Abstract
Smooth muscle myosin II undergoes an additional movement of the regulatory domain with ADP release that is not seen with fast skeletal muscle myosin II. In this study, we have examined the interactions of smooth muscle myosin subfragment 1 with ADP to see if this additional movement corresponds to an identifiable state change. These studies indicate that for this myosin:ADP, both the catalytic site and the actin-binding site can each assume one of two conformations. Relatively loose coupling between these two binding sites leads to three discrete actin-associated ADP states. Following an initial, weakly bound state, binding of myosin:ADP to actin shifts the equilibrium toward a mixture of two states that each bind actin strongly but differ in the conformation of their catalytic sites. By contrast, fast myosins, including Dictyostelium myosin II, have reciprocal coupling between the actin- and ADP-binding sites, so that either actin or nucleotide, but not both, can be tightly bound. This uncoupling, which generates a second strongly bound actomyosin ADP state in smooth muscle, would prolong the fraction of the ATPase cycle time that this actomyosin spends in a force-generating conformation and may be central to explaining the physiologic differences between this and other myosins.
Highlights
Smooth muscle myosin II undergoes an additional movement of the regulatory domain with ADP release that is not seen with fast skeletal muscle myosin II
An increase in the duty ratio of smooth muscle myosin II implies a longer lifetime of the strong binding state, which under physiologic conditions means a longer lifetime of ADPbound state(s) [4]
We find evidence for only one myosin:ADP state for myosin II from Dictyostelium, corresponding studies on smooth muscle myosin II clearly demonstrate the presence of two such states, both of which are on the pathway to ADP release
Summary
S1, myosin subfragment 1; 2ЈdmD, 2Јdeoxy mant-ADP; mant, N-methylanthraniloyl; MgADP, magnesium salt of adenosine 5Ј-diphosphate. Dantzig et al [16] examined the effects of MgADP on rigor tension in gizzard smooth muscle and found that, contrary to expectation, its addition did not reduce tension They postulated that the additional movement of the regulatory domain may contribute to the latch state, by slowing release of ADP from attached heads. We find evidence for only one myosin:ADP state for myosin II from Dictyostelium, corresponding studies on smooth muscle myosin II clearly demonstrate the presence of two such states, both of which are on the pathway to ADP release. The relevance of these results to models of strain-dependent release will be discussed
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