Dimerization/Docking domain of Cyclic GMP-dependent Protein Kinase I beta
Cyclic GMP-dependent Protein Kinase I (PKG1 or cGKI) is a Serine/Threonine Kinase (STK), catalyzing the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. cGKI exists as two splice variants, cGKI-alpha and cGKI-beta. They contain an N-terminal regulatory domain containing a dimerization/docking region and an autoinhibitory pseudosubstrate region, two cGMP-binding domains, and a C-terminal catalytic domain. Binding of cGMP to both binding sites releases the inhibition of the catalytic center by the pseudosubstrate region, allowing autophosphorylation and activation of the kinase. cGKI is a soluble protein expressed in all smooth muscles, platelets, cerebellum, and kidney. It is involved in the regulation of smooth muscle tone, smooth cell proliferation, and platelet activation. The dimerization/docking (D/D) domain is a leucine/isoleucine zipper that mediates both homodimerization and interaction with isotype-specific G-kinase-anchoring proteins (GKAPs). The D/D domain of the two variants (alpha and beta) differ, allowing for their targeting to different subcellular compartments and intracellular substrates. cGKI-beta binds specifically to inositol triphosphate receptor-associated PKG substrate (IRAG) and the transcriptional regulator TFII-I. Phosphorylation of IRAG by cGKI-beta contributes to smooth muscle relaxation while phosphorylation of TFII-I modulates its co-activator functions for serum response factor and Smad transcription factors.
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