The functional effects of mutations Thr673-->Asp and Ser702-->Asp at the Pro-directed kinase phosphorylation sites in the C-terminus of chicken gizzard caldesmon.

We expressed the C-terminal 99 amino acids of chicken gizzard caldesmon (658C) and two point mutants in which the preferred phosphorylation sites of MAP kinase and p34cdc2 kinase, Ser702 and Thr673 were altered to aspartic acid. The T673D mutant was indistinguishable from 658C but S702D was not phosphorylated by MAP kinase, was significantly less potent as an inhibitor of actin-tropomyosin activation of myosin MgATPase, and bound less actin-tropomyosin at low concentrations. Thus Ser702 is involved in the tropomyosin-dependent inhibitory mechanism of caldesmon, and its phosphorylation by MAP kinase or p34cdc2 kinase could modulate caldesmon function.