Isolation and characterization of a myosin‐like protein from bovine adrenal medulla

In recent years it has become apparent that proteins similar to those found in the contractile apparatus of muscle-tissue also occur in a variety of secretory cells [l] . These findings have led to a comparison between the stimulus-secretion coupling of secretory tissues and the excitation-contraction coupling in muscle [2] . Specifically, these similarities include a requirement for Ca2’ and metabolic energy, perhaps in the form of ATP. Furthermore, both processes can be inhibited in the presence of actin-filament disrupting agents such as cytochalasin B. As these same contractile proteins are involved in the chemical to mechanical energy transformation of skeletal-muscle movement, the idea has developed that an analogous muscle contractile apparatus might be implicated in the process of secretion [2-41 . The suggestion has been made that the release of neurotransmitters by the adrenal medulla is mediated by such a contractile event [5]. Recently an actomyosin-like protein has been described in the adrenal medulla which has Mg2+and Ca2’activated ATPase activity and which displays both the ATP-sensitive super-precipitation and viscometric properties which characterize muscle actomyosin [6,7]. The proposal has also been made that the regulatory role of Ca2+in the two processes may be mediated by similar mechanisms [8] . Indeed a calciumbinding protein has recently been isolated and purified from adrenal medulla which shows a striking resem-