Myosin Cross-Bridge Kinetics and the Mechanism of Catch

Abstract

Catch force in molluscan smooth muscle requires little, if any, energy input and is controlled by the phosphorylation state of the thick filament-associated mini-titin, twitchin. The kinetic parameters of myosin cross-bridge turnover in permeabilized catch muscle, and how they are potentially modified by the catch mechanism, were determined by single turnover measurements on myosin-bound ADP. Under isometric conditions, there are fast and slow components of cross-bridge turnover that probably result from kinetic separation of calcium-bound and calcium-free cross-bridge pools. The structure responsible for catch force maintenance at intermediate [Ca+2] does not alter the processes responsible for the fast and slow components under isometric conditions. Also, there is no measurable turnover of myosin-bound ADP during relaxation of catch force by phosphorylation of twitchin at pCa>8. The only effects of the catch link on myosin-bound ADP turnover are 1), a small, very slow extra turnover when catch force is maintained at very low [Ca+2] (pCa>8); and 2), attenuation of the shortening-induced increase in turnover at subsaturating [Ca+2]. These limited interactions between the catch link and myosin cross-bridge turnover are consistent with the idea that catch force is maintained by a thick and thin filament linkage other than the myosin cross-bridge.