A 35-Å movement of smooth muscle myosin on ADP release

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Abstract
MYOSIN II crossbridges interact with F-actin producing power-strokes of around 100 Å (refs 1, 2), during which the products of ATP hydrolysis are released3–5. This has been postulated to involve an articulation of the myosin head (S1) on actin, or substantial conformational changes in S1 itself6–8. Small movements of the regulatory light chain have been detected (see, for example, refs 9, 10), but most data suggest that the bulk of S1 does not move on actin during crossbridge cycling8,11. Here we present three-dimensional maps of S1-decorated F-actin in the presence and absence of MgADP. The myosin motor domain is similar in both states but there are major orientational differences in the light-chain-binding domain. This domain acts as a rigid level arm12,13 pivoting about the end of the motor domain and swinging ∼ 23°, resulting in a ∼ 35-Å step. Small, nucleotide-mediated conformational changes in the motor domain14–16may thus be converted by the light-chain domain into large movement steps.