The Biology of Cyclic GMP-dependent Protein Kinases

Abstract

Basic Properties of cGMP-dependentProtein Kinase Isoforms cGKs 1 belong to the family of serine/threonine kinases and are present in a variety of eukaryotes ranging from the unicellular organism Paramecium to Homo sapiens (1, 2). Mammals have two cGK genes, prkg1 and prkg2, that encode cGKI and cGKII. The N terminus (the first 90 -100 residues) of cGKI is encoded by two alternatively spliced exons that produce the isoforms cGKI␣ and cGKI␤. The enzymes have a rod-like structure and are activated at submicromolar to micromolar concentrations of cGMP (3, 4). They are composed of three functional domains: an N-terminal (A) domain, a regulatory (R) domain, and a catalytic (C) domain (for details see Refs. 1 and 2). The regulatory domain contains two tandem cGMP-binding sites that interact allosterically and bind cGMP with high and low affinity. Occupation of both binding sites induces a large change in secondary structure (5) to yield a more elongated molecule (6, 7). The catalytic domain contains the MgATP-and peptide-binding pockets. Binding of cGMP to both sites in the R domain releases the inhibition of the catalytic center by the N-terminal autoinhibitory/pseudosubstrate site and allows the phosphorylation of serine/threonine residues in target proteins and in the N-terminal autophosphorylation site. Activation of heterophosphorylation may be preceded by autophosphorylation. Autophosphorylation increases the spontaneous activity of cGKI and cGKII (8 -11) and is initiated by the binding of low cGMP concentrations to the high affinity site of cGKI (12,13). In addition to controlling activation and inhibition of the catalytic center, the N terminus has two other functions: dimerization, cGKs are homodimers that are held together by a leucine zipper present in the N terminus, and targeting, the enzymes are targeted to different subcellular localizations by their N termini.Tissue Distribution cGKI is present in high concentrations (Ͼ0.1 M) in all smooth muscles, platelets, cerebellum, hippocampus, dorsal root ganglia, neuromuscular endplate, and kidney. Low levels have been identified in cardiac muscle, vascular endothelium, granulocytes, chondrocytes, and osteoclasts. The I␣ isozyme is found in lung, heart, dorsal root ganglia, and cerebellum. Together with the I␣ isozyme, the I␤ isozyme is highly expressed in smooth muscle, including uterus, vessels, intestine, and trachea (14). Platelets, hippocampal neurons, and olfactory bulb neurons contain mainly the I␤ isozyme (14). The I␣ and I␤ cGKs are soluble enzymes and interact with different proteins through their distinct N termini. cGKII is expressed in several brain nuclei, intestinal mucosa, kidney, adrenal cortex, chondrocytes, and lung (15)(16)(17)(18). cGKII is anchored at the plasma membrane by myristoylation of the N-terminal Gly-2 residue. Only the membrane-bound cGKII phosphorylates cystic fibrosis transmembrane conductance regulator (19). Roles of cGKIControl of Smooth Muscle Tone by cGKI-NO and other NOgenerating organic nitrates stimulate the soluble gua...

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