In order to investigate the sorting specificity of myosin essential light chain (ELC) isoproteins for myosin heavy chain (MHC) of the nonmuscle type of stress fibers, epitope-tagged ELC cDNAs of the nonmuscle and fast skeletal muscle types were force-expressed in cultured cardiac fibroblasts from chicken embryos. Although the expressed ELC of the former type was found in the cytoplasm and nucleus as well as on stress fibers, that of the latter was found only along stress fibers. Thus, compared to ELC of the nonmuscle type, that of the fast skeletal muscle type has a higher affinity for the nonmuscle MHC present endogenously in fibroblast stress fibers. Together with the observation that this isoform has the highest affinity for MHC of the cardiac muscle type in myofibrils of caridomyocytes (Komiyama
et al., 1996, J. Cell Sci. 109, 2089-2099), it can be concluded that ELC of the fast skeletal muscle type is an evolved isoform possessing the ability to associate tightly with a variety of MHC isoforms.
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