Purification of myelin basic protein from bovine brain

Myelin basic protein (MBP) is a commonly used substrate for in vitro determination of numerous protein kinase activities. Herein we describe a rapid method for isolating relatively large amounts of MBP from bovine brain with a purity greater than that currently available from commercial sources. Lipids were first extracted from the CNS tissue by homogenization in sec-butanol. Washes under neutral and mildly basic conditions were employed to remove neutral and acidic proteins from the defatted residue. MBP was subsequently extracted under acidic conditions and further purified by chromatography on CM Sephadex C-25. Potential contaminating enzyme activities were destroyed by heart treatment. This method typically yields a recovery of 1.0-1.5 mg MBP per gram of starting material with a purity of greater than 95%. The MBP prepared in this manner was suitable for determination of kinase activities by both solution and the "in gel" kinase assay systems.     

Extensive degradation of myelin basic protein isoforms by calpain following traumatic brain injury

Axonal injury is one of the key features of traumatic brain injury (TBI), yet little is known about the integrity of the myelin sheath. We report that the 21.5 and 18.5-kDa myelin basic protein (MBP) isoforms degrade into N-terminal fragments (of 10 and 8 kDa) in the ipsilateral hippocampus and cortex between 2 h and 3 days after controlled cortical impact (in a rat model of TBI), but exhibit no degradation contralaterally. Using N-terminal microsequencing and mass spectrometry, we identified a novel in vivo MBP cleavage site between Phe114 and Lys115. A MBP C-terminal fragment-specific antibody was then raised and shown to specifically detect MBP fragments in affected brain regions following TBI. In vitro naive brain lysate and purified MBP digestion showed that MBP is sensitive to calpain, producing the characteristic MBP fragments observed in TBI. We hypothesize that TBI-mediated axonal injury causes secondary structural damage to the adjacent myelin membrane, instigating MBP degradation. This could initiate myelin sheath instability and demyelination, which might further promote axonal vulnerability.     

Degradation of bovine P2 protein by bovine brain cathepsin D

Bovine brain cathepsin D cleaved bovine P2 protein to produce three major and several minor peptides. The major P2 peptides formed were shown by amino acid analysis and partial sequencing to be peptides 17–54, 20–58 and 65–131 with the latter predominating. In preliminary experiments, P2 peptide 65–131 did not induce experimental allergic neuritis in Lewis rats in equimolar amounts to the neuritogenic P2.Special issue dedicated to Dr. Elizabeth Roboz-Einstein.     

Functional degradation of myelin basic protein. The proteomic approach

Proteolytic degradation of autoantigens is of prime importance in current biochemistry and immunology. The fundamental issue is the functional role of peptides produced in the process of change of the hydrolysis specificity during the transition from the normal to a pathologic state. In some cases identification of specific peptide fragments can be a diagnostic and prognostic criterion of the pathology progress. The subject of this work is the comparative study of degradation peculiarities of one of the major neuroantigens, myelin basic protein, by proteases activated upon the development of a pathological demyelinating process, and by proteasomes of different origin. Comparison of the specificity of the tested biocatalysts in some cases demonstrated critical changes in the set of myelin basic protein fragments capable of being presented on the major histocompatibility complex class I upon neurodegeneration, which may promote the development of autoimmune pathological processes.     

Enzymatic and nonenzymatic degradation of myelin basic protein

A procedure for large scale isolation of myelin basic protein (BP) has been modified to insure BP preparations free of neutral proteinase activity. Fractions were monitored by electrophoretic analysis of BP solutions incubated under various conditions of temperature and pH. Maximum degradation of human BP prepared by the old batch procedure occurs at pH 7, 47°C. BP preparations obtained by the new procedure, as well as BP preparations purified by CM-cellulose chromatography, are stable under these conditions. The latter, however, do undergo significant breakdown at pH 9, 100°C. The results suggest that the degradation observed under these conditions is non-enzymatic in nature.Special Issue dedicated to Dr. Elizabeth Roboz-Einstein.     

Susceptibility of myelin proteins to a neutral endoproteinase: The degradation of myelin basic protein (MBP) and P2 protein by purified bovine brain multicatalytic proteinase complex (MPC)

Multicatalytic proteinase complex (MPC) was isolated from bovine brain and the susceptibility of myelin basic protein (MBP) and P₂ protein of bovine central and peripheral nervous system was examined. SDS-polyacrylamide electrophoretic analysis of purified MPC revealed protein bands of molecular weight ranging from 22–35 kDa. The enzyme is activated by SDS at a concentration less than 0.01%. Upon incubation with MPC, purified MBP and P₂ proteins were degraded into smaller fragments. There was a 57% and 100% loss of MBP at 2 and 6 hours of incubation. The P₂ protein which is not susceptible to any endogenous non-lysosomal enzyme thus far studied was digested into small peptide fragments only in the presence of SDS (0.01%) and not in its absence. These results indicate that MPC which is active at physiological conditions may have a role in the turnover of myelin proteins and in demyelinating diseases.     

The effects of bovine myelin basic protein on the phase transition properties of sphingomyelin

The basic protein of myelin can spontaneously associate with the synthetic phospholipid N-palmitoyl-sphingosinephosphatidylcholine. The protein alters the phase transition properties of the lipid from a single transition at 41.5 degrees C to two overlapping transitions, one being slightly above and the other slightly below the transition temperature of the pure lipid. The effect was not seen upon the addition of poly(L-lysine) to this lipid nor does the myelin basic protein alter the phase transition properties of dimyristoylphosphatidylcholine. The results thus demonstrate that the myelin basic protein can interact with a major zwitterionic lipid component of myelin in addition to acidic phospholipids.     

The effect of bovine myelin basic protein on the phase transition properties of sphingomyelin

The basic protein of myelin can spontaneously associate with the synthetic phospholipid N-palmitoylsphingosinephosphatidylcholine. The protein alters the phase transition properties of the lipid from a single transition at 41.5°C to two overlapping transitions, one being slightly above and the other slightly below the transition temperature of the pure lipid. The effect was not seen upon the addition of poly(l-lysine) to this lipid nor does the myelin basic protein alter the phase transition properties of dimyristoylphosphatidylcholine. The results thus demonstrate that the myelin basic protein can interact with a major zwitterionic lipid component of myelin in addition to acidic phospholipids.     

Functional degradation of myelin basic protein. Proteomic approach

Proteolytic degradation of autoantigens is of prime importance in current biochemistry and immunology. The most fundamental issue in this field is the functional role of peptides produced when the specificity of hydrolysis changes during the shift from health to disease and from normal state to pathology. The identification of specific peptide fragments in many cases proposes the diagnostic and prognostic criterion in the pathology progression. The aim of this work is comparative study of the degradation peculiarities of one of the main neuroantigen, myelin basic protein by proteases, activated during progress of pathological demyelinating process, and by proteasome of different origin. The comparison of specificity of different studied biocatalysts gives reason to discuss the critical change in the set of myelin basic protein fragments capable to be presented by major histocompatibility complex class I during neurodegeneration, which can promote the progress of autoimmune pathological process.     

Phosphorylation sites of bovine brain myelin basic protein phosphorylated with Ca2+-calmodulin-dependent protein kinase from rat brain

The phosphorylation sites of myelin basic protein from bovine brain were determined after phosphorylation with Ca2+-calmodulin-dependent protein kinase. Four phosphorylated peptides were selectively and rapidly separated by reversed-phase high-performance liquid chromatography. Partial sequencing of the phosphorylated peptides by automated Edman degradation revealed that Ca2+-calmodulin-dependent protein kinase phosphorylated serine-16, serine-70, and threonine-95 specifically, as well as serine-115, which is located on the experimental allergic encephalitogenic determinant of the protein. Of the four amino acid sequences determined, two sequences surrounding phosphorylated amino acids, -Lys-Tyr-Leu-Ala-Ser(P)16-Ala- and -Arg-Phe-Ser(P)115-Trp-Gly-, have both sides of each phosphoserine residue occupied by hydrophobic amino acids, and a basic amino acid, arginine or lysine, is located at the position 2 or 4 residues amino-terminal to the phosphoserine residue. In contrast, the two other sequences surrounding phosphorylated amino acids, -Tyr-Gly-Ser(P)70-Leu-Pro-Glu-Lys- and -Ile-Val-Thr(P)95-Pro-Arg-, have a basic amino acid at the position 2 or 4 residues carboxyl-terminal to the phosphoamino acid residue.     

Microheterogeneity of bovine myelin basic protein studied by nuclear magnetic resonance spectroscopy

Myelin basic protein isolated from bovine white matter is known to consist of a mixture of three or more “charge isomers”, which can be separated by cation-exchange chromatography. We are using 360-MHz ¹H-nmr spectroscopy to establish the chemical and structural differences among them. Preliminary studies by difference spectroscopy between two of the isomers suggest (a) all aromatic residues, and probably their nearest-neighbors, are unchanged; (b) the less cationic isomer lacks one (or two) of its C-terminal Arg residues; and (c) a significant fraction of the two Met residues in the less cationic isomer is present as methionine sulfoxide.     

Purification and kinetic mechanism of S-adenosylmethionine: myelin basic protein methyltransferase from bovine brain.

The enzyme S-adenosylmethionine (AdoMet): myelin basic protein (MBP) methyltransferase was purified 250-fold from bovine brain with an overall yield of 130%, relative to crude supernatant. The purification involves acid-base and (NH4)2SO4 precipitation, chromatography over Sephadex G-100 and DEAE-cellulose, followed by preparative isoelectric focusing. The enzyme has a pI of 5.60 +/- 0.05, and the Mr is estimated to be between 71,000 (from SDS/polyacrylamide-gel electrophoresis) and 74,500 (from gel filtration). The enzyme is stable at 37 degrees C for over 2 h, is stable frozen and does not require metal ions or reductants. The enzyme shows a high specificity for MBP and does not accept polyarginine as a substrate; F1 histone is methylated at 37% of the rate of MBP. Methylation occurs on an arginine residue in a single h.p.l.c.-resolvable peptide from the tryptic cleavage of MBP. Simple saturation kinetics are observed with respect to both substrates, with Km values of 18 microM and 32 microM for MBP and AdoMet respectively. The simplest kinetic mechanism that is consistent with the data requires ordered rapid-equilibrium binding, with AdoMet as the first substrate. The enzyme isolated in this work is different, both physically and kinetically, from the histone-specific arginine methyltransferases described by other workers. A new, simple, assay system for the methylation of MBP is described.     

Deimination of myelin basic protein. 1. Effect of deimination of arginyl residues of myelin basic protein on its structure and susceptibility to digestion by cathepsin D

The effect of deimination of arginyl residues in bovine myelin basic protein (MBP) on its susceptibility to digestion by cathepsin D has been studied. Using bovine component 1 (C-1) of MBP, the most unmodified of the components with all 18 arginyl residues intact, we have generated a number of citrullinated forms by treatment of the protein with purified peptidylarginine deiminase (PAD) in vitro. We obtained species containing 0-9.9 mol of citrulline/mol of MBP. These various species were digested with cathepsin D, a metalloproteinase which cleaves proteins at Phe-Phe linkages. The rate of digestion compared to component 1 was only slightly affected when 2.7 or 3.8 mol of citrulline/mol of MBP was present. With 7.0 mol of citrulline/mol of MBP, a large increase in the rate of digestion occurred. No further increase was observed with 9.9 mol of citrulline/mol of MBP. The immunodominant peptide 43-88 (bovine sequence) was released slowly when 2.7 and 3.8 mol of citrulline/mol of MBP was present, but it was released rapidly when 7.0 mol of citrulline/mol of MBP was present. The dramatic change in digestion with 7.0 mol of citrulline/mol of MBP or more could be explained by a change in three-dimensional structure. Molecular dynamics simulation showed that increasing the number of citrullinyl residues above 7 mol/mol of MBP generated a more open structure, consistent with experimental observations in the literature. We conclude that PAD, which deiminates arginyl residues in proteins, decreases both the charge and compact structure of MBP. This structural change allows better access of the Phe-Phe linkages to cathepsin D. This scheme represents an effective way of generating the immunodominant peptide which sensitizes T-cells for the autoimmune response in demyelinating disease.     

Dependence on lipid structure of the coil-to-helix transition of bovine myelin basic protein

In aqueous solution bovine myelin basic protein has a close-to-random coil structure that is partially transformed to helix on interaction with lipids. Circular dichroism spectra have been used to follow this conformational transition which, with phospholipids, decreases in the order phosphatidylglycerol, phosphatidic acid approximately equal to phospholipids, decreases in the order phosphatidylethanolamine. There appears to be a strong correlation between the extent of alpha-helix formation and the degree of penetration of the hydrophobic region of the bilayer, as assessed by other methods. Cholesterol mixed in bilayers with phosphatidylserine has little effect on the protein secondary structure. Although basic protein binds strongly to cerebroside and to cerebroside sulphate, two of the other major myelin lipids, the intrinsic chirality of these lipids precludes assessment of their effect on the protein conformation. No significant changes in the circular dichroism spectra accompany the protein association with either of the zwitterionic bilayer-forming lipids, phosphatidylethanolamine and phosphatidylcholine. This seems to exclude extensive penetration into bilayers of these lipids and hence to exclude appreciable hydrophobic interactions; on the other hand, it is argued that little evidence exists for ionic attractions to these lipids. The optical activity of peptides derived from the basic protein by cleavage at the 42-43 and 88-89 peptides bonds (with cathepsin D) and at the 115-116 bond (with a skatole derivative) has also been measured in an attempt to locate the helix-forming regions within the primary structure.