AMINO-ACID-SEQUENCE OF MYOSIN LIGHT-CHAIN KINASE FROM BOVINE STOMACH WITH SPECIAL REFERENCES TO ITS ACTIN-BINDING DOMAIN

Bovine stomach contained two proteins of 155 kDa and 130 kDa, respecti vely, that showed myosin light chain kinase activity. Compared on the basis of equivalent myosin light chain kinase activity, the 155 kDa pr otein of bovine stomach exhibited much higher actomyosin-activating ac tivity than the 130 kDa protein (6). To inquire into whether or not th e 155 kDa protein was a specific actin-linked factor (leiotonin), its primary structure was studied. Both the motif of kinase activity and t he motif of calmodulin binding were found in this protein. The 130 kDa protein is shown to be the proteolytic product of the 155 kDa protein , of which the amino-terminal 25 kDa portion had been cut off by an en dogenous protease. From these facts, it is concluded that leiotonin an d myosin light chain kinase are the same molecule. Part of the amino-t erminal sequence of the 155 kDa protein was, however, similar to that of the actin-binding domains of smooth muscle caldesmon, suggesting th at this protein has additional functions other than its enzymatic acti vity. The protein was completely precipitated together with F-actin by ultracentrifuge, whereas the 130 kDa protein remained in the supernat ant after centrifuging and only F-actin was found in the precipitate. These results are the direct evidence that the 155 kDa protein is an a ctin-bound factor in smooth muscle and that the amino-terminal region of this protein is essential for the interaction with F-actin.