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

Abstract

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 proteinprotein interactions of a recombinant head-rod fragments of rat cardiac β-myosin heavy chain aa664915 with alanine-mutated IQ2 domain (rβ-MYH664915IQala4) 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 Ca2+ (Fura-2) of adult rat cardiomyocytes infected with adenoviral (Ad) vectors using hALC-1 or β-galactosidase as expression cassettes. hALC-1 bound more strongly (greater than three-fold lower KD) to rβ-MYH664915 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 Ca2+ signal remained unchanged. ConclusionIntense myosin binding of hALC-1 provides a mechanism for preferential sarcomeric sorting and Ca2+-independent positive inotropic activity. © The Author 2011.