Differential phosphorylation of LZ+/LZ- MYPT1 isoforms regulates MLC phosphatase activity

The vascular response to NO is due, in part, to a Ca(2+) independent activation of myosin light chain (MLC) phosphatase, a trimeric enzyme of 20kDa, 38kDa catalytic and 110-130kDa myosin targeting (MYPT1) subunits. Alternative mRNA splicing produces MYPT1 isoforms that differ by the presence or absence of a central insert (CI) and a leucine zipper (LZ), and the presence of a LZ+ MYPT1 isoform is important for protein kinase G (PKG) mediated activation of MLC phosphatase. This study was designed to determine the molecular basis for the differential sensitivity of the vasculature to NO. Our results demonstrate that the presence of the MYPT1 LZ domain is required for PKG to both phosphorylate MYPT1 at S668 and activate MLC phosphatase. Further for LZ+ MYPT1 isoforms, an S668A MYPT1 mutation prevents the PKG mediated, Ca(2+) independent activation of MLC phosphatase. These data demonstrate that differential PKG mediated S668 phosphorylation of LZ+/LZ- MYPT1 isoforms could be important for determining the diversity in the sensitivity of the vasculature to NO mediated vasodilatation. Thus, the relative expression of LZ+/LZ- MYPT1 isoforms, in part, defines the vascular response to NO and NO based vasodilators, and therefore, plays a role in the regulation of vascular tone in both health and disease.