An actin-binding protein in human platelets. Interactions with alpha-actinin on gelatin of actin and the influence of cytochalasin B.

A protein (molecular weight, approximately 250,000) with actin-binding properties has been isolated from human platelets. Addition of the actin-binding protein (ABP) to semiviscous solutions of purified actin containing troponin-tropomyosin (TM-TP) complex resulted in formation of viscous gels consisting of randomly associated actin TM-TP filaments. alpha-Actinin (alphaA), a muscle protein recently detected in platelets, also induced random cross-linking of dissociated actin into gels. Sequential addition of ABP and alphaA resulted in gels consisting of parallel associated actin TM-TP filaments in bundles, suggesting a cooperative interaction. Cytochalasin B (CB) had no apparent effect on the cross-linking of randomly associated actin TM-TP filaments induced by either protein alone but prevented development of bundles of parallel filaments when ABP and alphaA were added sequentially. In addition, CB disrupted the bundles of parallel associated actin TM-TP filaments when added to gels already formed by the dual action of ABP and alphaA and caused simultaneous release of alphaA from the complexes to the supernatant. The findings suggest that platelet ABP and alphaA may induce actin filaments to form the parallel associations observed in platelet pseudopods.