% less acidic phospholipids but neutral phospholipids were increased by 24%. The lipid abnormalities in "pt" rabbit myelin and in the composition of protein-lipid complex may be responsible for the disturbances of myelin formation and compaction in "pt" rabbit.

ESR imaging of myelin basic protein induced vesicle aggregation

Using a modulated magnetic field gradient technique, the conventional ESR spectrum of well-defined spatial sections and the one-dimensional-ESR image of the nitroxide centre line of spin-labeled stearic acid in phospholipid vesicles were recorded with a spatial resolution of 4.10(-5) m after pelleting the vesicles inside 1 mm (i.d.) sample capillaries in a slow centrifuge (2500 X g). The sedimentation characteristics of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol vesicles were quantitatively compared with particular reference to vesicle aggregation induced by myelin basic protein. Protein-induced changes in the effective molecular mass were determined from ESR images of sedimentation profiles. The present data lend further support to the notion that the primary target of myelin basic protein-lipid interaction is the acidic lipid pool of myelin.

A novel fragmentation of human myelin basic protein: identification of phosphorylated domains

Human myelin basic protein (MBP) was fragmented into three major polypeptides comprised of a NH2-terminal domain (residues 1-83), a middle domain (residues 84-119) which contains an experimental allergic encephalitogenic determinant and a highly conserved triproline sequence, and a COOH-terminal domain (residues 120-170) by Staphylococcus aureus V8 protease at pH 4.0. These three polypeptides could be identified and purified by reversed-phase high-performance liquid chromatography. Analysis of the sites of phosphorylation of the component 1 of human MBP, the most cationic species, catalyzed by a purified Ca2+-activated and phospholipid-dependent protein kinase and cAMP-dependent protein kinase revealed that although these protein kinases could incorporate approximately 6 and 4 mol 32P, respectively, into MBP, none of the potential sites were located within the middle domain.

The association of myelin basic protein with itself and other proteins

Chromatographic studies were performed to measure myelin basic protein (MBP) interactions by covalently binding a number of different proteins to Sepharose and passing radioactive bovine MBP over these columns. Studies at a variety of pH values, ionic strengths and temperatures revealed that the bovine MBP could interact with itself as well as cytochrome c, lysozyme, and ovalbumin. Chromatographic profiles of elution volume vs. pH revealed that the interaction between MBP and these immobilized proteins was biphasic. The self-association of MBP was found to be strongest between pH 7.4 and 8.1 and at an elevated temperature. Titration of the amino acid residues responsible for the association of MBP with other proteins revealed apparent pKs ranging from 6.10 to 6.70. A pH dependence study at an elevated temperature shifted the apparent pK of the MBP interaction to a lower value with all the proteins except ovalbumin. After destroying 60% of the histidine residues in MBP by photooxidation and passing 125I-labeled photooxidized MBP over Sepharose columns containing immobilized protein, the second phase in binding was decreased significantly with immobilized cytochrome c, lysozyme, and MBP and to a smaller extent with ovalbumin. These results are consistent with the involvement of deprotonated histidine residues in the MBP-protein associations.

Spontaneous vesicularization of myelin lipids is counteracted by myelin basic protein

Hand-vortexed dispersions of several lipids (cerebrosides, sulfatides, PC, PE, PS and sphingomyelin), mixed in the ratios found for these categories of lipids in myelin, exhibit 31P-NMR spectra which have contributions from both isotropic and lamellar resonances. Investigation of this system by freeze-fracture electron microscopy and X-ray diffraction revealed that this lipid mixture has spontaneously formed small unilamellar vesicles (SUVs) (diam. approximately 400 A) and large highly convoluted unilamellar vesicles (LUVs) (diam. approximately 1000 A), the latter possibly resulting from aggregation and fusion of the SUV structures. This vesicularization of the myelin lipids was reversed by the addition of myelin basic protein: only large multilamellar aggregates were formed in the presence of protein, as shown by all three experimental methods. Although no rigorous physical-chemical explanation for these phenomena is yet available, the possibility is suggested that the high concentration of cerebrosides and/or phosphatidylethanolamine in this particular mixture of myelin lipids play pivotal roles in the formation of these unusual vesicles. Spontaneous vesicularization of myelin lipids is discussed as a potential pathway toward destabilization of the myelin sheath.

Modulation of myelin basic protein-induced aggregation and fusion of liposomes by cholesterol, aliphatic aldehydes and alkanes

The effect of cholesterol on myelin basic protein-induced aggregation of zwitterionic phospholipid vesicles was studied by turbidimetry, quasi-elastic light scattering and centrifugation techniques. Without cholesterol, the degree of vesicle aggregation caused by myelin basic protein is relatively low and is only slightly increased using cholesterol concentrations up to approx. 25-30 mol%. When the cholesterol content in the bilayer exceeds approx. 30 mol%, there is a dramatic increase in the susceptibility of the vesicles to aggregation in the presence of myelin basic protein. Palmitoyl aldehyde and eicosane, substances resembling products of lipid degradation, increase myelin basic protein promoted fusion of vesicles. The fusion is accompanied by increased leakage of entrapped carboxyfluorescein. In the presence of cholesterol, myelin basic protein-induced fusion of the liposomes becomes much more sensitive to the presence of aliphatic aldehydes or alkanes. The results suggest that cholesterol has an important role in promoting membrane adhesion in biological systems but these structures become unstable in the presence of small amounts of products of lipid degradation. The findings have important implications to the understanding of the stability of the myelin membrane.

Effect of pH on interlamellar spacing in rat sciatic myelin

Tissue sections of rat sciatic nerves were incubated at various pH ranges to study the effects of proton titration on myelin fine structure by transmission electron microscopy. It was found that the major dense line of the sheath can open and close reversibly in response to proton titration at a pH range compatible with in vivo conditions. Opened and closed major dense lines coexisted in the sheath, but the number of opened lines increased with increasing pH. There was a pattern of periodicity in the opening of dense lines in that either one, two, or three opened lines alternated with one closed line. The local opening or closing of a dense line, furthermore, was often associated with reciprocal changes in the two adjacent dense lines, highly suggestive of the interaction of electric fields between the myelin membranes. These observations show that energy can be stored within the myelin leaflet.

Polylysine induces pH-dependent fusion of acidic phospholipid vesicles: a model for polycation-induced fusion

Polylysine induced aggregation and fusion of negatively charged small unilamellar phosphatidylcholine vesicles containing at least 10% anionic lipid. Aggregation was followed by absorbance changes and fusion was assayed both by electron microscopy and by fluorescence energy transfer between lipid probes. A method for preparing asymmetric vesicles, where the fluorescent probes were present only in the inner monolayer of the vesicle membrane, was developed. These vesicles were used to distinguish the inner and outer monolayer when measuring lipid mixing between vesicles. Since polylysine induced lipid mixing of both monolayers equally, fusion of these vesicles did occur. The extent of fusion was dependent on the charge ratio between bound polylysine and phosphatidylserine (PS) in the outer monolayer and was optimal at a ratio of about 1:1. Excess polylysine inhibited fusion. At a given concentration of polypeptide, fusion increased as the pH was lowered toward 3 with an apparent pKa near 4. Since this value is close to the pKa of the PS-carboxyl groups and far from the pKa of the lysine epsilon-amino groups, the pH dependence observed for fusion resides in the lipids rather than in the peptide. Fusion was dependent on the available lysine and not the size or molarity of the polypeptide. The data indicate that there must be sufficient sites on the vesicles and sufficient polypeptide to achieve effective aggregation. For fusion to occur after aggregation, charges on the vesicles must be neutralized either by polypeptide-PS interaction or by protonation of the PS carboxyl groups. Optimal conditions for fusion occur when charge neutralization is possible without completely covering the vesicles with polypeptide. The results are consistent with the notion that the polypeptide is necessary for fusion because of requirements for crosslinking, but limits fusion by steric inhibition.

NMR studies of myelin basic protein. XIII. Assignment of histidine residues in rabbit, bovine and porcine proteins

Myelin basic protein from three species (rabbit, cow and pig) and peptides from enzymatic digests or cleavage of the proteins have been examined in aqueous solutions by proton nuclear magnetic resonance (NMR) at 400 MHz. The epsilon 1-CH and delta 2-CH resonances of all the histidine residues in the three proteins have been assigned and the pK values have been measured. The heterogeneity of chemical shifts among these resonances can be variously ascribed to persistent localized secondary structures and to effects arising from charged side-chains, particularly those of aspartic acid residues, and from side-chains of aromatic moieties.

Gramicidin induced aggregation and size increase of phosphatidylcholine vesicles

To investigate the role of membrane proteins in the fusion process, linear hydrophobic polypeptide gramicidin was used as fusogenic agent in small unilamellar vesicles (SUV) constituted of saturated lecithins. It was found that gramicidin, externally added to a suspension of vesicles, induces a reversible vesicles aggregation. When incorporated into the bilayer, gramicidin induces increase in vesicle size. The vesicle size increase was monitored by column chromatography and transmission electron microscopy. The process of vesicle size increase occurs only when the lipid membrane is in the gel state. A maximum is observed in the kinetics at a temperature of approx. 25 degrees C lower than the phase transition temperature of lipids. Higher rates of vesicle size increase are obtained as the lipid chain length increases. The process is accompanied by a release of internal vesicle content and by membrane lipid mixing.

A comparison of the dodecyl sulfate-induced precipitation of the myelin basic protein with other water-soluble proteins

The interactions of sodium dodecyl sulfate with a number of proteins were examined at a variety of pH values ranging from 4.8 to 11.6. The dodecyl sulfate-induced precipitation of some of these proteins was observed within a relatively limited range of total dodecyl sulfate concentration. Most of the basic proteins precipitated at low pH but as the isoelectric point of the protein was approached the amount of protein that precipitated decreased. Bovine myelin basic protein was unique in that it precipitated at all pH values examined both above and below its isoelectric point. Thus, the dodecyl sulfate-induced precipitation of myelin basic protein appears to be different from the dodecyl sulfate-induced precipitation of most proteins. A comparison of protein precipitation at equivalent dodecyl sulfate:protein molar or weight ratios revealed very little difference in the precipitation behavior of the proteins studied. When the bovine myelin basic protein was cleaved at its single tryptophan residue, the N-terminal fragment (1-115) formed insoluble dodecyl sulfate complexes at pH values ranging from 4.8 to 9.2. The C-terminal fragment (116-169) precipitated almost completely at pH 4.8 but to a lesser extent at pH 7.4 and 9.2. Equimolar mixtures of the N- and C-terminal fragments precipitated in the presence of dodecyl sulfate at pH 7.4 and 9.2 to an extent greater than the C-terminal fragment alone but comparable to the N-terminal fragment alone or the whole basic protein. These results suggest: that the mechanism by which dodecyl sulfate induces the precipitation of myelin basic protein may be unique compared to other proteins and that the intact myelin basic protein is not necessary for its precipitation by dodecyl sulfate.

Myelin basic protein ability to organize lipid bilayers: structural transition in bilayers of lysophosphatidylcholine micelles

Myelin basic protein isolated by a single step with the cationic detergent cethyltrimethylammonium bromide in a lipid-bound form is able to induce structural transition of lysophosphatydilcholine micelles into multi-laminar vesicles. This finding, observed through electron microscopy, is discussed in the light of the assumed ability of the basic protein to organize myelin lipids.

Apocytochrome c induces pH-dependent vesicle fusion

The ability of apocytochrome c and the heme containing respiratory chain component, cytochrome c, to induce fusion of phosphatidylcholine (PC) small unilamellar vesicles containing 0-50 mol % negatively charged lipids was examined. Both molecules mediated fusion of phosphatidylserine (PS):PC 1:1 vesicles as measured by energy transfer changes between fluorescent lipid probes in a concentration- and pH-dependent manner, although cytochrome c was less potent and interacted over a more limited pH range than the apocytochrome c. Maximal fusion occurred at pH 3, far below the pKa of the 19 lysine groups contained in the protein (pI = 10.5). A similar pH dependence was observed for vesicles containing 50 mol % cardiolipin (CL), phosphatidylglycerol (PG), and phosphatidylinositol (PI) in PC but the apparent pKa values varied somewhat. In the absence of vesicles, the secondary structure of apocytochrome c was unchanged over this pH range, but in the presence of negatively charged vesicles, the polypeptide underwent a marked conformational change from random coil to alpha-helix. By comparing the pH dependencies of fusion induced by poly-L-lysine and apocytochrome c, we concluded that the pH dependence derived from changes in the net charge on both the vesicles and apocytochrome c. Aggregation could occur under conditions where fusion was imperceptible. Fusion increased with increasing mole ratio of PS. Apocytochrome c did induce some fusion of vesicles composed only of PC with a maximum effect at pH 4. Biosynthesis of cytochrome c involves translocation of apocytochrome c from the cytosol across the outer mitochondrial membrane to the outer mitochondrial space where the heme group is attached. The ability of apocytochrome c to induce fusion of both PS-containing and PC-only vesicles may reflect characteristics of protein/membrane interaction that pertain to its biological translocation.

Aliphatic aldehydes promote myelin basic protein-induced fusion of phospholipid vesicles

Myelin basic protein induces slow and limited fusion of phospholipid vesicles composed of a mixture of phosphatidylcholine and phosphatidylethanolamine. Addition of palmitoyl aldehyde to these vesicles dramatically increases their ability to fuse in the presence of myelin basic protein. Compared to aliphatic aldehydes, fatty acids are much less potent promoters of myelin basic protein-induced membrane fusion. The ability of aliphatic aldehydes to promote myelin basic protein-induced membrane fusion may be of relevance to myelin structure and function and, particularly, to the pathology of demyelinating diseases such as multiple sclerosis.

The encephalitogenic protein of myelin forms hexamers in which the polypeptides have a pleated-sheet structure

Sedimentation equilibrium data are shown to be consistent with the existence in solution of an equilibrium between monomers and hexamers of bovine myelin basic protein, without significant accumulation of intermediates. At low concentrations circular dichroism spectra were indicative of an aperiodic coiled secondary structure. At higher concentrations, where the protein self-associates, they showed formation of a beta-pleated sheet conformation. At low molar ratios myristoyllysophosphatidylcholine promotes protein self-association and the concomitant conformational transition. The data are consistent with the existence of an equilibrium mixture of relatively unstructured monomers and hexamers in which the polypeptides have a well-defined three-dimensional structure.

Effect of bovine basic protein charge microheterogeneity on protein-induced aggregation of unilamellar vesicles containing a mixture of acidic and neutral phospholipids

Two of the charge isomers (components 1 and 2) normally found as microheteromers of myelin basic protein were isolated, and their abilities to aggregate vesicles consisting of mixed phospholipids were studied. Component 1 (the most cationic of the microheteromers) aggregated phosphatidylcholine (PC) vesicles containing 7.8 mol% phosphatidylserine (PS) more rapidly and at lower protein concentrations than component 2, which differs from component 1 by 1 net positive charge. Modification of components 1 and 2 in vitro by phosphorylation with rabbit muscle protein kinase decreased the ability of both components to aggregate vesicles. The greater the extent of phosphorylation, the less effective were the isomers at inducing aggregation. Decreasing the charge of either component 1 or component 2 by removal of the two C-terminal arginyl residues also decreased the ability of the isomers to induce aggregation. Therefore, charge microheterogeneity, whether arising in vivo or generated in vitro, markedly affected the ability of these microheteromers to aggregate PC vesicles containing 7.8 mol% PS. Because a small difference in the charge of the protein had a marked effect on vesicle aggregation, we propose that charge microheterogeneity may play an important and dynamic role in the structure and function of normal myelin.