Conformers of myosin from scallop striated adductor muscle.

摘要

Myosin from the striated adductor muscle of scallop (Pecten maximus) folds into a compact 10S conformer, as has been characterized for smooth muscle and non-muscle myosins. The 10S conformer of scallop myosin is favoured at physiological ionic strengths in the absence of Ca2+ and in the presence of nucleotide triphoshate. The folded transition is accompanied by the trapping of the nucleotide at the active site to give a species with a half-time of about an hour at 20C. Ca2+ binding to the specific, regulatory sites on a myosin head promotes unfolding to the extended 6S conformer and activates product release by around 60-fold. The unfolding transition, however, remains much slower than the contraction-relaxation cycle of scallop striated muscle and could not play a role in the regulation of these events. The turnover of nucleotide by the 10S conformer is an order of magnitude slower than the turnover by native scallop filaments. The latter have very similar kinetic properties to that of scallop heavy meromyosin suggesting that the myosin-linked regulatory system requires only the head and neck domain to function properly. Thus there is no evidence, from nucleotide turnover measurements, for an intermolecular interaction occurring between the neck and tail regions of neighbouring myosin in the filament equivalent to that observed as an intramolecular interaction in the 10S conformer. The role of the 10S conformer in striated muscle may therefore be associated with events which occur on a slower time-scale than the contraction-relaxation cycle, such as transport of myosin molecules from their site of synthesis to the myofibril during growth and development of the muscle. Removal of either one or both of the regulatory light chains from scallop myosin prevents, or at least disfavours, formation of the folded 10S conformer. Readdition of the native regulatory light chains, or those from other other molluscan species, restores to the myosin its ability to fold. Labelling the reactive heavy chain thiol of myosin also disfavours formation of the 10S conformer and allows separation of the modified protein from the native molecules.