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J. Biol. Chem., Vol. 276, Issue 9, 6073-6082, March 2, 2001

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Identification, Characterization, and Functional Analysis of Heart-specific Myosin Light Chain Phosphatase Small Subunit^*

Takuro Arimura, Nobuhiro Suematsu^§, Ying-Bi Zhou^¶, Junji Nishimura^¶, Shinji Satoh^§, Akira Takeshita^§, Hideo Kanaide^¶, and Akinori Kimura^**

From the  Department of Molecular Pathogenesis, Division of Adult Diseases, Medical Research Institute, Tokyo Medical and Dental University, Tokyo 101-0062, the ^§ Department of Cardiovascular Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, the ^¶ Division of Molecular Cardiology, Research Institution of Angiocardiology, Faculty of Medicine, Kyushu University, Fukuoka 812-8582, and the  Etiology and Pathogenesis Research Unit, Medical Research Institute, Tokyo Medical and Dental University, Tokyo 101-0062, Japan

Myosin light chain phosphatase consists of three subunits, a 38-kDa catalytic 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 characterized. The small subunit has been cloned and studied from smooth muscle, but little is known about its function and specificity in the other muscles such as cardiac muscle. In this study, cDNAs for heart-specific small subunit isoforms, hHS-M₂₁, were isolated and characterized. Evidence was obtained from an analysis of genome to suggest that the small subunit was the product of the same gene as the large subunit. Using permeabilized renal artery preparation and permeabilized cardiac myocytes, it was shown that the small subunit increased sensitivity to Ca²⁺ in muscle contraction. It was also shown using an overlay assay that hHS-M₂₁ bound the large subunit. Mapping experiments demonstrated that the binding domain and the domain involved in the increasing Ca²⁺ sensitivity mapped to the same N-terminal region of hHS-M₂₁. These observations suggest that the heart-specific small subunit hHS-M₂₁ plays a regulatory role in cardiac muscle contraction by its binding to the large subunit.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EMBL Data Bank with accession number(s) AB050641 (for hHS-M₂₁ A) and AB050642 (for hHS-M₂₁ B).

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