Fast skeletal muscle isoforms exhibit the highest incorporation level intomyofibrils and stress fibers among members of myosin alkali light chain isoform family

Isoproteins of myosin alkali light chain (LC) were co-expressed in cultured chicken cardiomyocytes and fibroblasts and their incorporation levels into myofibrils and stress fibers were compared among members of the LC isoform family, In order to distinguish each isoform from the other, cDNAs of LC i soforms were tagged with different epitopes. Expressed LCs were detected wi th antibodies to the tags and their distribution was analyzed by confocal m icroscopy. In cardiomyocytes, the incorporation level of LC into myofibrils was shown to increase in the order from nonmuscle isoform (LC3nm), to slow skeletal muscle Isoform (LC1sa), to slow skeletal/ventricular muscle isofo rm (LC1sb), and to fast skeletal muscle isoforms (LC1f and LC3f). Thus, the hierarchal order of the LC affinity for the cardiac myosin heavy chain (MH C) is identical to that obtained in the rat (Komiyama et al,, 1996, J, Cell Sci,, 109: 2089-2099), suggesting that this order may be common for taxono mic animal classes, In fibroblasts, the affinity of LC for the nonmuscle MH C in stress fibers was found to increase in the order from LC3nm, to LC1sb, to LC1sa, and to LC1f and LC3f This order for the nonmuscle MHC is partly different from that for the cardiac MHC, This indicates that the order of t he affinity of LC isoproteins for MHC varies depending on the MHC isoform, Further, for both the cardiac and nonmuscle MHCs, the fast skeletal muscle LCs exhibited the highest affinity, This suggests that the fast skeletal mu scle LCs may be evolved isoforms possessing the ability to associate tightl y with a variety of MHC isoforms.