Binding of F-Actin to a Region between SH1 and SH2 Groups of Myosin Subfragment-1 which May Determine the High Affinity of Acto-Subfragment-1 Complex at Rigor1

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Abstract
Covalent cross-linking reaction between SH 1 and SH 2 groups in myosin subfragment-1 (S-1) by N,N'-p-phenylenedimaleimide (pPDM) was followed by the degree of inactivation of NH 4+ -EDTA ATPase activity. The rate of the cross-linking reaction decreased to less than a 20th in the presence of F-actin. The inhibitory effect of F-actin was not observed in the presence of MgATP. Binding of F-actin to S-1 was measured using ultracentrifugation. S-1 whose SH 1 and SH 2 , were covalently cross-linked by pPDM or 5, 5'-dithiobis(2-nitrobenzoic acid) (DTNB) did not bind F-actin. After the DTNB-cross-linked S-1 is reduced by dithiothreitol, the ability to bind F-actin is recovered. These results suggest that S-1 has a binding site for F-actin in the region between SH 1 and SH 2 . This site appears to determine the high affinity of acto-S-1 complex at the rigor while decreasing the affinity more than 10 2 times in the presence of MgATP.