Identification of two phosphorylated threonines in the tail region of Dictyostelium myosin II.

We have previously purified and characterized a Dictyostelium myosin II heavy chain kinase which phosphorylates threonine residues (Côté, G. P., and Bukiejko, U. (1987) J. Biol. Chem. 262, 1065-1072). The phosphorylated threonines are located within a 34-kDa fragment which can be selectively cleaved from the carboxyl terminal end of the Dictyostelium myosin II tail. Tryptic and chymotryptic digests of the 34-kDa fragment phosphorylated with the kinase have now been performed and the resulting phosphopeptides isolated and sequenced. Two phosphorylated threonine residues have been identified, corresponding to residues 1833 and 2029 in the complete amino acid sequence of the Dictyostelium myosin II heavy chain. These amino acids are 87 and 283 residues, respectively, distant from the carboxyl terminus of the Dictyostelium myosin II heavy chain and are present in sections of the tail which seem to be alpha-helical coiled coils. In contrast, the three Acanthamoeba myosin II heavy chain phosphorylation sites are located within 10 residues of each other in a small globular domain at the carboxyl terminal tip of the tail (Côté, G. P., Robinson, E. A., Appella, E., and Korn, E. D. (1984) J. Biol. Chem. 259, 12781-12787). This suggests that the mechanism by which heavy chain phosphorylation inhibits the actin-activated ATPase activity and filament-forming properties of the two myosins may be quite different.