Protein associations and basic protein conformation in the myelin membrane. The use of difluorodinitrobenzene as a cross-linking reagent

Single-molecule tracking of myelin basic protein during oligodendrocyte differentiation

This study aimed to expand our understanding of myelin basic protein (MBP), a key component of central nervous system myelin, by developing a protocol to track and quantifying individual MBP particles during oligodendrocyte (OL) differentiation. MBP particle directionality, confinement, and diffusion were tracked by rapid TIRF and HILO imaging of Dendra2 tagged MBP in three stages of mouse oligodendroglia: OL precursors, early myelinating OLs, and mature myelinating OLs. The directionality and confinement of MBP particles increased at each stage consistent with progressive transport toward, and recruitment into, emerging myelin structures. Unexpectedly, diffusion data presented a more complex pattern with subpopulations of the most diffusive particles disappearing at the transition between the precursor and early myelinating stage, before reemerging in the membrane sheets of mature OLs. This diversity of particle behaviors, which would be undetectable by conventional ensemble-averaged methods, are consistent with a multifunctional view of MBP involving roles in myelin expansion and compaction.

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.

Measurement of binding strength between prey proteins interacting with Toxoplasma gondii SAG1 and SAG2 using isothermal titration calorimetry (ITC)

Following the outcome from a previously performed yeast two-hybrid experiment, the binding strength between T. gondii SAG1 and SAG2 and their respective prey proteins were further confirmed in this study. The sag1, sag2 and their prey genes were amplified and cloned into a pGEMT vector. To express the recombinant proteins, the fragments were then subcloned into a pRSETA vector and transformed into E. coli BL21 (DE3) cells. The recombinant proteins were expressed optimally at 37°C and 1mM of IPTG. The 6X His-tag fusion proteins were purified, dialyzed and concentrated. To confirm the expressed proteins, the recombinant proteins were analysed by SDS-PAGE and Western blot. As expected, the size of SAG1, SAG2, HLY and HZF protein were 32, 23, 28 and 37 kDa, respectively. The purified proteins were loaded onto a MicroCal Auto-iTC200 calorimeter from MicroCal™ to quantify binding strength. ITC results indicated there was a typical binding curve for interactions between SAG1 and HLY protein. However, there was an atypical binding curve obtained for interactions between SAG2 and HZF protein. By observing the data obtained from the ITC assay, both of the human proteins (HLY and HZF) were demonstrated to bind to their respective SAG1 and SAG2 proteins.

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.

The Escherichia coli metal-binding chaperone SlyD interacts with the large subunit of [NiFe]-hydrogenase 3

The multi-step biosynthesis of the [NiFe]-hydrogenase enzyme involves a variety of accessory proteins. To further understand this process, a Strep-tag II variant of the large subunit of Escherichia coli hydrogenase 3, HycE, was constructed to enable isolation of protein complexes. A complex with SlyD, a chaperone protein implicated in hydrogenase production through association with the nickel-binding accessory protein HypB, was observed. A SlyD-HycE interaction preceding both iron and nickel insertion to the enzyme was detected, mediated by the chaperone domain of SlyD, and independent of HypB. These results support a model of several roles for SlyD during hydrogenase maturation.

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.

Identification of PINCH in Schwann cells and DRG neurons: shuttling and signaling after nerve injury

Particularly interesting new cysteine-histidine rich protein (PINCH) is a double zinc finger domain (LIM)-only adapter protein that functions to recruit the integrin-linked kinase (ILK) to sites of integrin activation. Genetic studies have shown that PINCH and ILK are required for integrin signaling. Since integrin activation is associated with Schwann cell migration, neurite outgrowth and regeneration, this study examined PINCH in the normal peripheral nervous system and after chronic constriction injury (CCI) in adult Sprague-Dawley rats. Immunohistochemistry identified PINCH immunoreactivity in cell bodies of dorsal root ganglia (DRG) neurons, axons, satellite cells, and Schwann cells. PINCH immunostaining was localized to the membrane of uninjured DRG cell bodies consistent with its localization at a site of integrin activation. In contrast, 5 days following CCI, PINCH immunostaining was diffuse throughout the DRG cell cytoplasm. Confocal microscopy of primary and transformed Schwann cells localized PINCH in cytoplasmic, perinuclear and nuclear areas. Examination of the PINCH sequence revealed a putative leucine-rich nuclear export signal (NES) and an overlapping basic nuclear localization signal (NLS). To demonstrate nuclear export of PINCH, rabbit anti-PINCH IgG was microinjected into Schwann cell nuclei and allowed to combine with PINCH contained within the nucleus. Immunofluorescence showed that the PINCH and anti-PINCH IgG complex rapidly translocated to the cytoplasm. Treatment with leptomycin B caused nuclear accumulation of PINCH, indicating that the CRM1 pathway mediates nuclear export of PINCH. ILK activity in Schwann cells was enhanced by platelet-derived growth factor (PDGF) and tumor necrosis factor alpha. PINCH immunoprecipitates from PDGF- and TNFalpha-stimulated Schwann cells contained several high-molecular-weight threonine-phosphorylated proteins. Taken together, these results indicate that PINCH is an abundant shuttling/signaling protein in Schwann cells and DRG neurons.

Myelin proteolipid protein, basic protein, the small isoform of myelin-associated glycoprotein, and p42MAPK are associated in the Triton X-100 extract of central nervous system myelin

To further our understanding of the functions of the major myelin proteins, myelin basic protein (MBP) and proteolipid protein (PLP), and other myelin proteins, such as 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and myelin-associated glycoprotein (MAG), bovine brain myelin was extracted with Triton X-100, and protein complexes in the detergent-soluble fraction were isolated by coimmunoprecipitation and sucrose density gradient sedimentation. MBP, PLP, and the small isoform of MAG (S-MAG) were coimmunoprecipitated from the detergent-soluble fraction by anti-PLP, anti-MBP or anti-MAG monoclonal antibodies. Additionally, a 30 kDa phosphoserine-containing protein and two phosphotyrosine-containing proteins (M(r) 30 and 42 kDa) were found in the coimmunoprecipitates. The 42 kDa protein is probably p42MAPK, in that MAPK was shown also to be present in the immunoprecipitated complex. CNP, the small PLP isoform DM20, the large MAG isoform L-MAG, MOG, CD44, MEK, p44MAPK, and actin were not present in the immunoprecipitates, although they were present in the detergent-soluble fraction. Lipid analysis revealed that the PLP-MBP-S-MAG coimmunoprecipitated with some phospholipids and sulfatide but not cholesterol or galactosylceramide. However, the complex had a high density, indicating that the lipid/protein ratio is low, and it was retained on a Sepharose CL6B column, indicating that it is not a large membrane fragment. Given that MAG is localized mainly in the periaxonal region of myelin, where it interacts with axonal ligands, the PLP-MBP-S-MAG complex may come from these regions, where it could participate in dynamic functions in the myelin sheath and myelin-axonal interactions.

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.

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.

Organization of the transcortin-binding domain on placental plasma membranes

Complex formation between transcortin (corticosteroid-binding globulin) and 20 kDa sialoglycoprotein from human syncytiotrophoblast plasma membranes (presumably a transcortin-recognizing subunit of the transcortin membrane receptor) was studied using FPLC and cross-linking with bifunctional reagents. The action of 1,5-difluoro-2,4-dinitrobenzene (DFDNB) on a solution of the purified 20 kDa sialoglycoprotein and transcortin resulted in formation of covalently linked complexes of 95 kDa and 140 kDa consisting of one transcortin molecule and either two or four molecules of the membrane sialoglycoprotein (the molecular mass of transcortin is 55 kDa). Additionally, cross-linking resulted in the appearance of a 43 kDa species which is the cross-linked dimer of the membrane protein. The dimer was also observed during chromatography on a Superose 12 column in the absence of DFDNB treatment. Treatment of intact syncytiotrophoblast membranes with DFDNB resulted in isolation of the transcortin binding protein dimer as the major portion of total pool of the protein. Formation of the transcortin complexes with two and four molecules of the membrane protein was also observed when the membranes were incubated with 125I-labeled transcortin and treated with DFDNB, but formation of the latter complexes predominated. The results obtained suggest that the recognizing and binding domain for transcortin in placental membranes is organized as dimers consisting of non-covalently linked sialoglycoprotein monomers of a 20 kDa each and that transcortin has two sites for interaction with this dimer. Apparently, binding of two dimers results in the formation of the functional form of the transcortin-receptor complex. The possible biological role of such a complex is discussed.

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 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.

Reactivity of two anti-galactosyl ceramide antibodies towards myelin basic protein

Two anti-galactosyl ceramide antibodies (polyclonal Ab142 and a monoclonal antibody) were characterized in terms of their reactivity towards purified lipids and myelin basic protein. Polyclonal Ab142 is a rabbit anti-mouse galactosyl ceramide (Gal C) IgG. Antigenic recognition is dependent on both galactose and ceramide since neither could inhibit galactosyl ceramide binding by more than 10%. MAb-Gal C is a monoclonal antibody raised in mice against galactosyl ceramide. Binding of MAb-Gal C to Gal-C was equally inhibited by ceramide and galactose to approximately 50%, indicating that both groups are important for antibody recognition. MAb-Gal C was also shown to be reactive with the structurally related lipids, sphingomyelin and sulfatide. Polyclonal Ab142, although raised against Gal C, was shown to be 3-fold more reactive with component 8 (C-8) of myelin basic protein than Gal C. On the other hand, the MAb-Gal C which also reacted with C-8 was 2-fold less reactive with C-8 than with Gal C. Neither of these antibodies were reactive with component 1 (C-1) of myelin basic protein. An anti-MBP IgG was shown to be reactive with C-1 and C-8 but unreactive with Gal C. In competitive inhibition ELISA, C-8 was able to compete out 44% and 41% of Gal C binding to polyclonal Ab142 and MAb, respectively. The reverse competition demonstrated that Gal C could inhibit 75% of C-8 binding to both antibodies. D-galactose was unable to inhibit C-8 binding to either antibody, whereas ceramide was as efficient as Gal C.(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.

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.

Interaction of myelin basic protein and proteolipid protein

The interaction of myelin basic protein (MBP) and proteolipid protein (PLP) was studied using a microtitre well binding assay and the ligand-blot overlay technique. The binding of iodinated PLP to MBP that was immobilized on microtitre wells was saturable and reversible. Its selectivity was investigated by the ligand-blot overlay technique. Iodinated PLP was found to bind MBP but not any other CNS myelin proteins. This interaction was not dependent on the phosphoryl moiety of MBP. Binding of PLP to histone H4 also occurred, but the amount of PLP bound per unit MBP was greater than for this histone.

Triton X-100 extractions of central nervous system myelin indicate a possible role for the minor myelin proteins in the stability in lamellae

Isolated CNS myelin membranes were extracted with Triton X-100 under conditions previously established for the isolation of cytoskeletal proteins. Treated myelin retained much of its characteristic lamellar structure despite the removal of most of the major myelin basic protein (18.5 kDa) and the proteolipid protein, which together normally constitute 60% of the total myelin protein. The SDS-PAGE profile of this extract residue demonstrated an enrichment in proteins of Mr 30 to 60 kilodaltons (the Wolfgram group). The major myelin proteins were identified by antibodies on Western immunoblots, as were the 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP), actin, tubulin, myelin-associated glycoprotein (MGP) and the 21.5 kDA MBP. The overall behavior of CNP, the 21.5 kDa MBP, MGP and tubulin towards Triton extraction is reminiscent of the behavior of other membrane-skeletal complexes, supporting the idea that these and other minor myelin proteins might be part of heteromolecular complexes with interactions spanning several lamellae of the myelin sheath.

Use of protein-bearing nitrocellulose as immunogen for in vitro production of monoclonal antibodies: application to myelin basic protein electrophoretically separated from a complex brain protein mixture

A procedure for producing monoclonal antibody to myelin basic protein (MBP) using in vitro immunization with MBP transferred to nitrocellulose is described. Following the separation of brain proteins by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and electrophoretic transfer of the electrophoretogram onto nitrocellulose, the MBP band located by immunodetection was excised from the nitrocellulose, ground, and used as immunogen for in vitro stimulation of unprimed mouse spleen cells. While in vitro immunization with soluble MBP was able to generate many hybrids, all the wells in the fusions carried out with the immobilized MBP contained hybrids, 33 to 42% of which were positive to MBP. Among these, six were further characterized; all were IgM and all bound to epitopes common to the 18.5K and 21.5K MBP forms of several species. In view of its simplicity, this technique should have a wide application for the rapid production of monoclonal antibodies to selected proteins or their fragments present in small quantity or difficult to purify on a large scale.

Organization of the Sindbis virus nucleocapsid as revealed by bifunctional cross-linking agents

Purified Sindbis virus nucleocapsids were reacted with a variety of bifunctional protein-specific cross-linking agents. The products were analyzed in concentration-gradient polyacrylamide gels and amounts of various products determined. These studies indicated that available lysine residues within adjacent capsid proteins in purified intact nucleocapsids are separated by 6 A. The capsid proteins in intact nucleocapsids are cross-linked in a pattern predicted for discrete monomeric entities, rather than in dimeric or trimeric aggregates. Purified, soluble capsid protein exists in a conformation that differs from the arrangement of protein within nucleocapsids. These conformational differences suggest that topological changes may occur in the capsid protein during virus maturation. Cross-linked nucleocapsids that were treated with RNases resulted in the generation of RNA-free protein shells that retained hexagonal morphology, indicating that, together, the RNA and protein form the outer surface of the nucleocapsid. These data are used to produce a model of the Sindbis virus nucleocapsid in which the proteins are arranged quasi-equivalently in a T = 4 icosahedral shell.

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.

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.

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.

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.

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.

An X-ray diffraction analysis of oriented lipid multilayers containing basic proteins

X-ray diffraction techniques have been used to study the structures of lipid bilayers containing basic proteins. Highly ordered multilayer specimens have been formed by using the Langmuir-Blodgett method in which a solid support is passed through a lipid monolayer held at constant surface pressure at an air/water interface. If the lipid monolayer contains acidic lipids then basic proteins in the aqueous subphase are transferred with the monolayer and incorporated into the multi-membrane stack. X-ray diffraction patterns have been recorded from multilayers of cerebroside sulphate and 40% (molar) cholesterol both with and without polylysine, cytochrome c and the basic protein from central nervous system myelin. Electron density profiles across the membranes have been derived at between 6 A and 12 A resolution. All of the membrane profiles have been placed on an absolute scale of electron density by the isomorphous exchange of cholesterol with a brominated cholesterol analog. The distributions and conformations of the various basic proteins incorporated within the cerebroside sulphate/cholesterol bilayer are very different. Polylysine attaches to the surface of the lipid bilayer as a fully extended chain while cytochrome c maintains its native structure and attaches to the bilayer surface with its short axis approximately perpendicular to the membrane plane. The myelin basic protein associates intimately with the lipid headgroups in the form of an extended molecule, yet its dimension perpendicular to the plane of the membrane of approx. 15 A is consistent with the considerable degree of secondary structure found in solution. In the membrane plane, the myelin basic protein extends to cover an area of about 2500 A2. There is no significant penetration of the protein into the hydrocarbon region of the bilayer or, indeed, beyond the position of the sulphate group of the cerebroside sulphate molecule.

Amino-acid sequence of human and bovine brain myelin proteolipid protein (lipophilin) is completely conserved

Proteolipid protein (PLP) was isolated from white matter of human brain by chloroform/methanol extraction and further purified by chromatography. Performic acid oxidation yielded a product homogeneous in NaDodSO4-polyacrylamide electrophoresis with a molecular mass of 30 kDa. The carboxymethylated PLP was chemically cleaved with cyanogen bromide into four fragments: CNBr I 22-24 kDa, CNBr II 5 kDa, CNBr III 1.4 kDa and CNBr IV 0.7 kDa. HBr/dimethylsulfoxide cleavage at tryptophan residues released four fragments: Trp I 14-16 kDa, Trp II 2.0 kDa, Trp III 5 kDa and Trp IV 7 kDa. Hydrophilic fragments were enriched in 50% formic acid (CNBr II, III, IV and Trp II and III), whereas hydrophobic peptides precipitated from this solvent were CNBr I, Trp I and IV. The fragments were separated by gel filtration with 90% formic acid as solvent and finally purified by gel permeation HPLC (Si 60 and Si 100) for automated liquid and solid-phase Edman degradation. Large fragments were further cleaved with different proteinases (trypsin, V8-proteinase, endoproteinase Lys-C and thermolysin). We used an improved strategy in the sequencing of the human proteolipid protein compared with our approach to the structural elucidation of bovine brain PLP. The amino-acid sequence of human PLP contains 276 residues, the same as found in bovine proteolipid protein. The two sequences proved to be identical. The possible importance of the conservative structure of this integral membrane protein is discussed.

On the binding of brain myelin basic protein to chromatographic resins

Myelin basic protein, only in association with certain detergents, is able to bind irreversibly to the usual gel filtration media. While this binding is greatly advantageous in purification of proteolipid (the other major myelin protein), the question arises of the uncommon, nonphysiological behaviour of the basic protein. A relationship between binding property and basic protein structure is suggested.

Detection of various forms of brain myelin basic protein in vertebrates by electroimmunoblotting

An electroimmunoblot technique was used to detect various forms of myelin basic protein (MBP) in brain homogenates of 14 vertebrate species. Three antibodies were used to probe the immunoblots: a monoclonal anti-human MPB reacting with an antigenic determinant located at amino acid residues 131 to 136; a polyclonal anti-human MBP and a polyclonal anti-chicken MBP. Because no processing of the tissue is required prior to electrophoresis, in vitro artifacts are minimized. The 18.5 K form of MBP was present in all species except the shark. A 21.5 K MBP was observed in ovine, bovine, pig, rabbit, mouse, rat, monkey, but not in human, guinea pig, shark, toad and marsupial brains. A variant with a molecular weight between 17 K and 18 K was found in mouse, rat, bovine, human, monkey, pig, and chicken brains, and was the sole component in the shark brain. Marsupial brains had five or six forms of MBP between 14.5 K and 18.5 K.

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.

Lipid/myelin basic protein multilayers. A model for the cytoplasmic space in central nervous system myelin

A multilayered complex forms when a solution of myelin basic protein is added to single-bilayer vesicles formed by sonicating myelin lipids. Vesicles and multilayers have been studied by electron microscopy, biochemical analysis, and X-ray diffraction. Freeze-fracture electron microscopy shows well-separated vesicles before myelin basic protein is added, but afterward there are aggregated, possibly multilayered, vesicles and extensive planar multilayers. The vesicles aggregate and fuse within seconds after the protein is added, and the multilayers form within minutes. No intra-bilayer particles are seen, with or without the protein. Some myelin basic protein, but no lipid, remains in the supernatant after the protein is added and the complex sedimented for X-ray diffraction. A rather variable proportion of the protein is bound. X-ray diffraction patterns show that the vesicles are stable in the absence of myelin basic protein, even under high g-forces. After the protein is added, however, lipid/myelin basic protein multilayers predominate over single-bilayer vesicles. The protein is in every space between lipid bilayers. Thus the vesicles are torn open by strong interaction with myelin basic protein. The inter-bilayer spaces in the multilayers are comparable to the cytoplasmic spaces in central nervous system myelins . The diffraction indicates the same lipid bilayer thickness in vesicles and multilayers, to within 1 A. By comparing electron-density profiles of vesicles and multilayers, most of the myelin basic protein is located in the inter-bilayer space while up to one-third may be inserted between lipid headgroups. When cytochrome c is added in place of myelin basic protein, multilayers also form. In this case the protein is located entirely outside the unchanged bilayer. Comparison of the various profiles emphasizes the close and extensive apposition of myelin basic protein to the lipid bilayer. Numerous bonds may form between myelin basic protein and lipids. Cholesterol may enhance binding by opening gaps between diacyl-lipid headgroups.

Structure of the proteolipid protein extracted from bovine central nervous system myelin with nondenaturing detergents

As a basis for attempts to define the structures of the proteins within myelin, methods have been developed for their extraction and isolation in solutions of non-denaturing detergents. With use of solutions of deoxycholate or Triton X-100, up to 90% of the protein has been extracted from bovine CNS myelin, along with most of the phospholipid. The proteolipid protein has been purified in deoxycholate solutions by chromatography on a blue dye-ligand column, which retained all of the basic protein and 2',3'-cyclic nucleotide-3'-phosphodiesterase, and then on Sephacryl S300, which separated proteolipid protein from phospholipid and high-molecular-weight proteins. The proteolipid protein was isolated from Triton X-100 extracts of myelin by adsorption onto phosphocellulose resin, with subsequent elution by 0.5 M sodium chloride. Gel permeation chromatography was used as the final purification step. Sedimentation equilibrium experiments gave a monomer molecular weight of 134,000 +/- 8000 in deoxycholate and 145,000 +/- 17,000 in Triton X-100 solutions. On the basis of an apparent subunit molecular weight of 23,500 it was deduced that the native protein is probably hexameric. Above 0.2 gL-1 in Triton X-100 solutions and 0.5 gL-1 in deoxycholate solutions the protein aggregated. In deoxycholate solutions the protein adopts the highly helical conformation expected for an intrinsic membrane protein.

Investigation of the organisation of the major proteins in bovine myelin membranes. Use of chemical probes and bifunctional crosslinking reagents

Bovine myelin was incubated with a variety of bifunctional reagents and chemical probes. The use of a photosensitive hydrophobic compound, 1-azido-[125I]iodobenzene, led to the suggestion that the proteolipid protein is deeply intercalated into the hydrophobic milieu of the membrane, but did not support the contention that regions of the basic protein behave in a similar fashion. Crosslinking studies indicated that both polypeptides may be present in the membrane as homodimers and these dimers may be part of much larger assemblies. These results give rise to a somewhat different model for the structural organisation of myelin to that proposed earlier.

Myelin basic protein-enhanced fusion of membranes

Myelin basic protein caused rapid aggregation of vesicles containing acidic phospholipids. Aggregation could be reversed by trypsin digestion of the myelin basic protein. Aggregated vesicles containing gel phase phospholipids or vesicles containing greater than 15 mol% lysolecithin underwent fusion. The extent of fusion was measured by irreversible changes in the light-scattering intensities or diffusion coefficients of the vesicles. Fusion was also measured by the fluorescence quenching which occurred when vesicles containing a covalently bound fluorophore. N-4-nitrobenzo-2-oxa-1,3-diazole, were fused with vesicles containing the covalently bound spin label, 4,4-dimethyl-oxazolidine-N-oxyl. The kinetics of fusion were first order in phospholipid and had half-times of 0.5-5 min depending on lysolecithin composition. This protein-enhanced membrane fusion may provide a valuable model system for studying some types of biological membrane fusions.

Self-association of myelin basic protein: enhancement by detergents and lipids

Self-association of basic protein has been proposed to be of functional significance in central nervous system myelin. In aqueous solution this protein self-associates, previous data being consistent with the formation of dimers, which then undergo an indefinite isodesmic self-association [Smith, R. (1980) Biochemistry 19, 1826-1831]. As this protein is membrane bound in vivo, we have now examined the effects of amphiphiles on the self-association equilibria. Contrary to the expected effects, at low molar ratios dodecyl sulfate, deoxycholate, Triton X-100, and lysophosphatidylcholine increased protein intermolecular attraction. The anionic detergents led to partial precipitation even at 1:1 protein:detergent molar ratios whereas the zwitterionic lipid and the nonionic detergent exerted less pronounced effects. Sedimentation velocity and equilibrium measurements have been used to define quantitatively the effects of lysophosphatidylcholine. The sedimentation coefficient increases up to a lipid:protein ratio of 4:1 and then remains constant up to a ratio of 12:1. The sedimentation equilibrium data suggest that the mode of protein-protein interaction is the same as in the absence of lipid but with substantially increased association constants. The dimerization constant is increased from 1.20 X 10(2) M-1 to 1.0 X 10(3) M-1 and the isodesmic association constant from 3.4 X 10(4) M-1 to 1.2 X 10(5) M-1. The effects of detergents on myelin basic protein are compared with the effects on other proteins, and the implications for the state of the protein with myelin are discussed.

Tentative identification of a second central nervous system myelin membrane autoantigen (M2) by a biochemical comparison with the basic protein (BP)

Two central nervous system myelin autoantigens, M2 and basic protein (BP), were examined, using complement-fixing antibodies against each autoantigen as markers on myelin. M2 activity was very labile and very insoluble, PB activity was very resistant. Trypsin reduced both activities an this reduction was greater after phospholipase treatment. Both activities were slightly solubilized in 8 M urea. It is known that BP is not present on the surface of myelin and is considered a peripheral membrane protein. M2 appears to be a surface and integral membrane protein, and as such resembles Folch Pi proteolipid protein. The relationship between M2 and BP requires further study.

Preparation and properties of an immunosorbent column specific for the myelin basic protein

An immunosorbent column specific for the myelin basic protein (BP) was prepared by coupling purified anti-BP antibodies to cyanogen bromide (BrCN)-activated Sepharose 4B. The BP-immunosorbent column bound BP between pH 4.5 and pH 6.8. In its working range the column bound approximately 400-475 microgram of BP at pH 6.8 and 250 microgram at pH 4.5 with recoveries of 72-77%. The BP-immunosorbent column could effectively separate BP from simple mixtures of BP and proteins of similar size and charge and from acid extracts of bovine brain. The results indicate that the BP-immunosorbent column can be used to isolate BP from a mixture of proteins and may be adapted for use in the small-scale purification of the myelin basic proteins involving a minimum number of steps.