The Association of Creatine Phosphokinase with the Mitotic Spindle

The M-isozyme of creatine Phosphokinase (M-CPK) has been proposed as a phosphorylation intermediate in the respiratory control of striated muscle (Bessman et al., 1980). This isozyme has been shown previously to be a constituent of the M line of avian cardiac (Wallimann et al., 1978a) and skeletal muscle (Wallimann et al., 1978a,b). Additionally, M-CPK has been demonstrated in the region of the A-band (Sharov et al., 1977) and associated with myosin (Turner et al., 1973), as well as being found in abundance within mitochondria (Jacobus and Lehninger, 1973; Sharov et al., 1977). In striated muscle, a close functional relationship has been suggested between myofibrillar CPK and ATPase in that ATP generated from creatine phosphate apparently has a preferred access to the myofibrillar ATPase system (Bessman et al., 1980). M-CPK has been found in association with cytoskeletal elements in both muscle (Fuseler and Shay, 1980) and nonmuscle cells (Eckert et al., 1980), and is predominantly associated with intermediate filaments in the cytoplasm of muscle (Fuseler et al., 1981) and nonmuscle (Eckert et al., 1980) cells grown in culture. Additionally, this isozyme can associate with actin stress fibers in mammalian cardiac cells in primary culture, but does not associate with cytoplasmic microtubules in various cell types (Fuseler and Shay, 1980; Fuseler et al., 1981). These observations have given rise to the hypothesis that cytoskeletal elements may function to support a CPK-catalyzed energy-generating system. By such a scheme, the distribution and organization of the elements of the cytoskeleton or an organelle could influence local levels of available ATP and hence energy in the cytoplasm.