Steroid-sensitive gene 1 is a novel cyclic GMP-dependent protein kinase I substrate in vascular smooth muscle cells

Abstract

Background: Protein kinase GI (PKGI) regulates multiple cardiovascular processes, but its effectors remain incompletely understood. Results: We identified steroid-sensitive gene 1 (SSG1) as being regulated by PKG in vascular cells. Conclusion: SSG1 functions as a PKGI kinase target and interacting partner in cardiovascular tissues. Significance: Identifying new PKGI effectors may improve the understanding of the pathophysiology of cardiovascular diseases. NO, via its second messenger cGMP, activates protein kinase GI (PKGI) to induce vascular smooth muscle cell relaxation. The mechanisms by which PKGI kinase activity regulates cardiovascular function remain incompletely understood. Therefore, to identify novel protein kinase G substrates in vascular cells, a λ phage coronary artery smooth muscle cell library was constructed and screened for phosphorylation by PKGI. The screen identified steroid-sensitive gene 1 (SSG1), which harbors several predicted PKGI phosphorylation sites. We observed direct and cGMP-regulated interaction between PKGI and SSG1. In cultured vascular smooth muscle cells, both the NO donor S-nitrosocysteine and a trial natriuretic peptide induced SSG1 phosphorylation, and mutation of SSG1 at each of the two predicted PKGI phosphorylation sites completely abolished its basal phosphorylation by PKGI. We detected high SSG1 expression in cardiovascular tissues. Finally, we found that activation of PKGI with cGMP regulated SSG1 intracellular distribution. © 2013 by The American Society for Biochemistry and Molecular Biology, Inc.