Degradation of smooth-muscle myosin by trypsin-like serine proteinases.

Abstract

Hydrolysis of the myosins from smooth and from skeletal muscle by a rat trypsin-like serine proteinase and by bovine trypsin at pH 7 is compared. Proteolysis of the H chains of both myosins by the rat enzyme proceeds at rates .apprx. 20 times faster than those obtained with bovine trypsin. Whereas cleavage of skeletal muscle myosin H chain by both enzymes results in the generation of conventional products, i.e., heavy meromyosin and light meromyosin, the H chain of smooth muscle myosin is degraded into a fragment of MW 150,000. This is dissimilar from heavy meromyosin and cannot be converted into heavy meromyosin. Proteolysis of the H chain takes place in the head region. The regulatory L chain (20 k Da) [kilo daltons] of smooth muscle myosin is degraded very rapidly by the rat proteinase. The ability of smooth muscle myosin to have its ATPase activity activated by actin in the presence of a crude tropomyosin fraction on introduction of Ca2+ is diminished progressively during exposure to the rat proteinase. The rate of loss of the Ca2+-activated actomyosin ATPase activity is very similar to the rate observed for proteolysis of the H chain and 3-4 times slower than the rate of removal of the so-called regulatory L chain. The significance of these findings in terms of the functional organization of the smooth muscle myosin molecule is discussed. Since the degraded myosin obtained after exposure to very small amounts of the rat proteinase is not longer able to respond to Ca2+, i.e., the functional activity of the molecule has been removed, the implications of a similar type of proteolysis operating in vivo are considered for myofibrillar protein turnover in general, but particularly with regard to the initiation of myosin degradation, which is known to take place outside the lyosome (i.e., at neutral pH).