In a previous study, protein kinase FJglycogen synthase kinase-3 (F,/GSK-3) was identified as a myelin basic protein (MBP) kinase associated with intact brain myelin. In this report, the phosphorylation sites of MBP by kinase F,/GSK-3 were further determined by two-dimensional electrophoresis/TLC, phosphoamino acid analysis, tryptic peptide mapping, Edman degradation, and direct sequencing. Kinase F,/GSK-3 phosphorylates MBP on both threonine and serine residues. Three tryptic phosphopeptide peaks were resolved by C,8 reverse-phase HPLC. Sequential manual Edman degradation together with direct sequence analysis revealed that T(p)PPPSQGK is the phosphorylation site sequence for the first major phosphopeptide peak. When mapping with the bovine brain MBP sequence, we finally demonstrate Thrg7-Pro, one of the in vivo phosphorylation sites in MBP, as the major site phosphorylated by kinase FJGSK-3, implicating a physiologically relevant role of F,/GSK-3 in the regulation of brain myelin function. By using the same approach, we also identified NIVTg4(p)PR as the phosphorylation site sequence in the second major tryptic phosphopeptide derived from [32P]MBP phosphorylated by kinase FJGSK-3, further indicating that kinase FJ GSK-3 represents a Thr-Pro motif-directed MBP kinase involved in the phosphorylation of brain myelin. Key Words: Myelin basic protein-Protein kinase F,/glycogen synthase kinase-3-Phosphorylation site-Consensus sequence motif. J. Neurochern. 62, 1596-1 603 (1 994).