New states of actomyosin.

Unstained frozen hydrated samples of myosin subfragment 1 (S-1) cross-linked to actin with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide have been examined by electron microscopy in an effort to probe structural states of the attached cross-bridge. The cross-linked complex in the absence of ATP has a rigor-like appearance. In contrast, both in the presence of ATP and after the N, N'-p-phenylenedimaleimide (pPDM) bridging of the reactive thiols of S-1, the covalently attached cross-bridges of the acto X S-1 complex appear more disordered and no longer assume the characteristic rigor 45 degrees angle with the actin filaments. The images both in the presence and absence of ATP bear a striking resemblance to those obtained by negative staining of the cross-linked acto X S-1 complex (Craig, R., Greene, L. E. & Eisenberg, E. (1985) Proc. Natl. Acad. Sci. U.S. A. 82, 3247-3251). The actin-bound pPDM S-1 complex, formed by treating the cross-linked complex with pPDM in the presence of ATP, is an expected analog of the weakly bound cross-bridge state. The disordered appearance of S-1 molecules of the cross-linked complex in the presence of ATP and after pPDM treatment may reflect the structural state of the weakly bound cross-bridge.