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Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.

The substrate specificity of bovine brain myelin basic protein (MBP)-specific protein methylase I (S-adenosyl-L-methionine:protein-L-arginine N-methyltransferase, EC 2.1.1.23), which methylates arginine residues of protein, has been studied using various MBPs, several synthetic peptides and heterogeneous nuclear ribonucleoprotein complex protein (hnRNP). (1) Among MBPs from different species of brain, the carp MBP was found to be the best substrate for MBP-specific protein methylase I. This high degree of methyl acceptability is most likely due to the fact that carp MBP is not in vivo methylated at the arginine residue (Deibler, G.E. and Martenson, R.E. (1973) J. Biol. Chem. 248, 2387-2391) and that the methylatable amino acid sequence is present in this protein. (2) In order to study the minimum chain length of MBP polypeptide which functions as the methyl acceptor, several synthetic polypeptides whose sequences are identical to the region surrounding the residue 107 of bovine MBP (the in vivo methylation site) were synthesized. It was found that the hexapeptide, Gly-Lys-Gly-Arg-Gly-Leu (corresponding to residues 104-109 of bovine MBP), was the shortest methyl accepting peptide, while the tetrapeptide, Gly-Arg-Gly-Leu (corresponding to residues 106-109) was inactive as a substrate. (3) hnRNP protein is known to contain methylarginine at residue 193 (Williams, K.R., Stone, K.L., LoPresti, M.B., Merrill, B.M. and Plank, S.R. (1985) Proc. Natl. Acad. Sci. USA 82, 5666-5670) which is post-translationally modified. Thus, the RNP protein overproduced in Escherichia coli and therefore did not contain methylarginine was examined for its methyl acceptability. It was found that neither MBP-specific nor histone-specific protein methylase I could methylate this methylarginine-less RNP protein. This suggests a possible existence of a distinct protein methylase I specific for this nuclear protein.

Myelin partially purified from spinal cords of dysmyelinating mutant (shiverer) mice had almost three-fold the specific activity of 5'-nucleotidase found in the respective myelin fraction from normal mice. The specific activities of two other normally myelin-associated enzymes, 2',3'-cyclic nucleotide-3'-phosphohydrolase and carbonic anhydrase, were only slightly higher in the myelin membranes from shiverers, compared to those from controls. In the mutants, the three enzymes probably occur in oligodendrocyte processes. Hypothetically, the 5'-nucleotidase in the myelin sheath in shiverer and normal mice may be localized in specialized structures.

Because of the implication of myelin basic protein in some neurological diseases its in vivo structure is of particular interest. The protein is usually isolated using organic solvents and acid solutions and has previously been shown to contain little alpha-helical or beta-structure; but it is not known how the extraction methods influence the structure. Following recent observations that deoxycholate generally causes minimal structural perturbation when used to dissolve membrane proteins, this detergent has been used to extract the basic protein from bovine myelin. The protein contained in deoxycholate washes of myelin has been purified by gel chromatography and its secondary structure examined by circular dichroism spectroscopy. This protein and conventionally prepared bovine and human basic protein to which 1% deoxycholate has been added appear to have the same structure: they contain 8-14% more helical structure than the chloroform/methanol-extracted protein in pH 4.8 acetate buffer or in pH 9.15 Tris buffer. This conformational change is unaffected by addition of 0.25 M NaCl. The helical content will approach the upper limit if, as is expected, these ordered segments are short. It is suggested that basic protein may adopt this more ordered structure in myelin and possess activity not apparent in its water-soluble unordered conformation. Retention of its encephalitogenic activity following severe treatment may result from an ability to rapidly refold to the original conformation rather than from this activity being inherent in the unordered form.

The localization of proteins in myelin was studied by the use of a non-penetrating reagent. Tritiated 4,4'-diisothiocyano-2,2'-ditritiostilbene disulfonic acid was used to label the isolated myelin membrane. The membrane was labelled, the basic protein and the hydrophobic protein, lipophilin, were isolated. After 10 min of exposure to the reagent, the specific activity of lipophilin was found to be 10 times greater than that of the basic protein. Water shock did not alter the specific activities. However, sonication increased the specific activity of lipophilin but not that of basic protein. When the isolated proteins were labelled with 3H-labelled 4,4'-diisothiocyano-2,2'-ditritiostilbene disulfonic acid, the specific activity of the basic protein was 10 times that of lipophilin. We concluded that the low specific activity of basic protein isolated from the labelled membrane was due to the inaccessible position of this protein in the membrane bilayer.