Temperature-induced changes in the flexibility of the loop between SH1 (Cys-707) and SH3 (Cys-522) in myosin subfragment 1 detected by cross-linking.

The ability of dibromobimane to cross-link SH1 (Cys-707) in the 21-kDa C-terminal segment to SH3 (Cys-522) in the 50-kDa middle segment of the myosin S1 heavy chain has been examined as a function of nucleotide binding and temperature. The results obtained indicate that, while the reagent rapidly reacts with SH1 at both 25 and 4 degrees C, its ability to cross-link to SH3 is highly dependent on temperature. At 25 degrees C, substantial cross-linking from monofunctionally labeled SH1 to SH3 occurs, in agreement with recent work of Mornet, Ue, and Morales (1985, Proc. Natl. Acad. Sci, USA 82, 1658-1662) and of Ue (1987, Biochemistry 26, 1889-1894) and with their conclusion that a loop, allowing SH1 and SH3 to reside at the cross-linking span of dibromobimane, preexists in the protein. At 4 degrees C, however, negligible amounts of cross-linking are observed whether or not a nucleotide is present, despite indications that SH1 is labeled rapidly by the reagent at this temperature. The inability to form this cross-link is not due to an alternate cross-link between monofunctionally labeled SH1 and another thiol in the 21-kDa segment. These results indicate that this loop exists at 25 degrees C and does not exist (or exists only transiently) at the lower temperature.