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In all eukaryotes, actin and myosin play a role in the maintenance of cell shape and in cellular movement (e.g. reviewed by Korn, 1978). In the muscles of multicellular organisms actomyosin is the principal structural component of the sarcomere, the unit of the muscle fibre. Movement is effected by the sliding of actin and myosin filaments over each other, with accompanying hydrolysis of ATP providing energy for this process, which is regulated by calcium-sensitive myofibrillar proteins (see Bagshaw, 1982, for review). The myosin molecule consists oftwo heavy chains (MHC, ofMr about 220000) each with a globular head region where the ATPase activity is situated, and a filamentous, rod-like tail, and four associated light chains (of Mr 15000-30000),two so-called regulatory or DTNB light chains (MLC2) and two alkali light chains (MLC1, MLC3). It is the alkali myosin light chain proteins and their genes which are the subject of this review. The function of the alkali light chains is not clear. It has been shown that, although these light chains are associated with the globular region of the myosin molecule, probably near the head/tail junction (Flicker et al., 1983; Sellers & Harvey, 1984), they do not modify the ATPase activity of isolated myosin heavy chains (e.g. Wagner & Giniger, 1981; Sivaramakrishnan & Burke, 1982). Other experiments (e.g. Wagner & Weeds, 1977; Winstanley et al., 1979; Trayer & Trayer, 1985) would suggest that they play a role in the interaction of the head region of myosin with actin. It has become clear in the last few years, however, that distinct isoforms of the myosin alkali light chains are present in different muscles and in non-muscle cells, suggesting that these isoforms are associated with the different contractile properties of these tissues. This is also the case for the actins and myosin heavy chains. The presence of different isoforms of the contractile proteins in different adult and developing muscles (for review see Buckingham & Minty, 1983) raises interesting questions, not only about their function, but also about the regulation ofthe corresponding genes. Recombinant DNA technology has made it possible to isolate the mRNA and gene sequences of families such as that of the myosin alkali light chains, which can be used to address such questions. The contribution of this type of approach to research on muscle, and the interest of muscle as a model system for the molecular biologist in which to ask fundamental questions about gene regulations, are complementary aspects discussed in this review. The myosin alkali light chain isoforms and their genes

The myosin alkali light chain proteins and their genes