DETERMINATION OF THE SITES OF POSTTRANSLATIONAL MODIFICATIONS IN THE CHARGE ISOMERS OF BOVINE MYELIN BASIC-PROTEIN BY CAPILLARY ELECTROPHORESIS MASS-SPECTROSCOPY

The posttranslational modifications in each of the 18.5 kDa bovine mye lin basic protein charge isomers C-1 to C-6 have been determined by th e use of capillary electrophoresis-mass spectroscopy. The pattern of m odifications is viewed as being unique to each charge isomer and is th ought to reflect a specific placement and function for each isomer in the myelin membrane. Several of the sites of posttranslational phospho rylation were found to differ from a number of the reported sites that were phosphorylated in vitro by various kinases. These differences su ggest that an extremely cautious approach be taken in identifying in v ivo posttranslationally modified amino acid residues from residues tha t have been modified in vitro by various kinases. We have identified t he following posttranslationally phosphorylated and deamidated, modifi ed sites in the bovine MBP components C1-C6. C1 has no modification; C 2 represents a deamidation of Gin 146; in C3, Thr 97 and Ser 164 are p hosphorylated; in C4, Ser 54, Thr 97, and Ser 160 are phosphorylated; in C5 Ser 7, Ser 54, Thr 97, and Ser 164 are phosphorylated; anti in C 6, Ser 7, Ser 54, Thr 97, Ser 160, and Ser 164 are phosphorylated.