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J. Biol. Chem., Vol. 277, Issue 6, 4422-4427, February 8, 2002

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Agonist-induced Force Enhancement
THE ROLE OF ISOFORMS AND PHOSPHORYLATION OF THE MYOSIN-TARGETING SUBUNIT OF MYOSIN LIGHT CHAIN PHOSPHATASE^*

Christopher T. Richards, Ozgur Ogut, and Frank V. Brozovich^§¶

From the Departments of ^§ Medicine and  Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106

The magnitude of agonist-induced Ca²⁺ sensitization of force is tissue-dependent, but an explanation for this diversity is unknown. Ca²⁺ sensitization is thought to involve a G-protein-mediated inhibition of myosin light chain phosphatase activity by phosphorylation of the myosin-targeting subunit (MYPT). The MYPT has two isoforms that differ by a central insert, which lies near this phosphorylation site. Expression of MYPT isoforms is both developmentally regulated and tissue-specific. We hypothesized that the presence or absence of the central insert determines the magnitude of agonist-induced Ca²⁺ sensitization. Throughout development, the chicken aorta exclusively expresses the splice-in MYPT isoform, and guanosine 5'-O-(thiotriphosphate) (GTPS) produces a significant force enhancement. Early during development, the chicken gizzard expresses the splice-in MYPT isoform, and GTPS produced a Ca²⁺ sensitization. In the gizzard coincident with the shift in expression from the splice-in to splice-out MYPT isoform, GTPS no longer produced force enhancement. In addition, adenosine 5'-O-(thiotriphosphate) (ATPS) phosphorylated only adult gizzard tissue, the only tissue that did not demonstrate a Ca²⁺ sensitization. These results suggest that the relative expression of splice-in/splice-out MYPT isoforms determines the magnitude of agonist-induced force enhancement and that MYPT phosphorylation is not required for Ca²⁺ sensitization.

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