Prediction of the secondary structure of myelin basic protein

NMR and molecular dynamics studies of an autoimmune myelin basic protein peptide and its antagonist: structural implications for the MHC II (I-Au)-peptide complex from docking calculations

Experimental autoimmune encephalomyelitis can be induced in susceptible animals by immunodominant determinants of myelin basic protein (MBP). To characterize the molecular features of antigenic sites important for designing experimental autoimmune encephalomyelitis suppressing molecules, we report structural studies, based on NMR experimental data in conjunction with molecular dynamic simulations, of the potent linear dodecapeptide epitope of guinea pig MBP, Gln74-Lys75-Ser76-Gln77-Arg78-Ser79-Gln80-Asp81-Glu82-Asn83-Pro84-Val85 [MBP(74-85)], and its antagonist analogue Ala81MBP(74-85). The two peptides were studied in both water and Me(2)SO in order to mimic solvent-dependent structural changes in MBP. The agonist MBP(74-85) adopts a compact conformation because of electrostatic interactions of Arg78 with the side chains of Asp81 and Glu82. Arg78 is 'locked' in a well-defined conformation, perpendicular to the peptide backbone which is practically solvent independent. These electrostatic interactions are, however, absent from the antagonist Ala81MBP(74-85), resulting in great flexibility of the side chain of Arg78. Sequence alignment of the two analogues with several species of MBP suggests a critical role for the positively charged residue Arg78, firstly, in the stabilization of the local microdomains (epitopes) of the integral protein, and secondly, in a number of post-translational modifications relevant to multiple sclerosis, such as the conversion of charged arginine residues to uncharged citrullines. Flexible docking calculations on the binding of the MBP(74-85) antigen to the MHC class II receptor site I-A(u) using haddock indicate that Gln74, Ser76 and Ser79 are MHC II anchor residues. Lys75, Arg78 and Asp81 are prominent, solvent-exposed residues and, thus, may be of importance in the formation of the trimolecular T-cell receptor-MBP(74-85)-MHC II complex.

Effects of the osmolyte trimethylamine-N-oxide on conformation, self-association, and two-dimensional crystallization of myelin basic protein

The osmolyte trimethylamine-N-oxide (TMAO) is a naturally in vivo occurring "chemical chaperone" that has been shown to stabilise the folding of numerous proteins. Myelin basic protein (MBP) is a molecule that has not yet been suitably crystallized either in three dimensions for X-ray crystallography or in two dimensions for electron crystallography. Here, we describe lipid monolayer crystallization experiments of two species of recombinant murine MBP in the presence of TMAO. One protein was unmodified, whereas the other contained six Arg/Lys-->Gln substitutions to mimic the effects of deimination (i.e., the enzymatic modification of Arg to citrulline), which reduces the net positive charge. Planar arrays of both proteins were formed on binary lipid monolayers containing a nickel-chelating lipid and a phosphoinositide. In the presence of TMAO, the diffraction spots of these arrays became sharper and more distinct than in its absence, indicating some improvement of crystallinity. The osmolyte also induced the formation of epitaxial growth of protein arrays, especially with the mutant protein. However, none of these assemblies was sufficiently ordered to extract high-resolution structural information. Circular dichroic spectroscopy showed that MBP gained no increase in ordered secondary structure in the presence of TMAO in bulk solution, whereas it did in the presence of lipids. Dynamic light-scattering experiments confirmed that the MBP preparations were monomodal under the optimal crystallization conditions determined by electron microscopy trials. The salt and osmolyte concentrations used were shown to result in a largely unassociated population of MBP. The amino acid composition of MBP overwhelmingly favours a disordered state, and a neural-network-based scheme predicted large segments that would be unlikely to adopt a regular conformation. Thus, this protein has an inherently disordered nature, which mitigates strongly against its crystallization for high-resolution structure determination.

Analogous structural motifs in myelin basic protein and in MARCKS

Myelin basic protein (MBP) and myristoylated alanine-rich C-kinase substrate (MARCKS) are similar in terms of having extended conformations regulated by their environment (i.e., solubilised or lipid-associated), N-terminal modifications, a dual nature of interactions with lipids, binding to actin and Ca2+-calmodulin, and being substrates for different kinds of protein kinases. The further sequence similarities of segments of MBP with lipid effector regions of MARCKS, and numerous reports in the literature, support the thesis that some developmental isoform of MBP functions in signal transduction.

Cryoelectron microscopy of protein-lipid complexes of human myelin basic protein charge isomers differing in degree of citrullination

Myelin basic protein (MBP) is considered to be essential for the maintenance of stability of the myelin sheath. Reduction in cationicity of MBP, especially due to conversion of positively charged arginine residues to uncharged citrulline (Cit), has been found to be associated with multiple sclerosis (MS). Here, the interactions of an anionic phosphatidylserine/monosialoganglioside-G(M1) (4:1, w:w) lipid monolayer with 18.5-kDa MBP preparations from age-matched adult humans without MS (no Cit residues), with chronic MS (6 Cit), and with acute Marburg-type MS (18 Cit) were studied by transmission and ultralow dose scanning transmission electron microscopy under cryogenic conditions. Immunogold labeling and single particle electron crystallography were used to define the nature of the complexes visualized. These electron microscopical analyses showed that the three different MBP charge isomers all formed uniformly sized and regularly shaped protein-lipid complexes with G(M1), probably as hexamers, but exhibited differential association with and organization of the lipid. The least cationic Marburg MBP-Cit(18) formed the most open protein-lipid complex. The data show a disturbance in lipid-MBP interactions at the ultrastructural level that is related to degree of citrullination, and which may be involved in myelin degeneration in multiple sclerosis.

Three-dimensional structure of myelin basic protein. I. Reconstruction via angular reconstitution of randomly oriented single particles

Myelin basic protein (MBP) plays an integral role in the structure and function of the myelin sheath. In humans and cattle, an 18.5-kDa isoform of MBP predominates and exists as a multitude of charge isomers resulting from extensive and varied post-translational modifications. We have purified the least modified isomer (named C1) of the 18.5-kDa isoform of MBP from fresh bovine brain and imaged this protein as negatively stained single particles adsorbed to a lipid monolayer. Under these conditions, MBP/C1 presented diverse projections whose relative orientations were determined using an iterative quaternion-assisted angular reconstitution scheme. In different buffers, one with a low salt and the other with a high salt concentration, the conformation of the protein was slightly different. In low salt buffer, the three-dimensional reconstruction, solved to a resolution of 4 nm, had an overall "C" shape of outer radius 5.5 nm, inner radius 3 nm, overall circumference 15 nm, and height 4.7 nm. The three-dimensional reconstruction of the protein in high salt buffer, solved to a resolution of 2.8 nm, was essentially the same in terms of overall dimensions but had a somewhat more compact architecture. These results are the first structures achieved directly for this unusual macromolecule, which plays a key role in the development of multiple sclerosis.

Three-dimensional structure of myelin basic protein. II. Molecular modeling and considerations of predicted structures in multiple sclerosis

A computational model of myelin basic protein (MBP) has been constructed based on the premise of a phylogenetically conserved beta-sheet backbone and on electron microscopical three-dimensional reconstructions. Many residues subject to post-translational modification (phosphorylation, methylation, or conversion of arginines to citrullines) were located in loop regions and thus accessible to modifying enzymes. The triproline segment (residues 99-101) is fully exposed on the back surface of the protein in a long crossover connection between two parallel beta-strands. The proximity of this region to the underlying beta-sheet suggests that post-translational modifications here might have potential synergistic effects on the entire structure. Post-translational modifications that lead to a reduced surface charge could result first in a weakened attachment to the myelin membrane rather than in a gross conformational change of the protein itself. Such mechanisms could be operative in demyelinating diseases such as multiple sclerosis.

Immunolocalization of 17 and 21.5 kDa MBP isoforms in compact myelin and radial component

Our previous biochemical analyses revealed that the levels of the minor MBP isoforms 21.5 and 17 kDa are elevated relative to the 14 and 18.5 kDa MBP isoforms in the fraction of isolated myelin of murine CNS that is enriched in interlamellar junctions (or radial component). To substantiate the localization of 21.5 and 17 kDa MBP in the myelin sheath, we used immunoelectron microscopy on thin-sections of mouse optic nerve. Two different polyclonal antibodies were used to distinguish 21.5 and 17 kDa MBP from 14 and 18.5 kDa MBP: Ab-MBP21.5, which was raised against a synthetic peptide corresponding to the exon II amino acid sequence 61-83 of mouse 21.5 kDa MBP (LKQSRSPLPSHARSRPGLCHMYK), and Ab-MBP14, which is immunoreactive to all four isoforms of mouse MBP. Our SDS-PAGE/immunoblotting demonstrated that Ab-MBP21.5, unlike Ab-MBP14, recognized only the 21.5 and 17 kDa MBP isoforms from isolated mouse CNS myelin. Immunolabelling of tissue sections indicated that Ab-MBP14 bound tenfold more to junction-free compact myelin than to radial component, whereas Ab-MBP21.5 bound about equally to the two regions of the myelin sheath. In addition, within the junction-free compact myelin, both antibodies bound nearly three fold more to the major dense line than to the intraperiod line.

Myelin basic protein purified on an ion-exchange continuous polymer bed in the presence of ethylene glycol and salt possesses activity against p-nitrophenyl acetate

In this paper we describe a fast and mild method based on the use of a unique cation exchanger and buffers containing ethylene glycol and salt for the purification of the myelin basic protein (MBP; MW 18.5 kDa). MBP thus purified hydrolyses catalytically p-nitrophenyl acetate. This esterase activity facilitates not only the purification of MBP but also indicates that probably it is in its native state, i.e. there is a good chance that the purified molecules are structurally and chemically identical. This is a prerequisite to obtain crystals appropriate for x-ray diffraction and other studies.

Limited postnatal ethanol exposure permanently alters the expression of mRNAS encoding myelin basic protein and myelin-associated glycoprotein in cerebellum

Experiments were designed to test the hypothesis that ethanol exposure during development can selectively affect the expression of specific isoforms of myelin protein gene expression in the rat cerebellum. We focused on myelin basic protein (MBP) and myelin-associated glycoprotein (MAG) gene expression. Both of these genes are alternatively spliced to yield 4 (MBP) or 2 (MAG) mRNA isoforms. Prenatal ethanol exposure, delivered to the dams in a liquid diet, did not significantly alter the expression of MBP or MAG gene expression in the cerebellums of 15-day-old pups, as measured by quantitative in situ hybridization using specific oligodeoxynucleotide probes. In contrast, postnatal ethanol exposure delivered directly to the pups over a 6-day period by gastrostomy tube (PN days 4-10) reduced the expression of specific MBP and MAG isoforms in the cerebellum of animals in adulthood. These data demonstrate that ethanol exposure, especially during the period of rapid myelination, has selective effects on mRNA isoforms encoding specific MBPs and MAG.

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.

Is myelin basic protein crystallizable?

Myelin basic protein (MBP) is the predominant extrinsic protein in both central and peripheral nervous system myelins. It is thought to be involved in the stabilizing interactions between myelin membranes, and it may play an important role in demyelinating diseases such as multiple sclerosis. In spite of the fact that this abundant protein has been known for almost three decades, its three-dimensional crystal structure has not yet been determined. In this study we report on our extensive attempts to crystallize the major 18.5 kDa isoform of MBP. We used MBP having different degrees of purity, ranging from crude MBP (that was acid or salt extracted from isolated myelin), to highest purity single isoform. We used convention strategies in our search for a suitable composition of a crystallization medium. We applied both full and incomplete factorial searches for crystallization conditions. We analyzed the available data on proteins which have previously resisted crystallization, and applied this information to our own experiments. Nevertheless, despite our efforts which included 4600 different conditions, we were unable to induce crystallization of MBP. Previous work on MBP indicates that when it is removed from its native environment in the myelin membrane and put in crystallization media, the protein adopts a random coil conformation and persists as a population of structurally non-identical molecules. This thermodynamically preferred state presumably hinders crystallization, because the most fundamental factor of protein crystallization - homogeneity of tertiary structure--is lacking. We conclude that as long as its random coil flexibility is not suppressed, 18.5 kDa MBP and possibly also its isoforms will remain preeminent examples of proteins that cannot be crystallized.

Morphometric Analysis of Normal, Mutant, and Transgenic CNS: Correlation of Myelin Basic Protein Expression to Myelinogenesis

The neurological mutant mice shiverer (shi) and myelin deficient (shimld) lack a functional gene for the myelin basic proteins (MBP), have virtually no myelin in their CNS, shiver, seize, and die early. Mutant mice homozygous for an MBP transgene have MBP mRNA and MBP in net amounts approximately 25% of normal, have compact myelin, do not shiver or seize, and live normal life spans. We bred mice with various combinations of the normal, transgenic, shi, and shimld genes to produce mice that expressed MBP mRNA at levels of 0, 5, 12.5, 17.5, 50, 62.5, and 100% of normal. The CNS of these mice were analyzed for MBP content, tissue localization of MBP, degree of myelination, axon size, and myelin thickness. MBP protein content correlated with predicted MBP gene expression. Immunocytochemical staining localized MBP to white matter in normal and transgenic shi mice with an intensity of staining comparable to the degree of MBP gene expression. An increase in the percentage of myelinated axons and the thickness of myelin correlated with increased gene expression up to 50% of normal. The percentage of myelinated axons and myelin thickness remained constant at expression levels greater than 50%. The presence of axons loosely wrapped with oligodendrocytic membrane in mice expressing lower amounts of MBP mRNA and protein suggested that the oligodendroglia produced sufficient MBP to elicit axon wrapping but not enough to form compact myelin. Mean axon circumference of myelinated axons was greater than axon circumference of unmyelinated axons at each level of gene expression, further evidence that oligodendroglial cells preferentially myelinate axons of larger caliber.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Conservation throughout vertebrate evolution of the predicted beta-strands in myelin basic protein

To identify functionally important parts of the 18.5-kDa myelin basic protein (MBP), the amino acid sequences from 10 species ranging from shark to human were aligned using the SEQHP computer program. The residues that are invariant or very conservatively substituted (Arg/Lys, Ser/Thr, Ile/Leu, Asp/Glu) among all 10 proteins were scored. Of the 72 conserved residues in the 170-residue human protein (42% conserved), 32 are found within the five beta-strands previously predicted (45 residues, 71% conserved), 23 within the small-loops region (42 residues, 55% conserved), but only 17 within the large-loops region (83 residues, 20% conserved). Of the 22 hydrophobic residues within the predicted beta-sheet of human MBP, 20 hydrophobic residues remain in the shark protein, 19 of them in the same positions. In contrast, there are 10 hydrophobic residues elsewhere in the human protein, but only 7 remain in the shark protein and only 1 of them is in the same position. The triprolyl sequence found in all mammalian MBPs and in the chicken MBP is not conserved in the shark protein. The four alternately spliced forms of mouse MBP can be accommodated by the beta-structural model, but not the 17-kDa human MBP, which lacks exon 5. These findings confirm the crucial role of the hydrophobic residues in the predicted beta-sheet for the structure and function of the protein. It seems likely that the conserved portions of the protein make an important contribution to the highly ordered lamellar structure of myelin.

Conformational correlates of the epitopes of human myelin basic protein peptide 80-89

Different epitopes residing within the decapeptide of residues 80-89 of human myelin basic protein (MBP) exist in the MBP-like material detected in human CSF and urine. In the present study, the structure of human MBP peptide 80-89 was examined by a combination of physical measurements and correlated with its varying immunochemical reaction with three polyclonal antisera. At least two epitopes are present in the decapeptide. Progressive shortening and reduction in net negative charge of MBP peptide 80-89 to form peptides 81-89, 82-89, 83-89, and 84-89 revealed an epitope not present in intact MBP. Circular dichroism and Fourier-transform infrared of these MBP peptides in water demonstrated random structure that was partially changed to beta-structure in the shorter peptides. In methanol, used as a model for a lipid environment, the random structure was diminished and was replaced by alpha-helix and beta-structure, especially in the shorter peptides. The findings indicate that the range of epitopes present in this decapeptide is influenced by conformation, which, unexpectedly, becomes progressively less random as the peptide becomes smaller, especially in a hydrophobic environment. This behavior has implications for the immunochemical detection of small antigens or antibodies to them in tissue extracts or body fluids.

Myelin basic protein: interaction with calmodulin and gangliosides

The structural characteristics of myelin basic protein (MBP) involved in protein-protein and protein-lipid interactions were investigated. Rabbit MBP could bind calmodulin (CaM) in the presence of Ca2+ to form a complex that remained undissociated in 8 M urea. However, no tight complex formation was observed when the divalent cation was absent. These results suggest that MBP may contain a hydrophobic domain similar to those in the other well-characterized CaM-binding proteins. The stoichiometry of calmodulin binding to MBP was approximately 1:1. Prior limited proteolysis of MBP with trypsin abolished the formation of the MBP-CaM complex, indicating that the entire MBP polypeptide may be involved in the recognition of the hydrophobic clefts in CaM. MBP also formed tight complexes with gangliosides, but the presence of Ca2+ was not required. Binding of gangliosides to MBP-CaM complex released CaM from the complex. The ganglioside-binding sites in MBP were determined after trisecting the protein at two glutamic acid residues with Staphylococcus aureus V8 protease. Subsequent binding studies revealed that a 9.5-kDa polypeptide, which may correspond to the NH2-terminal domain (residues 1-83) of MBP, had higher affinity for the binding of lucifer yellow CH-labeled GM1 than did the other two polypeptides, of apparent molecular mass (Mr) 5,500 and 4,500, respectively. Among the various proteins in purified guinea pig brain myelin, synaptosomes, and synaptosomal membranes, MBP was found to have the highest affinity in binding lucifer yellow CH-GM1.

The myelin proteins of the shark brain are similar to the myelin proteins of the mammalian peripheral nervous system

The two major structural proteins in the shark CNS are similar to the structural proteins, Po and myelin basic protein (MBP), found in the mammalian peripheral nervous system (PNS). Shark Po is 46% similar to its mammalian counterpart. The extracellular domain of shark Po also appears to be organized as an immunoglobulin-like domain that mediates homotypic interactions. The intracellular domain of shark Po also is very basic and may play a role in myelin condensation analogous to that of MBP. Shark MBP is 44% similar to mammalian MBP. Both MBPs show conserved interspersed regions and are present in multiple forms that arise by alternative splicing of a single transcript. These structural analyses indicate that the complexities seen in mammalian myelin arose early during vertebrate evolution.

Covalent linkage of phosphoinositides to myelin basic protein: in vitro incorporation of [32P] phosphoinositides to myelin basic protein

We have previously reported that the covalent attachment of phosphoinositides to myelin basic protein (MBP) occurs both in vivo and in vitro [Smith, R. A. et al. (1986) Biochemistry 25:2677-2681; Biochemistry 25:2682-2686; and Biochem. Biophys. Res. Comm. 316:426-432]. Phosphoinositidation of MBP was also detected when [32P] phosphoinositides were incubated with myelin pretreated with Triton X-100 and EGTA. Less than 10% of this covalent linkage of phosphoinositides to MBP survived after acidic treatment (0.1 N HCl at 37 degrees C for 10 min). MBP is predicted to lack sufficient hydrophobicity to bind to membranes as shown by analysis of its amino acid sequence for hydrophobic regions and thus its phosphoinositidation may provide an anchor for this purpose.

Evidence for the expression of four myelin basic protein variants in the developing human spinal cord through cDNA cloning

Four human myelin basic protein (MBP) variants with molecular masses of 21.5, 20.2, 18.5, and 17.3 kilodaltons (kDa) have been identified in the developing human spinal cord and their structures determined through an analysis of cDNA clones of their mRNAs. The 20.2-kDa MBP mRNA encoded a novel MBP variant, the structure of which has not been reported in any species. Its amino acid sequence was identical with that of the 21.5-kDa MBP except for a deletion of 11 amino acid residues encoded by exon 5 of the MBP gene. All four human MBP variants were identical except for the insertion of deletion of two peptide fragments corresponding to those encoded by exons 2 and 5 of the MBP gene. In this study, no mature human MBP cDNAs missing exon 6 sequences were identified. This suggests that, unlike the mouse, the four human MBP mRNAs encoding these MBP variants arise by the alternative splicing of only exons 2 and 5 from the primary MBP gene transcript. This indicates that the predominant MBP splicing pathways in human and mouse are different. Immunoblots of human fetal spinal cords (11-21 weeks) indicated that MBP expression turned on abruptly between 14 and 16 weeks. Expression of the 20.2-kDa MBP variant was most evident at 16 weeks and its relative proportion declined thereafter, suggesting that its expression was developmentally regulated.

Evidence for specific polypeptide chain folding in myelin basic protein from reactions between fragments of the protein and monoclonal antibodies

The specificities of two monoclonal IgM antibodies (18.25 and 21.14.2) evoked in mice with guinea pig myelin basic protein were examined and interpreted in terms of a specific folding of the protein's polypeptide chain. Studies with guinea pig and rabbit myelin basic protein fragments showed that a region encompassing the central Phe-Phe (87-88) sequence is obligatory, but not sufficient, for reactivity with antibody 18.25. Appreciable reactivity was observed for rabbit peptides 22-95 and 45-151, and lower, but significant, reactivity was shown by peptide 32-95. Only very weak reactivity was seen with peptide 44-95. No reactivity was observed with peptide 1-95 after its lysine residues were acetylated, acetamidinated, or guanidinated. These results have been interpreted in terms of a polypeptide chain folding that creates an epitope within sequence Val-Val-His-Phe-Phe-Lys-Asn-Ile-Val (84-92). The specific conformation of this epitope, which includes probably the Lys-89 and possibly the Asn-90 and Val-92 side chains, could be formed by the association of sequence 84-92 with either sequence Ile-Leu-Asp-Ser-Ile-Gly-Arg-Phe-Phe (37-45) or with sequence Val-Leu-Ser-Arg-Phe (108-112) to form beta-sheet structures essentially identical with those that appear to be present in the intact BP [Martenson R.E.J. Neurochem. 46, 1612-1622 (1986)]. The second monoclonal antibody, no. 21.14.2, reacts only with guinea pig myelin basic protein and fragments containing the species-restricted sequence Arg-Ala-Asp-Tyr-Lys-Ser-Lys (129-135).(ABSTRACT TRUNCATED AT 250 WORDS)

Colvalent linkage of phospholipid to myelin basic protein: identification of phosphatidylinositol bisphosphate as the attached phospholipid

Evidence presented demonstrates a covalent attachment of a phospholipid to bovine myelin basic protein. Partial characterization of the phospholipid moiety was performed on myelin basic protein obtained from 32P-phosphorylated whole myelin that was first delipidated by two ether/ethanol (3:2 v/v) extractions, ether extraction, and acetone extraction and then purified by preparative sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The myelin basic protein was precipitated with aqueous acetone and treated with proteases. Treatment with carboxypeptidase Y or trypsin for several hours released a lipophilic fragment, which was purified by reverse-phase high-performance liquid chromatography to yield two "lipopeptides". Such lipopeptides were obtained from both the major and minor myelin basic proteins of rat and bovine brain. Treatment with either mild base or phospholipase C removes the lipophilic character of the peptide fragment. The lipophilic fragment is a substrate for phospholipase D, but it does not comigrate on thin-layer chromatography with any 32P-labeled lipid obtained from myelin incubated with [gamma-32P]ATP. Polyphosphoinositides were shown to be released by mild acid treatment of myelin basic protein that had been extracted with organic solvent and then purified by SDS-polyacrylamide gel electrophoresis. Along with the fact that inositol monophosphate was identified in the partial acid hydrolysate of the lipopeptide, we have concluded that polyphosphoinositide (phosphatidylinositol 4-phosphate and/or phosphatidylinositol 4,5-bisphosphate) was the original phospholipid portion of the lipopeptide.

Possible hydrophobic region in myelin basic protein consisting of an orthogonally packed beta-sheet

Theoretical analysis was carried out to determine how the approximately 20% of beta-structure observed in the 18.5 kilodalton (kDa) myelin basic protein (MBP) could be organized into a relatively stable beta-sheet. The beta-sheet is presumed to consist of the five most hydrophobic segments of polypeptide chain, which have beta-structure potential. These correspond approximately to sequences 15-21, 37-45, 84-92, 106-112, and 148-154 (rabbit MBP sequence numbering) and constitute beta-strands a, b, c, d, and e, respectively. A number of constraints are imposed upon the sheet; e.g., it should have the same topology in all MBP forms (21.5, 18.5, 17, and 14 kDa); strand e should lie at the sheet edge; strands b, c, and d should be ordered sequentially; the sheet formed by strands a, b, c, and d should be antiparallel; a maximum of the nonpolar surface area should be removed from the aqueous milieu; and charged side chains should be solvent-accessible. On the basis of these constraints it is possible to propose six orthogonally packed beta-sheets having different topologies. If strand e is restricted to an antiparallel alignment, the number of different sheets is reduced to four. Each of these sheets can form a relatively compact hydrophobic globular region. Two of the strands (a and e) can undergo transitions to alpha-helix without disrupting the structure of the remaining sheet bcd or producing major topologic rearrangements of the polypeptide chain.

Monoclonal and polyclonal antibodies to myelin basic protein determinants

A detailed immunochemical examination of monoclonal and polyclonal antibody responses to myelin basic protein (MBP) and its peptides has revealed the existence of as many as 27 antigenic determinants, many of them conformational. Topological mapping of the potential antigenic determinants onto a model of MBP secondary structure places these determinants within 11 separate regions of the molecule, including those portions that have been found to be encephalitogenic. MBP and its peptides, therefore, fall under the umbrella of the Multideterminant-Regulatory Model of Benjamin et al. (1984). However, in the case of MBP, multideterminant immunogenicity appears to represent mainly an escape from tight regulation through the avenue of conformational change.

Complement potentiates the degradation of myelin proteins by plasmin: implications for a mechanism of inflammatory demyelination

A previous finding, that the basic protein in lyophilized bovine myelin was degraded by macrophage-conditioned media in the presence of plasminogen, suggested that the macrophage-secreted plasminogen activator, along with plasminogen, might have a role in destruction of myelin during inflammatory demyelination. To approximate more closely the conditions expected in vivo, plasmin, or macrophage supernatants plus plasminogen, were incubated with freshly homogenized bovine white matter or freshly isolated myelin, as distinguished from lyophilized myelin. Under these conditions basic protein was not degraded. Phospholipase or lysolecithin potentiated the degradation of basic protein in fresh bovine myelin by plasmin; however, the cultured macrophages did not secrete significant amounts of phospholipase and plasminogen activator simultaneously into the culture media after activation with any of several different agents. Recently myelin was shown to activate complement. After preincubation of fresh myelin with guinea pig serum, as a source of complement, the basic and proteolipid proteins were vulnerable to plasmin or to macrophage-conditioned media plus plasminogen. C3-depleted and C4-deficient sera were not effective, suggesting that these complement components were required for the serum effect. Hypothetically, then, degradation of myelin proteins in the CNS could be initiated by plasminogen activator, secreted by infiltrating macrophages, plus complement and plasminogen, which could enter the CNS through lesions in the blood-brain barrier.

Alternative splicing accounts for the four forms of myelin basic protein

We have isolated cDNA clones encoding the four different forms of mouse myelin basic protein (MBP) and have analyzed the structure of the MBP gene. The three larger forms of MBP differ from the smallest by the inclusion of either or both of two short amino acid sequences at positions 57 and 124 of the smallest protein. The mouse genome contains a single MBP gene comprised of seven exons. The two amino acid sequences present only in the larger MBPs are encoded by separate exons. Furthermore, all exons in the coding region begin or end in complete codons so that alternative splicing does not alter the reading frame. We conclude that the four forms of this myelin protein are encoded in separate mRNAs, each derived by a simple alternative splicing of the primary MBP gene transcript. Comparison of the amino acid sequence encoded by each exon with a recent model of the secondary structure of MBP suggests that each of the seven exons encodes one or two of the predicted structural motifs of the protein.

Membrane proteins in reverse micelles: myelin basic protein in a membrane-mimetic environment

The solubility, reactivity, and conformational dynamics of myelin basic protein (MBP) from bovine brain were studied in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-isooctane and water. Such a membrane-mimetic system resembles the aqueous spaces of native myelin sheath in terms of physicochemical properties as reflected in the high affinity of MBP for interfacial bound water. This is marked by the unusual profile of the solubility curve of the protein in reverse micelles, which shows optimal solubility at a much lower molar ratio of water to surfactant ([ H2O]/[AOT] = w0) than that reported for other water-soluble proteins. The role of counterions and/or charged polar head groups in the solubilization process is revealed by comparison of the solubility of MBP in nonionic surfactant micellar solutions. Whereas MBP is unfolded in aqueous solutions, insertion into reverse micelles generates a more folded structure, characterized by the presence of 20% alpha-helix. This conformation is unaffected by variations in the water content of the system (in the 2.0-22.4 w0 range). The reactivity of epsilon-amino groups of lysine residues with aqueous solutions of o-phthalaldehyde demonstrates that segments of the peptide chain are accessible to water. Similar results were obtained with the sequence involved in heme binding. In contrast, the sole tryptophan residue, Trp-117, is shielded from the aqueous solvent, as indicated by lack of reaction with N-bromosuccinimide. The invariance of the wavelength maximum emission in the fluorescence spectra as a function of w0 is consistent with this result.(ABSTRACT TRUNCATED AT 250 WORDS)

Microcalorimetric studies of the heats of solution of bovine myelin basic protein

Heats of solution for myelin basic protein have been determined using microcalorimetry. All aqueous systems studied yielded negative heats of solution; in contrast, trifluoroethanol produced a small positive heat of solution, while reaction with dimethyl sulfoxide was strikingly exothermic. The heat of interaction for native myelin basic protein with 8 M urea at pH 4.0, 29 degrees C, was found to be -79 +/- 16 kcal/mol. The significance of these results in terms of the protein's structural organization is discussed.

Spectroscopic assessment of secondary and tertiary structure in myelin basic protein

Myelin basic protein conformation and hydrophobicity, along with the protein's behavior in the presence of the fluorescent probe 6-(p-toluidino)-2-naphthalenesulfonate, have been studied by using Fourier transform infrared (FT-IR) and Raman spectroscopy. The FT-IR and Raman spectra provided compelling evidence for the presence of a small amount of beta structure, ca. 25%, in the aqueous solution and solid-state forms of myelin basic protein. The enhanced fluorescence and shift in the emission maximum of 6-(p-toluidino)-2-naphthalenesulfonate when bound to myelin basic protein are consistent with the presence of at least one hydrophobic region in the molecule. Loss of the fluorescence enhancement in the presence of denaturing agents indicates that native myelin basic protein has a folded structure in solution. All of the results provide support for conformational predictions derived from the application of Edmundson wheels to the primary structure.

Hydrophobic regions in myelin proteins characterized through analysis of "hydropathic" profiles

Computer-generated "hydropathic" profiles were constructed for graphic comparison of the amino acid sequences for P2 protein, 18.5 kilodalton (kDa) myelin basic protein (BP), and myelin proteolipid protein (PLP). Profiles were also obtained for cytochrome b5, a membrane protein known to be capable of reversible association with lipid bilayers and of a size comparable to that of the myelin BPs. Analysis of the PLP sequence produced profiles generally compatible with the suggestions that PLP has three transbilayer and two bilayer intercalating segments. Profiles for P2 and 18.5 kDa BP were found to contain hydrophilic segments separated by relatively short hydrophobic regions. Whereas hydropathic indices in hydrophobic regions of P2, 18.5 kDa BP, and PLP fall in the value ranges recently reported for cores of globular proteins and intrabilayer domains of membrane proteins, hydrophobic sections of P2 and 18.5 kDa BP have hydropathic indices similar to those in the hydrophobic core (transprotein) regions of globular proteins. None of them are comparable to the region of cytochrome b5 known to anchor that protein in its membrane or to the segments of PLP sequence proposed as intrabilayer domains. This comparison suggests that neither BP has structural characteristics compatible with insertion into the hydrocarbon core of the myelin lipid bilayer, a conclusion that is consistent with a recently published study that identified the bilayer penetrating proteins of myelin with a hydrophobic probe. The above findings suggest an enhancement for some details of myelin architecture and a cautious approach to interpreting data for BP intercalation into bilayers.

NMR investigation of the charge isomers of bovine myelin basic protein

Myelin basic protein (MBP) isolated from bovine white matter is obtained as a mixture of molecules which can be separated by cation-exchange chromatography at basic pH into three or more charge isomers. The three principal charge isomers of the microheterogeneous myelin basic protein have been isolated, and compared individually by high-resolution H NMR spectroscopy (360 and 400 MHz). In addition to confirming sources of MBP charge microheterogeneity such as fractional deamidation of Gln and loss of C-terminal Arg, NMR difference and spin-echo spectra further suggested (i) the presence of significant oxidation of (both) MBP Met residues to methionine sulfoxide; (ii) the three charge isomers contain equal ratios and absolute contents of mono- and dimethylated Arg; and (iii) the most-cationic isomer is deficient in its content of a putative extra Ala residue vs the other two isomers. Spectral analysis suggested that each MBP charge isomer is itself not a unique molecule, but more likely a mixture of molecules of equal net charge which are modified at any of the indicated functional side chains throughout the 169-residue protein. The results are discussed with respect to the possible consequences of MBP microheterogeneity to protein conformation and function.

Immunoperoxidase methods and advances in skin biology

This review presents an abbreviated discussion of a technic the application of which is having profound effects on approaches to pathologic diagnosis, classification of tumors, analysis of cell infiltrates in the skin, and a variety of research problems. Its use will continue to increase as more physicians, pathologists, and researchers become acquainted with its potential.

Predicted folding of beta-structure in myelin basic protein

Predictions of myelin basic protein secondary structure have not previously considered a major role for beta-structure in the organization of the native molecule because optical rotatory dispersion and circular dichroism studies have provided little, if any, evidence for beta-structure, and because a polycationic protein is generally considered to resist folding into a compact structure. However, the Chou-Fasman, Lim, and Robson algorithms identify a total of five beta-strands in the amino acid sequence. Four of these hydrophobic amino acid sequences (37-45, 87-95, 110-118, and 150-158) could form a hairpin intermediate that initiates folding of a Greek-key-type beta-structure. A second fold on the more hydrophobic side, with the addition of a strand from the N-terminus (residues 13-21), would complete the five-stranded antiparallel beta-sheet. A unique strand alignment can be predicted by phasing the hydrophobic residues. The unusual triproline sequence of myelin basic protein (100-102) is enclosed in the 14-residue hairpin loop. If these prolines are in the trans conformation, models show that a reverse turn could occur at residues 102-105 (Pro-Ser-Gln-Gly). Algorithms do not agree on the prediction of alpha-helices, but each of the two large loops could accommodate an alpha-helix. Myelin basic protein is known to be phosphorylated in vivo on as many as five Ser/Thr residues. Phosphorylation might alter the dynamics of folding if the nascent polypeptide were phosphorylated in the cytoplasm. In particular, phosphorylation of Thr-99 could neutralize cationic residues Lys-106 and Arg-108 within the hairpin loop. In addition, the methylation of Arg-108 might stabilize the hairpin loop structure through hydrophobic interaction with the side chain of Pro-97. The cationic side chains of arginine and lysine residues located on the faces of the beta-sheet (Arg-43, Arg-114, Lys-13, Lys-92, Lys-153, and Lys-156) could provide sites for interaction with phospholipids and other anionic structures on the surface of the myelin lipid bilayer.

A heme binding site on myelin basic protein: characterization, location, and significance

Myelin basic protein (MBP), an extrinsic membrane protein from the myelin sheath, binds dicyanohemin. The binding generates absorption bands in the Soret region and quenches the fluorescence emitted by the sole tryptophan residue. The absorption titration curves in the Soret demonstrate that the binding is stoichiometric, one heme per protein, with a large value of the extinction coefficient (8 X 10(4) M-1 cm-1 at 420 nm). Fluorescence quenching titration curves indicate an identical stoichiometry and a low quenching efficiency of 20%. From the heme titration curve the association constant between dicyanohemin and MBP is estimated to be greater than or equal to 10 nM-1 in 50 mM 4-morpholinepropanesulfonic acid buffer, pH 7.0, at 20 degrees C. Digestion of MBP by Staphylococcus aureus V8 protease yields a peptide (38-118) whose heme binding properties are identical to those of MBP. In contrast, peptides obtained by digestion of MBP with cathepsin D do not exhibit any specific binding of dicyanohemin. The cleavage of the Phe-Phe (42-43) bond appears to be critical in this respect. A comparison of the sequence immediately preceding, including these residues with a probable heme binding site of a mitochondrial cytochrome b, reveals a high degree of homology. The possible significance of heme binding is discussed.

The structure of bovine brain myelin proteolipid and its organization in myelin

A model, based on amino acid sequence data, is proposed for the organization of the myelin proteolipid in myelin membrane. The model has three distinctive features: three trans-membrane segments that traverse the lipid bilayer, two cis-membrane domains that enter and exit the same side of the membrane, and a highly charged segment resembling myelin basic protein on the cytoplasmic side of the membrane. It is proposed that the cis-membrane domain(s) can promote the formation and stabilization of the multilamellar myelin structure by hydrophobic interaction with the apposite bilayer across the extracellular space.

Cleavage of rabbit myelin basic protein by thrombin

Rabbit myelin basic protein (BP) contains several Arg-X bonds with differing susceptibilities to thrombic cleavage as measured by the yields of the various cleavage products obtained under three different conditions. Under conditions where the thrombin-to-substrate ratio was very low (1 NIH unit/mg BP), the concentration of substrate was relatively low (4 mg BP/ml), and the incubation time was short (2 h), the rabbit BP was cleaved essentially completely and specifically at a single site, the Arg(95)-Thr(96) bond. The BPs of other species (beef, pig, guinea pig, rat) were similarly cleaved, no doubt because all have the same amino acid sequence in this region of the protein. Under conditions in which the enzyme-to-substrate ratio and the substrate concentration were higher (2 NIH units/mg BP, 8 mg BP/ml) and the incubation time was long (24 h), additional, partial cleavages occurred, principally at the Arg(43)-Phe(44) and Arg(128)-Ala(129) bonds, but with some cleavage at the Arg(31)-His(32) and Arg(63)-Thr(64) bonds as well. Under conditions in which all three variables were elevated (5 NIH units/mg peptide, 20 mg peptide/ml, 24 h), more extensive cleavage occurred at the above sites. In peptide (96-168), which we examined in detail, nearly complete cleavage of the Arg(128)-Ala(129) bond occurred, with partial cleavage at the unmethylated Arg(105)-Gly(106), Arg(111)-Phe(112), Arg(150)-Leu(151), and Arg(160)-Ser(161) bonds. The susceptibilities to cleavage of the Arg-X bonds in the BP can be explained with varying degrees of success in terms of the known specificity of thrombin. Cleavage of two of the bonds, Arg(128)-Ala(129) and Arg(160)-Ser(161), suggests the occurrence of a chain reversal or beta-turn in the sequence preceding the scissile bonds. Most cleavages of the BP with thrombin do not occur in the more hydrophobic regions; in particular, the hydrophobic region in the center of the molecule that includes the Phe-Phe(87-88) sequence is left intact.

Characterization of mouse myelin basic protein messenger RNAs with a myelin basic protein cDNA clone

Using a family of synthetic tetradecamer oligonucleotides as a primer for cDNA synthesis and a second family of tetradecamers as a hybridization probe, we have prepared and isolated a cDNA clone of mouse myelin basic protein (MBP). The clone, pNZ111, corresponds to the region of the mRNA that codes for an amino acid sequence present in all four major forms of MBP. The relative abundance of MBP mRNA, estimated by dot blot hybridization, increased with the age of the mouse to a maximum at 18 days, then decreased to about one-fourth of that amount at later ages. Mouse MBP mRNAs, selected by their ability to hybridize to the clone, translate into the four forms of myelin basic protein. In RNA blot analyses, pNZ111 hybridized to multiple species of mouse mRNA. The predominant hybridization is to a broad band of RNAs ranging in length from 2,350 to 2,100 bases. These mRNA species are extremely long, considering that the largest MBP could be encoded by approximately 600 bases. In addition to these, there are also minor bands that hybridize with pNZ111, including a band of 4,100 bases and smaller ones of 1,900, 1,500, and 1,200 bases.

A general model of the P2 protein of peripheral nervous system myelin based on secondary structure predictions, tertiary folding principles, and experimental observations

The amino acid sequence of the P2 protein of peripheral myelin was analyzed with regard to regions of probable alpha-helix, beta-structure, beta-turn, and unordered conformation by means of several algorithms commonly used to predict secondary structure in proteins. Because of the high beta-sheet content and virtual absence of alpha-helix shown by the circular dichroic spectra of the protein, a bias was introduced into the algorithms to favor the beta-structure over the alpha-helical conformation. In order to define those beta-sheet residues that could lie on the external hydrophilic surface of the protein and those that could lie in its hydrophobic interior, the predicted beta-strands were examined for charged and uncharged amino acids located at alternating positions in the sequence. The sequential beta-strands in the predicted secondary structure were then ordered into beta-sheets and aligned according to generally accepted tertiary folding principles and certain chemical properties peculiar to the P2 protein. The general model of the P2 protein that emerged was a "Greek key" beta-barrel, consisting of eight antiparallel beta-strands with a two-stranded ribbon of antiparallel beta-structure emerging from one end. The model has an uncharged, hydrophobic core and a highly hydrophilic surface. The two Cys residues, which form a disulfide, occur in a loop connecting two adjacent antiparallel strands. Two hydrophilic loops, each containing a cluster of acidic residues and a single Phe, protrude from one end of the molecule. The general model is consistent with many of the properties of the actual protein, including the relatively weak nature of its association with myelin lipids and the positions of amino acid substitutions. Alternative beta-strand orderings yield three specific models having different interstrand connections across the barrel ends.

Immunocytochemical localization of basic protein in major dense line regions of central and peripheral myelin

To localize basic protein (BP) in the lamellar structure of central and peripheral myelin, we perfused newborn and 7-11-day rat pups with a phosphate-buffered fixative that contained 4% paraformaldehyde and 0.05 or 0.2% glutaraldehyde. Teased, longitudinally split or "brush" preparations of optic and trigeminal nerves were made by gently teasing apart groups of myelinated fibers with fine forceps or needles. Some of these preparations were immunostained without pretreatment in phosphate-buffered antiserum to BP according to the peroxidase-antiperoxidase method. Others were pretreated in ethanol before immunostaining. Then, all of them were dehydrated, embedded in Epon, and sectioned for electron microscopic study. In optic and trigeminal nerves that were not pretreated, myelin, glial cells, and their organelles were well preserved. BP immunostaining was present on cytoplasmic faces of oligodendroglial and Schwann cell membranes that formed mesaxons and loose myelin spirals. In compact central and peripheral myelin, reaction product was located in major dense line regions, and the myelin periodicity was the same as that observed in unstained control myelin that had been treated with preimmune serum. In ethanol-pretreated tissue, the myelin periodicity was reduced but dense line staining still was present. Our immunocytochemical demonstration of dense line localization of BP in both CNS and PNS myelin that was not disrupted or pretreated with solvents is important because of conflicting evidence in earlier immunostaining studies. Our results also support biochemical and histochemical evidence suggesting that BP exists in vivo as a membrane protein interacting with lipids on the cytoplasmic side of the bilayer in the spirally wrapped compact myelin membrane.

Isolation and identification of large overlapping fragments of rabbit myelin basic protein produced by limited peptic hydrolysis

Treatment of rabbit myelin basic protein component 1 with pepsin (enzyme:substrate, 1:500 w/w) in 0.5 M-ammonium formate (pH 6.00) for 15-20 min at room temperature resulted in limited cleavage of the protein. The resulting fragments were isolated by ion-exchange chromatography and gel filtration and identified by amino acid and COOH-terminal analyses and by tryptic peptide mapping. All of the possible products resulting from incomplete cleavages at the highly susceptible Phe44-Phe45, Phe87-Phe88, Leu109-Ser110, and Leu151-Phe152 bonds were isolated: peptides (1-151), (1-109), (1-87), (45-168), (45-151), (45-109), (88-168), (88-151), and (110-168). Of these, peptides (1-151), (1-87), and (88-151) were recovered in the greatest yield (0.14-0.19 mol per mol of starting protein). Relatively low yields (0.04 mol/mol starting protein) were obtained for peptides (1-109) and (110-168), indicating that the Leu109-Ser110 bond is somewhat more resistant to peptic cleavage than are the Phe-Phe and Leu-Phe bonds. Smaller fragments of the basic protein were also recovered: peptides (1-44), (1-28), (45-87), (88-109), (110-151), and (152-168). Many of the individual peptides could be readily identified in electrophoretograms of the total peptic digest. The relative electrophoretic mobilities of the above-mentioned peptides, together with the previously isolated peptides (1-14) and (15-44), were determined in 15% (w/w) polyacrylamide slab gels containing 1 M-acetic acid and 8 M-urea.