Limited digestion of guinea pig myelin basic protein and its carboxy-terminal fragment (residues 89-169) with Staphylococcus aureus V8 protease

Limited proteolysis of myelin basic protein in a system mimetic of the myelin interlamellar aqueous space

We have investigated the early steps of myelin basic protein (MBP) degradation in a membrane mimetic system (reverse micelles), resembling the interlamellar aqueous spaces where the protein is located in the myelin sheath. MBP, unfolded in buffer, refolds on incorporation into the micelles, resulting in reduced accessibility to three proteolytic enzymes, trypsin, cathepsin D, and Staphylococcus aureus V8 protease, in comparison with aqueous solution. Eleven cleavage sites seen in buffer are removed from proteolytic attack in micellar solution. These sites delineate a protected protein domain displaying a potential beta-sheet structure capable of interacting with the myelin membrane. An additional site not seen in buffer is attacked in the micelles. Experiments with a structure inducer, 15% 1-propanol in buffer, reveal that the refolding pattern of MBP in reverse micelles is specific to the membrane biomimetic system and is not produced by organic solvent per se. Micellar digestions of MBP generate long peptides, two of which, isolated after tryptic digestion, have been found to be immunodominant in multiple sclerosis patients. The findings suggest the structure induced in MBP by the micelles resembles that leading to production of the self-peptides recognized by T cells during proteolytic breakdown of MBP in autoimmune demyelinating diseases.

Electroimmunoblotting of myelin basic protein peptides: a novel approach to the rapid characterisation of antigenic specificities of monoclonal and polyclonal anti-MBP antibodies

A rapid and sensitive method for the identification of antigenic determinants recognised by monoclonal and polyclonal antibodies directed against myelin basic protein (MBP) is described. By electroimmunoblotting a series of overlapping peptides covering the entire MBP molecule with monoclonal anti-MBP antibodies, the binding pattern of immunoreactive peptides can be rapidly determined and the reactive antigenic determinant identified. This procedure, which can be performed with both native and synthetic peptides, can also with appropriate modification, be applied to the analysis of naturally occurring or experimentally induced polyclonal anti-MBP autoantibodies.

Basic protein dissociating from myelin membranes at physiological ionic strength and pH is cleaved into three major fragments

Experiments were performed with isolated human myelin membrane preparations to analyse factors that may modulate association of myelin basic protein (MBP) with the membranes and could contribute to demyelinating processes. Transfer of membranes (5 mg protein ml-1) at 37 degrees C and pH 7.4 from a hypotonic medium, in which they were relatively stable, to one of physiological ionic strength produced three major effects: (1) initial dissociation of MBP from the membranes by a nonenzymatic process that was doubled in the presence of millimolar Ca2+/Mg2+; (2) within 10 min, the appearance in the medium of three major MBP fragments (14.4, 10.3, and 8.4 kilodaltons); and (3) progressive acidification of dissociated MBP and its fragments, probably due to deamidation. Between 1 and 6 h a steady state was apparent in which protein equivalent to 15% of the MBP originally bound to the membranes was found in the medium. The three major MBP fragments formed two-thirds of this solubilised material and appeared metabolically stable for 24 h. The kinetics of peptide formation suggested that dissociated, rather than membrane-bound, MBP was cleaved by myelin-associated neutral proteases. Two-dimensional electrophoresis and immunoblotting using two monoclonal antibodies indicated that proteolysis occurred in the vicinity of residues 35 and 75. Evidence was also obtained for removal of C-terminal arginines and relatively rapid deamidation in the C-terminal half of MBP. These modifications of MBP might also occur if extracellular fluid gained access to the compacted cytoplasmic space of the myelin sheath.

Specificity of prolyl endopeptidase

A series of tetrapeptides, Cbz(Bz)-Gly-X-Leu-Gly, were synthesized and the kinetic parameters, kcat and kcat/Km, determined for their hydrolyses by prolyl endopeptidase from Flavobacterium. The peptides with X = N-Me-Ala, Sar and Ala as well as the standard substrate (X = Pro) were found to be good substrates, while those with X = alpha-aminobutyryl, Hyp, Ser and Gly were poor substrates, and those with X = pipecolyl, alpha-aminoisobutyryl, N-Me-Val, N-Me-Leu, Hyp(O-Bzl) and Ser(O-Bzl) were not cleaved at all. These results suggest that the specificity-determining site or S1 subsite of the enzyme is designed to fit exactly the proline residue of the substrate with allowance for the residues carrying substituents at the N and/or C alpha which must not exceed the size of the pyrrolidine ring of proline.

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)

Cleavage of rabbit myelin basic protein by plasmin: isolation and identification of the major products

Rabbit myelin basic protein (BP) was subjected to partial cleavage with plasmin, and 15 cleavage products were isolated by a combination of gel filtration and ion-exchange chromatography. Their identification was achieved by amino acid analysis and tryptic peptide mapping, supplemented in some instances by carboxy-terminal analyses with carboxypeptidases A, B, and Y and amino-terminal analyses with dipeptidyl aminopeptidase I. The results showed that major plasmic cleavage sites included the Lys89-Asn90, Lys133-Ser134, and Lys153-Leu154 bonds. Cleavages also occurred at the Arg31-His32, Lys53-Arg54, and Arg25-His26 bonds, but these appeared to be less extensive. A large number of additional peptides were produced in relatively low yield. The smaller of these were isolated from heterogeneous fractions by high-voltage electrophoresis-TLC. Amino acid analysis of these peptides showed that minor cleavage sites included the Arg9-His10, Lys13-Tyr14, Lys103-Gly104, Lys137-Gly138, Lys140-Gly141, and Arg160-Ser161 bonds. In spite of a lower selectivity toward peptide bonds in BP as compared with pepsin, cathepsin D, and thrombin, plasmin has the advantage over the former proteinases in that it does not cleave at or near the Phe44-Phe45 bond. Instead it cleaves at the Arg31-His32 and Lys53-Arg54 bonds, thus preserving the entire hydrophobic sequence Ile-Leu-Asp-Ser-Ile-Gly-Arg-Phe-Phe as well as short sequences to either side.

Sites in myelin basic protein that react with monoclonal antibodies

The epitopes (antigenic sites) for seven monoclonal antibodies (MAbs) evoked in rats or mice by guinea pig or monkey myelin basic protein (BP) have been located in four different sequences of the BPs extracted from various species. Six of the MAbs were evoked by guinea pig BP. (1) One epitope, possibly a pair, is included within residues 1-14 of all BPs tested and reacts with two rat IgG MAbs. (2) A definite pair of overlapping epitopes includes the central Phe91-Phe92 sequence. One epitope is contained entirely within sequence 90-99 and reacts with a rat IgG MAb. The substitution of Ser in chicken BP for Thr97 destroys this epitope. The other epitope appears to include residues on the amino side of Phe44 and even of His32 and suggests some tertiary structure in BP. This epitope reacts with a mouse IgM MAb that does not recognize the chicken substitution. (3) The third epitope lies within residues 114-121, specifically including Trp118, and reacts with a rat IgG MAb. A cross-reacting epitope probably includes residues 44-45 in certain species (guinea pig and bovine but not rabbit). (4) Another pair of epitopes is located within residues 131-140 but is severely species-restricted. This region in guinea pig BP evoked a species-specific mouse IgM MAb. The same region in monkey BP evoked the seventh MAb, a mouse IgG, which reacts with human, chimpanzee, monkey, bovine, and rat-18.5 kDa BPs and to a lesser extent rabbit BP but not with guinea pig, pig, or chicken BPs. Some tertiary structure in guinea pig BP is also suggested by the reactivities with the IgM MAb. All of the MAbs react with myelin in histologic preparations, but the optimum method of preparation of the tissue varies with each.

Sequence of guinea pig myelin basic protein

This paper proposes a tentative amino acid sequence of guinea pig myelin basic protein obtained by comparison of peptide fragments of the guinea pig and bovine proteins. Analyses of the tryptic peptides confirmed the known sequence differences in the NH2-terminal half of the molecule and showed that in the COOH-terminal half of the guinea pig protein Ser131 was missing, Ala136 - His137 was deleted, Leu140 was replaced by Phe, and an extra Ala was inserted somewhere within sequence 142-151 (tryptic peptide T23 ). Sequence determination of guinea pig tryptic peptides corresponding to residues 130-134 ( T20 ), 135-138 ( T21 ), and 142-151 ( T23 ) of the bovine protein confirmed the above sequence changes and placed the extra Ala between Gly142 and His143 . The sequence of the region corresponding to bovine residues 130-143 is thus Ala-Asp-Tyr-Lys-Ser-Lys-Gly-Phe-Lys-Gly-Ala-His. No species differences were observed in the amino acid compositions of the remaining tryptic peptides obtained from the COOH-terminal half of the molecule. Based upon these results, the guinea pig basic protein contains 167 amino acid residues and has a molecular weight of 18,256.

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.

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.

Antigenic determinant recognized by a monoclonal antibody to human myelin basic protein

From an examination of electroimmunoblots and peptide maps, a mouse monoclonal antibody to human myelin basic protein MBP was shown to react with the amino acid sequence Ala-Ser-Asp-Tyr-Lys-Ser which is located in the C-terminal half of MBP. Although a completely different immunization schedule was used by Sires et al. (1981) they obtained a monoclonal antibody reacting with the same determinant. In contrast to results with other monoclonal antibodies to globular proteins (Todd et al. 1982) this monoclonal antibody seems to react with a sequential rather than a topographical determinant.