Proton N.M.R. evidence for secondary and tertiary structure in myelin basic proteins

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.

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.

Characterization of a recombinant murine 18.5-kDa myelin basic protein

A recombinant hexahistidine-tagged 18.5-kDa isoform of murine myelin basic protein has been characterized biochemically and immunogenically, by mass spectrometry, by circular dichroism under various conditions (in aqueous solution, with monosialoganglioside G(M1), and in 89% 2-propanol), and by transmission electron microscopy. The preparations of this protein indicated a high degree of purity and homogeneity, with no significant posttranslational modifications. Circular dichroic spectra showed that this preparation had the same degree of secondary structure as the natural bovine 18.5-kDa isoform of myelin basic protein. Incubation of the recombinant protein with lipid monolayers containing a nickel-chelating lipid resulted in the formation of fibrous assemblies that formed paracrystals of spacings 4.8 nm between fibers and 3-4 nm along them.

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.

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.

Magnetic resonance of membranes

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A mitogen for Schwann cells is derived from myelin basic protein

Evidence is presented that a mitogen can be produced from myelin basic protein (MBP) which may be related to the Schwann cell proliferation characteristic of Wallerian degeneration. Myelin derived from the shiverer mutant which is devoid of MBP is also devoid of mitogenic activity. Absorption of the mitogen with a polyclonal antisera to MBP abolishes the mitogenic effect. In addition, only liposomes containing MBP are mitogenic to cultured Schwann cells; liposomes containing other myelin-specific proteins do not stimulate Schwann cell division. These results directly demonstrate the MBP origin of a mitogen for Schwann cells.

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

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

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.

Conformation of two antigenic regions in myelin basic protein

Four different regions of myelin basic protein from various species have been reported to be the antigenic sites (epitopes) for seven monoclonal antibodies evoked in rats or mice by guinea pig or monkey basic protein. The structures of the epitopes located in the amino-terminal region and in the eight-residue sequence including S-133, were examined by proton n. m. r at 400 MHz in aqueous solutions of peptides obtained by enzymatic cleavage of the rabbit protein. The data suggest conformational similarities between the two regions.

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.

Interaction of ganglioside GM1 and myelin basic protein studied by carbon-13 and proton nuclear magnetic resonance spectroscopy

The interaction of the myelin basic protein (MBP) and the major endogenous ganglioside GM1 in myelin of the central nervous system has been investigated using both 500-MHz 1H and 67.89 MHz 13C NMR. Titration of MBP by GM1 resulted in 13C NMR signal shifts for the I1e and His residues of MBP at a GM1/MBP mole ratio of one or less. The carbohydrate head group of GM1 was also found to be perturbed. 1H NMR results obtained in a similar manner demonstrated the perturbation of His and Phe residues. At a GM1/MBP mole ratio of 0.5, small perturbation of Trp #116 was observed, and at mole ratios of two and beyond significant involvement of Phe residues and methylated Arg #107 was found. Met #167 was more perturbed than Met #20; hence, more extensive interaction of the lipid is occurring with the C-terminus of the protein than with the N-terminus. No resonances from GM1 bound to MBP at mole ratios of up to one appeared in the spectra. However, as the GM1/MBP mole ratio was increased to eight or greater a major conformational change of MBP was detected. An upfield shift of the GM1 midchain methylene resonance was observed for the GM1/MBP complex. This observation provides strong evidence that the state of GM1 interacting with MBP is different from that of GM1 micelles. The number of saturable GM1 binding sites on MBP is estimated to be four. The data also favor a rapid exchange between bound GM1 and GM1 micelles. Interaction of MBP with the oligosaccharide derived from GM1 was found to be weaker than with GM1. Based on our data, a model for the interaction can be proposed: the first GM1 molecule is bound to the protein molecule through its head group and hydrocarbon chains, followed by the formation of a GM1/MBP complex with a concomitant conformational change of MBP as more GM1 is added.

NMR studies of myelin basic protein. X. Conformation of a determinant encephalitogenic in the rabbit

Residues 67 to 75 in myelin basic protein from several species comprise the sequence Thr-His-Tyr-Gly-Ser-Leu-Pro-Gln-Lys that acts as an encephalitogenic determinant in the rabbit. Proton magnetic resonance spectra of human, bovine and porcine proteins display nuclear Overhauser effects between the delta-CH of Tyr-69 and the delta-CH3 of Leu-72, which indicate reverse-turn conformations about the Gly-Ser residues. This effect occurs also in physiological saline solution at pH 6.0 but in dimethylsulfoxide solution the nuclear Overhauser effect disappears. Circular dichroism indicates that the protein when bound to ganglioside micelles acquires 30-40% alpha-helical conformation, but the reverse turn still persists in the sequence of the rabbit encephalitogen. These results suggest that the encephalitogenic region of the protein remains at the aqueous interface of the micelles.

NMR studies of myelin basic protein. IX. Complete assignments of the tyrosine residues by proton NMR of proteins from six species

All the proton resonances from the tyrosine residues are assigned in 400 MHz NMR spectra in aqueous solution of myelin basic proteins from human, cow, pig, rabbit, rat (small protein) and chicken. Assignments are based on species comparisons, spectra of enzymatic cleavage products of the basic protein, pH titrations, broadening effects of Gd(III), and nuclear Overhauser effects. The mobile extended polypeptide chain structure of the protein facilitates the detection of interactions between nearest neighbors. Evidence is found for reverse turns in the structure in regions of encephalitogenic determinants.

Interaction of the mouse and bovine myelin basic proteins and two cleavage fragments with anionic detergents

The binding of deoxycholate and dodecyl sulfate to the mouse and bovine myelin basic proteins and two peptide fragments, obtained by cleavage of the bovine basic protein at its single tryptophan residue, was examined. Complete equilibrium binding isotherms for both detergents were obtained by examining their binding to each of the polypeptides immobilized on agarose. The bulk of the binding of dodecyl sulfate was found to be highly cooperative, and at saturation all four polypeptides bound far more detergent than globular, water-soluble proteins. The sum of the dodecyl sulfate bound by each of the two bovine basic protein cleavage fragments was almost twice that bound by the intact protein at saturation, suggesting that cleavage of the bovine basic protein exposes sites for additional binding of dodecyl sulfate. At pH values below pH 8.0, an additional cooperative transition was observed below the critical micelle concentration of sodium dodecyl sulfate in the binding isotherms of all four polypeptides. The midpoint of this transition corresponded to an apparent pK of approximately 5.5; however, the destruction of 90% of the histidine residues in the bovine basic protein had no effect on this transition. At pH 9.2 and moderate ionic strength (I = 0.1), the bulk of the binding of deoxycholate to the mouse and bovine basic proteins occurred at and above the critical micelle concentration of the detergent; and saturation values of deoxycholate binding to these two proteins were considerably higher than that reported for globular, water-soluble proteins. In marked contrast to the results with dodecyl sulfate, neither cleavage fragment was observed to bind deoxycholate. The results suggest that the higher ordered structure of the bovine basic protein may play an important role in the binding of anionic amphiphiles to the protein.