Smooth muscle myosin light chain kinase expression in cardiac and skeletalmuscle

The purpose of this study was to characterize myosin light chain kinase (ML CK) expression in cardiac and skeletal muscle. The only classic MLCK detect ed in cardiac tissue, purified cardiac myocytes, and in a cardiac myocyte c ell line (AT1) was identical to the 130-kDa smooth muscle MLCK (smMLCK). A complex pattern of MLCK expression was observed during differentiation of s keletal muscle in which the 220-kDa-long or "nonmuscle" form of MLCK is exp ressed in undifferentiated myoblasts. Subsequently, during myoblast differe ntiation, expression of the 220-kDa MLCK declines and expression of this fo rm is replaced by the 130-kDa smMLCK and a skeletal muscle-specific isoform , skMLCK in adult skeletal muscle. These results demonstrate that the skMLC K is the only tissue-specific MLCK, being expressed in adult skeletal muscl e but not in cardiac, smooth, or nonmuscle tissues. In contrast, the 130-kD a smMLCK is ubiquitous in all adult tissues, including skeletal and cardiac muscle, demonstrating that, although the 130-kDa smMLCK is expressed at hi ghest levels in smooth muscle tissues, it is not a smooth muscle-specific p rotein.