A fluorescence temperature-jump study of conformational transitions in myosin subfragment 1

The tryptophan fluorescence of unmodified myosin subfragment I (SI) from ra bbit and chicken skeletal muscle with various nucleotides and phosphate ana logues bound was measured after rapid temperature jumps. The fluorescence d ecreased during the temperature rise. Under some conditions, this decrease was followed by an increase, reflecting structural transitions within the p rotein. With adenosine 5'-[beta,gamma -imido]triphosphate (p[NH]ppA) or wit h ADP and BeFx bound, this rise was very rapid (reciprocal time constant ap prox. 2000 s(-1)) and varied only slightly with starting temperature, sugge sting that, with these ligands, two different protein conformations were pr esent in rapid equilibrium over a large temperature range. In the presence of ATP, the transient included several relaxation processes. Overall, the r esults suggest that complexes of Sl with ATP or with a number of other liga nds exist as a mixture of two forms in temperature-dependent equilibrium. T he results throw light on the finding of different forms of S1 in recent cr ystallographic studies and indicate a surprising lack of strong coupling be tween myosin's structural state and the nature of the nucleotide bound.