Fragmin, a microfilament regulatory protein from Physarum polycephalum, is phosphorylated by casein kinase II-type enzymes.

Fragmin is a 42 kDa regulatory protein involved in actin microfilament organization in Physarum polycephalum. We show that fragmin is a target of casein kinase II (CK II) enzymes isolated from evolutionarily divergent species. In Physarum microplasmodia, two such kinases were identified. A serine residue located in the sequence Gly-Gly-Ser-Asp-Leu-Glu constitutes the phosphorylation site and was identified by phosphopeptide sequencing, mass spectometry analysis, and inhibition studies with a synthetic peptide corresponding to this site. Interestingly, the actin-fragmin dimer (A--F) as well as the actin2-fragmin trimer (A2--F) are equally efficient targets, and phosphorylation had no effect on the actin-binding properties of fragmin. Actin-fragmin isolated from microplasmodia revealed a minor acidic fragmin isoform, suggesting that fragmin is phosphorylated in vivo. The actin-fragmin complex is also phosphorylated on the actin subunit by an endogenous actin-fragmin kinase [Gettemans, J., De Ville, Y., Vandekerckhove, J., & Waelkens, E. (1992) EMBO J. 11, 3185-3191]. We show that the two phosphorylation events act independently of each other.