The tryptophan fluorescence of unmodified myosin subfragment 1 (S1) from rabbit and chicken skeletal muscle with various nucleotides and phosphate analogues bound was measured after rapid temperature jumps. The fluorescence decreased during the temperature rise. Under some conditions, this decrease was followed by an increase, reflecting structural transitions within the protein. With adenosine 5'-[beta,gamma-imido]triphosphate (p[NH]ppA) or with ADP and BeF(x) bound, this rise was very rapid (reciprocal time constant approx. 2000 s(-1)) and varied only slightly with starting temperature, suggesting that, with these ligands, two different protein conformations were present in rapid equilibrium over a large temperature range. In the presence of ATP, the transient included several relaxation processes. Overall, the results suggest that complexes of S1 with ATP or with a number of other ligands exist as a mixture of two forms in temperature-dependent equilibrium. The results throw light on the finding of different forms of S1 in recent crystallographic studies and indicate a surprising lack of strong coupling between myosin's structural state and the nature of the nucleotide bound.
The tryptophan fluorescence of unmodified myosin subfragment 1 (S1) from rabbit and chicken skeletal muscle with various nucleotides and phosphate analogues bound was measured after rapid temperature jumps. The fluorescence decreased during the temperature rise. Under some conditions, this decrease was followed by an increase, reflecting structural transitions within the protein. With adenosine 5′-[β,γ-imido]triphosphate (p[NH]ppA) or with ADP and BeFx bound, this rise was very rapid (reciprocal time constant approx. 2000s−1) and varied only slightly with starting temperature, suggesting that, with these ligands, two different protein conformations were present in rapid equilibrium over a large temperature range. In the presence of ATP, the transient included several relaxation processes. Overall, the results suggest that complexes of S1 with ATP or with a number of other ligands exist as a mixture of two forms in temperature-dependent equilibrium. The results throw light on the finding of different forms of S1 in recent crystallographic studies and indicate a surprising lack of strong coupling between myosin's structural state and the nature of the nucleotide bound.
Present evidence on crossbridge behaviour is reviewed from the viewpoint of the Eisenberg-Hill concepts. We conclude that a cartoon illustrating the operation of these ideas in practice should be drawn so as to include morphological changes in the body of the myosin molecule as part of the power-stroke. The bulk of structural evidence on the operation of crossbridges in intact muscle supports such a description.
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