A Single Serine in the Carboxyl Terminus of Cardiac Essential Myosin Light Chain-1 Controls Cardiomyocyte Contractility In Vivo

Although it is well known that mutations in the cardiac essential myosin light chain-1 (cmlc-1 ) gene can cause hypertrophic cardiomyopathy, the precise in vivo structural and functional roles of cMLC-1 in the heart are only poorly understood. We have isolated the zebrafish mutantlazy susan (laz ), which displays severely reduced contractility of both heart chambers. By positional cloning, we identified a nonsense mutation within the zebrafishcmlc-1 gene to be responsible for thelaz phenotype, leading to expression of a carboxyl-terminally truncated cMLC-1. Whereas complete loss of cMLC-1 leads to cardiac acontractility attributable to impaired cardiac sarcomerogenesis, expression of a carboxyl-terminally truncated cMLC-1 inlaz mutant hearts is sufficient for normal cardiac sarcomerogenesis but severely impairs cardiac contractility in a cell-autonomous fashion. Whereas overexpression of wild-type cMLC-1 restores contractility oflaz mutant cardiomyocytes, overexpression of phosphorylation site serine 195–deficient cMLC-1 (cMLC-1S195A ) does not reconstitute cardiac contractility inlaz mutant cardiomyocytes. By contrast, introduction of a phosphomimetic amino acid on position 195 (cMLC-1S195D ) rescues cardiomyocyte contractility, demonstrating for the first time an essential role of the carboxyl terminus and especially of serine 195 of cMLC-1 in the regulation of cardiac contractility.