COOPERATIVE RIGOR BINDING OF MYOSIN TO ACTIN IS A FUNCTION OF F-ACTINSTRUCTURE

Many aspects of cooperative behavior within pure F-actin filaments hav e now been described. We have used two myosin fragments, heavy meromyo sin (HMM) and Subfragment 1 (S1), to look at the rigor binding to diff erent forms of F-actin. With Ca2+ bound at the high-affinity metal bin ding site in actin, there is a very large cooperativity in the binding of HMM, but no cooperativity for S1. With Mg2+ bound at the high affi nity bo site, or with conditions that stabilize the conformation of su bdomain-2 of actin, there is no cooperativity seen with either HMM or S1. These results show that the two heads of HMM can induce structural changes in F-actin that are not observed with the single head of S1. They also support the notion that the binding of myosin to F-actin ind uces a conformational change in subdomain-2 of actin, and that under c ertain conditions this conformational change can be cooperatively prop agated through an actin filament. (C) 1997 Academic Press Limited.