Myosin filament ATPase is enhanced by intramolecularly cross-linked actin

Reaction of rabbit skeletal muscle F-actin with the lysine-directed photolabile cross-linker, N-5-azido-2-nitrobenzoyloxy succinimide was limited to Lysine-328 and Lysine-326, with Lysine-328 being labelled to a greater extent. Photolysis of the modified actin enhanced the actin-activated MgATPase activity of filamentous scallop myosin 3-4-fold more than unmodified actin, without affecting calcium sensitivity. Unphotolysed modified actin behaved as untreated actin, indicating that photolysis was essential for the effect. The actin-activated ATPase of filamentous rabbit myosin was similarly increased by photolysed N-5-azido-2-nitrobenzoyloxy succinimide-modified actin. After photolysis in either the monomeric (G-) or filamentous (F-) form, N-5-azido-2-nitrobenzoyloxy succinimide-modified actin moved as a monomeric (42 kDa) species on SDS gels, and depolymerized and polymerized readily, demonstrating that any cross-linking event produced by photolysis must be intramolecular. In contrast to the substantial increase in actin-activated ATPase activity observed when photolysed ANB-NOS-modified actin was added to filamentous myosin, the enhancement was not observed with the soluble HMM and S-1 fragments of myosin. Photolysed modified actin showed only poor movement on a rabbit HMM-coated surface in vitro motility assays. These results can be explained if the internally cross-linked G-actin subunits which comprise only a fraction of the actin population, either weaken the actin-actin contacts or have an increased affinity for myosin.