Distribution of isoelectric variants of the 17,000-dalton myosin light chain in mammalian smooth muscle.

Isoelectric focusing of purified vascular smooth muscle myosin revealed two variants of the 17,000-dalton light chain subunits. The isoelectric points of the light chain variants were determined to be 4.13 (LC17a) and 4.19 (LC17b). Tryptic peptide maps of the two species of light chain generated by reverse-phase high performance liquid chromatography disclosed small but obvious differences in peptide composition while amino acid analyses of the variants were quite similar. Two-dimensional electrophoresis of extracts from various mammalian smooth muscles revealed tissue-specific differences in the relative content of LC17a and LC17b. Vascular (aorta, carotid, and pulmonary artery) muscles and tracheal smooth muscle contained both light chain variants while smooth muscle of the gastrointestinal tract (stomach and jejunum) contained LC17a only. The actin-activated Mg2+-ATPase activities of both phosphorylated and nonphosphorylated stomach (LC17b = 0) and aortic (LC17b = 40%) myosins were compared. In the presence of saturating tropomyosin, a 2-fold difference in Vmax was measured: phosphorylated, aortic, 0.119 +/- 0.009 versus stomach, 0.239 +/- 0.012 mumol of PO4 liberated/min/mg of myosin; nonphosphorylated, aortic, 0.065 +/- 0.004 versus stomach, 0.123 +/- 0.004 mumol of PO4 liberated/min/mg of myosin. In addition, the Vmax of myosin subfragment-1 ATPase from bovine aortic, pulmonary artery, and stomach myosins (LC17b contents, 40, 20, and 0%, respectively) was found to decrease in direct proportion to the LC17b content. Our results suggest that isoforms of the 17,000-dalton light chain subunits of mammalian smooth muscle myosin could play an important role in modulating actomyosin ATPase activity.