Mapping the binding site of thymosin β4 on actin by competition with G-actin binding proteins indicates negative co-operativity between binding sites located on opposite subdomains of actin

Abstract

The β-thymosins are small monomeric (G-)actin-binding proteins of 5 kDa that are supposed to act intracellularly as actin-sequestering factors stabilizing the cytoplasmic monomeric pool of actin. The binding region of thymosin β4 was determined by analysing the binding of thymosin β4 to actin complexed with DNase I, gelsolin or gelsolin segment 1. Binding was analysed by determining the increase in the critical concentration of actin polymerization by native gel electrophoresis or chemical cross-linking. The formation of a ternary complex including thymosin β4 should indicate that the actin-binding proteins attach to different sites on actin. Competition would be indicative of binding to identical or overlapping sites on actin or of a negative co-operative linkage between the two binding sites. Competition of thymosin β4 for actin binding was observed in the presence of intact gelsolin or the N-terminal gelsolin fragment, segment 1, indicating that thymosin β4 binds to a site close to or identical with the gelsolin segment 1-binding site. The ternary complex of actin-DNase I-thymosin β4 was obtained only when using the chemically cross-linked actin-thymosin β4 complex, indicating that thymosin β4 is dissociated by the binding of DNase I to actin. It is suggested that the dissociation of thymosin β4 by DNase I binding to actin is caused by negative co-operativity between their spatially separated binding sites on actin. A similar negative co-operativity was observed between DNase I and gelsolin segment 1 binding to actin. The results therefore indicate that the respective binding sites for DNase I and segment 1 on subdomains 1 and 2 of actin are linked in a negative co-operative manner.