Plant profilin induces actin polymerization from actin:β-thymosin complexes and competes directly with β-thymosins and with negative co-operativity with DNase I for binding to actin

Recombinant plant (birch) profilin was analyzed for its ability to promote actin polymerization from the actin:thymosin beta(4) and beta(9) complex. Depending an the nature of the divalent cation, recombinant plant (birch) profilin exhibited two different modes of interaction with actin, like mammalian profilin. In the presence of magnesium ions birch profilin promoted the polymerization of actin from A:T beta(4). In contrast, in the presence of calcium but absence of magnesium ions birch profilin was unable to initiate the polymerization of actin from the complex with T beta(4). However, under these conditions profilin. formed a stable stoichiometric complex with skeletal muscle alpha-actin, as verified by its ability to increase the critical concentration of actin polymerization. Chemical cross-linking indicated that birch profilin competes with T beta(4) for actin binding. Ternary complex formation of birch profilin with actin:DNase I complex was suggested by chemical cross-linking. However, the determination of the critical concentrations of actin polymerization in the simultaneous presence of birch profilin and DNase I indicated that profilin and DNase I did not form a ternary complex. These data indicated a negative co-operativity between the profilin and DNase I binding sites on actin. (C) 1998 Federation of European Biochemical Societies.