Unzipping the Role of Myosin Light Chain Phosphatase in Smooth Muscle Cell Relaxation

Recently, it has been hypothesized that myosin light chain (MLC) phosphatase is activated by cGMP-dependent protein kinase (PKG) via a leucine zipper-leucine zipper (LZ-LZ) interaction through the C-terminal LZ in the myosin-binding subunit (MBS) of MLC phosphatase and the N-terminal LZ of PKG (Surks, H. K., Mochizuki, N., Kasai, Y., Georgescu, S. P., Tang, K. M., Ito, M., Lincoln, T. M., and Mendelsohn, M. E. (1999) Science 286, 1583-1587). Alternative splicing of a 3′-exon produces a LZ⁺ or LZ ^- MBS, and the sensitivity to cGMP-mediated smooth muscle relaxation correlates with the relative expression of LZ⁺/LZ^- MBS isoforms (Khatri, J. J., Joyce, K. M., Brozovich, F. V., and Fisher, S. A. (2001) J. Biol. Chem. 276, 37250-37257). In the present study, we determined the effect of LZ⁺/LZ^- MBS isoforms on cGMP-induced MLC₂₀ dephosphorylation. Four avian smooth muscle MBS-recombinant adenoviruses were prepared and transfected into cultured embryonic chicken gizzard smooth muscle cells. The expressed exogenous MBS isoforms were shown to replace the endogenous isoform in the MLC phosphatase holoenzyme. The interaction of type I PKG (PKGI) with the MBS did not depend on the presence of cGMP or the MBS LZ. However, direct activation of PKGI by 8-bromo-cGMP produced a dose-dependent decrease in MLC₂₀ phosphorylation (p < 0.05) only in smooth muscle cells expressing a LZ⁺ MBS. These results suggest that the activation of MLC phosphatase by PKGI requires a LZ ⁺ MBS, but the binding of PKGI to the MBS is not mediated by a LZ-LZ interaction. Thus, the relative expression of LZ⁺/LZ^- MBS isoforms could explain differences in tissue sensitivity to NO-mediated vasodilatation.