Presence of another species of myelin basic protein (MBP) in the developing chick brain

It has been observed in the chick myelin that a protein with a molecular weight of 19.5K, in addition to the major species of myelin basic protein (MBP) (MBP(1), mol. wt 18.5K), reacts with anti-MBP serum directed against MBP(1). The larger MBP (MBP(2) mol. wt 19.5K), which seemingly does not correspond to any of the MBP species from other animal sources, is detected at high level in chick myelin from the initial stage of myelination. MBP(1) is easily extracted from the fluffy layer of Folch's upper layer, while MBP(2) is extracted by sonication of the acidified fluffy layer. Both extracted MBPs can be purified by using high performance liquid chromatography (HPLC) with a reverse phased column after Sephadex G-100 and CM cellulose column chromatographies. The amino acid compositions of both MBPs are similar to each other. Further, both MBPs are phosphorylated using protein kinase C with 5.3 and 3.6 mol of incorporated phosphates to 1 mol of MBP(1) and MBP(2), respectively. One dimensional peptide mapping of both MBPs with Staphylococcus aureus V8 protease, carboxypeptidase Y and 2-(2-nitrophenylsulfenyl)-3-methyl-3?-bromoindolenine reveals different degradation patterns. This difference is also demonstrated by cyanogen bromide fragmentation. The MBP(2) cleavage has a low degradation rate in contrast to the complete cleavage of MBP(1). The two fragments (mol. wt 8 and 9K) from MBP(1) and one fragment (mol. wt 7K) from MBP(2), which are purified by HPLC, can be sequenced from the N-terminus. It is suggested that the methionine site is different in both MBP(1) and MBP(2), and there are a small number of amino acid substitutions between both MBPs.