The proteins associated with contraction in lamellibranch ‘catch’ muscle

Abstract

Pecten 'actomyosin' as prepared by the usual dilution-precipitation procedure is reasonably pure only when obtained from the striated adductor; when prepared from the smooth tonic adductor it consists of a protein mixture of about 1/3 actomyosin (AM) and $\frac{2}{3}$ tropomyosin A (TM$_{A}$; paramyosin). This admixture accounts for the previously reported differences in adenosine triphosphatase (ATP-ase) activity of crude 'AM' from the two types of adductor muscles, since, after purification, all the biological activity characteristic of contractile proteins (ATP-ase activity, ATP-sensitivity and 'contractility') is associated with AM alone. In presence of ATP this latter can also be dissociated into actin and myosin. After purification by salting out, the myosin is quite distinct from TM$_{A}$ in its solubility properties and sedimentation constant, and, unlike tropomyosin but like rabbit myosin, it possesses ATP-ase activity and combines with rabbit actin to form a synthetic or 'hybrid' actomyosin with an ATP-sensitivity of 45%. The presence in adductor muscles of a conventional actomyosin and myosin in addition to TM$_{A}$ makes it unlikely that the latter protein replaces the classical contractile proteins functionally in 'catch' muscle.