| Abstract: | Morphologically similar short myosin and rod filaments (minifilaments) have been prepared in 10 mM Tris--citrate buffer, pH 8.0, in the absence of other myosin or rod forms. Both minifilament systems are dissociated in the same manner in the presence of ATP or pyrophosphate. Identical binding of these ligands to myosin and rod minifilaments suggests that myosin heads play no role in substrate-induced destabilization of the minifilaments. The effects of ATP and pyrophosphate on minifilaments are similar to their dissociating effect on synthetic filaments [Harrington, W. F., & Himmelfarb, S. (1972) Biochemistry 11, 2945--2952], thus justifying their use in conformational studies in lieu of filaments. In view of their small size and homogeneity, the minifilaments constitute an appropriate material for such studies. The binding of pyrophosphate to myosin and rod minifilaments decreases their alpha-helical content, as measured by circular dichroism. No change in the secondary structure of subfragment 1 and light meromyosin is observed upon binding of pyrophosphate, but substantial changes (10%) are detected in subfragment 2. The structural changes in myosin, possibly relevant to contraction, are localized in the subfragment 2 region of the molecule. These results emphasize the importance of charge interactions in the functional behavior of thick filaments. |
