Calcium-sensitive regulation of actin-myosin interactions in baby hamster kidney (BHK-21) cells (nonmuscle contraction/phosphorylation/adenosinetriphosphatase/myosin kinase/modulator)

A fraction has been obtained from baby ham- ster kidney (BHK-21) cells that will stimulate the actin-moder- ated ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity of both BHK-21 myosin and gizzard smooth muscle myosin. This acti- vation is associated with the specific phosphorylation of the myosin 20,000-dalton light chain. The BHK-21 myosin light chain kinase preparation contains a major protein of approxi- mately 105,000 molecular weight as determined by sodium dodecyl sulfate gel electrophoresis. Both the actin activation and phosphorylation events require the presence of Ca2+ and the so-called modulator or calcium-ependent regulator protein that has been isolated from smooth muscle, brain, and other tissues. On the basis of these results we propose that this kinase system constitutes a Ca2+?dependent regulatory mechanism for myosin-actin interactions in nonmusc le mammalian cells. Proteins similar to the myosins of muscle have been isolated from various mammalian cells, including platelets (1), leuko- cytes (2), macrophages (3), brain (4), liver (5), adrenal medulla (6), thyroid gland (7), and cultured fibroblasts (8-10). Actin has also been found in the cytoplasm of most eukaryotic cells. The simultaneous presence of these two proteins has led to the suggestion that cell motility, mitosis, and other cellular processes could be related to the in vivo formation of actomyosin com- plexes (11). By analogy with muscle systems, it is usually as- sumed that regulation of the actin-myosin interactions occurs