Identification, characterization, and functional analysis of heart-specific myosin light chain phosphatase small subunit

Myosin light chain phosphatase consists of three subunits, a 38-kDa catalyt ic subunit, a large 110-130-kDa myosin binding subunit, and a small subunit of 20-21 kDa. The catalytic subunit and the large subunit have been well c haracterized. The small subunit has been cloned and studied from smooth mus cle, but little is known about its function and specificity in the other mu scles such as cardiac muscle. In this study, cDNAs for heart-specific small subunit isoforms, hHS-M-21, were isolated and characterized. Evidence was obtained from an analysis of genome to suggest that the small subunit was t he product of the same gene as the large subunit. Using permeabilized renal artery preparation and permeabilized cardiac myocytes, it was shown that t he small subunit increased sensitivity to Ca2+ in muscle contraction. It wa s also shown using an overlay assay that hHS-M-21 bound the large subunit. Mapping experiments demonstrated that the binding domain and the domain inv olved in the increasing Ca2+ sensitivity mapped to the same N-terminal regi on of hHS-M-21. These observations suggest that the heart-specific small su bunit hHS-M-21 plays a regulatory role in cardiac muscle contraction by its binding to the large subunit.