Human essential myosin light chain isoforms revealed distinct myosin binding, sarcomeric sorting, and inotropic activity

Aims: In this paper we tested the hypothesis that different binding affinities of the C-terminus of human cardiac alkali (essential) myosin light chain (A1) isoforms to the IQ1 motif of the myosin lever arm provide a molecular basis for distinct sarcomeric sorting and inotropic activity. Methods and Results: We employed circular dichroism and surface plasmon resonance spectroscopy to investigate structural properties, secondary structures, and protein-protein interactions of a recombinant head-rod fragments of rat cardiac beta-myosin heavy chain aa664-915 with alanin mutated IQ2 domain (rbeta-MYH(664-915)IQ(ala4)) and A1 isoforms (human atrial -hALC1- and human ventricular -hVLC-1- light chains). Double epitope tagging competition was used to monitor the intracellular localization of exogenously introduced hALC-1 and hVLC-1 constructs in neonatal rat cardiomyocytes. Contractile functions of A1 isoforms were investigated by monitoring shortening and intracellular free Ca(2+) (Fura-2) of adult rat cardiomyocytes infected with adenoviral (Ad) vectors using hALC-1 or beta-galactosidase as expression cassettes. hALC-1 bound more strongly (>3fold lower K(D)) to rbeta-MYH(664-915) than did hVLC-1. Sorting specificity of A1 isoforms to sarcomeres of cardiomyocytes rose in the order hVLC-1 to hALC-1. Replacement of endogenous VLC-1 by hALC-1 in adult rat cardiomyocytes increased contractility while the systolic Ca(2+) signal remained unchanged. Conclusions: Intense myosin binding of hALC-1 provides a mechanism for preferential sarcomeric sorting and Ca(2+)-independent positive inotropic activity.