Thiol-specific cross-linkers of variable length reveal a similar separation of SH1 and SH2 in myosin subfragment 1 in the presence and absence of MgADP.

A series of thiol-specific cross-linking reagents were prepared for studying the kinetics of cross-linking between SH1 (Cys(707)) and SH2 (Cys(697)) in rabbit skeletal muscle myosin subfragment 1. The reagents were of the type RSS(CH(2))(n)()SSR, with R = 3-carboxy-4-nitrophenyl and n = 3, 6, 7, 8, 9, 10, and 12, spanning distances from 9 to 20 A. The reactions were monitored spectrophotometrically by measuring the release of 2-nitro-5-thiobenzoate. Reaction rates for modification of SH1 (k(1)) and for cross-linking (k(2)) were measured by the decrease of the K(+)(EDTA)-ATPase activity and the decrease of the Ca(2+)-ATPase activity, respectively, and corrected for the different reactivities of C(n). Cross-linking rates in the presence and absence of MgADP showed similar dependence on the length of the reagents: While the cross-linking rates for n = 3 or n = 6 were close to those for n = 0 (Ellman's reagent), those for n = 7 and 8 were significantly increased. Thus the distance between SH1 and SH2 appears to be equal in both states and can be estimated as >/=15 A, based on the length of the reagent with n = 8 in stretched conformation. Under rigor conditions, reactivity of SH1 differed significantly from that in the presence of MgADP, presumably because of shielding through a lipophilic domain. Similarly, the cross-linking rates k(2) for C(3), C(6), and C(7) in the absence of MgADP were ca. 15 times lower than in the presence of MgADP, suggesting a change in the structure of the SH2 region that depends on nucleotide binding. The results are discussed in terms of recent X-ray structures of S1 and S1-MgADP [Rayment et al. (1993) Science 261, 50-58; Gulick et al. (1997) Biochemistry 36, 11619-11628].