Isolation and characterization of the G-actin-myosin head complex

The two main proteins involved in muscular contraction and cell motility, myosin and actin, possess the intrinsic property of being able to form filamentous structures. This property poses a serious impediment to the study of their structures and interactions, and a considerable effort has thus been made to isolate their functional domains. The globular part of myosin, subfragment-1 (S1), which possesses ATPase and actin-binding sites as well as supporting the movement of actin filaments during in vitro assays, has been isolated. But because S1 is efficient in inducing actin polymerization, as is myosin, it has not been possible to prepare and characterize a complex of S1 with monomeric actin (G-actin). We have now used chromatographically purified proteins to show that only the S1 isoenzyme carrying the A1 light-chain subunit promotes actin polymerization. The other isoenzyme, S1 (A2), carrying the A2 light-chain subunit, binds to actin, forming a tight complex of G-actin and S1 in a 1:1 ratio. This new functional difference between myosin isoforms directly implicates the A1 light-chain in myosin-induced actin polymerization. Additionally, this finding should lead to the purification of the stable G-actin-S1 complex needed to resolve the structure and to understand the molecular dynamics of the actin-myosin system.