Excitation energy transfer studies on the proximity between SH1 and the adenosinetriphosphatase site in myosin subfragment 1.
Excitation energy transfer studies were carried out to determine the distance between the adenosinetriphosphatase (ATPase) site and a unique "fast-reacting" sulfhydryl (referred to as SH1) in myosin subfragment 1. The fluorescent moiety of the probe N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylene-diamine was used as the donor attached at SH1. The chromophoric nucleotide analogue 2'(3')-0-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate was used as the acceptor noncovalently bound at the ATPase site. The energy transfer efficiency was found to be 56% by measuring the decrease in donor fluorescence lifetime. The critical transfer distance, R0(2/3), was determined to be 40.3 A. Since both donor and acceptor are likely to be rigidly attached, a statistical interpretation of the data was applied (Hillel, Z., & Wu, C.-W. (1976) Biochemistry 15, 2105] to determine distances. The method yielded the following conclusions: most probable distance = 38.7 A; maximum possible distance = 52 A; 10% probability for the distance to be less than 20 A; 3% probability to be less than 15 A. It may be concluded that despite the great influence that the two sites exert on each other, it is not likely that SH1 interacts directly with the ATPase site in myosin subfragment 1. This conclusion is in agreement with the findings of Wiedner et al. [Wiedner, H., Wetzel, R., & Eckstein, F. (1978) J. Biol. Chem. 253, 2763] and Botts et al. [Botts, J., Ue., K., Hozumi, T., & Samet, J. (1979) Biochemistry 18, 5157].