Guanylate kinase <p>Guanylate kinase (<db_xref db="EC" dbkey="2.7.4.8"/>) (GK) [<cite idref="PUB00003284"/>] catalyzes the ATP-dependent phosphorylation of GMP into GDP. It is essential for recycling GMP and indirectly, cGMP. In prokaryotes (such as <taxon tax_id="562">Escherichia coli</taxon>), lower eukaryotes(such as yeast) and in vertebrates, GK is a highly conserved monomeric protein of about 200 amino acids. GK has been shown [<cite idref="PUB00000870"/>, <cite idref="PUB00001440"/>, <cite idref="PUB00005396"/>] to be structurally similar to protein A57R (or SalG2R) from various strains of <taxon tax_id="10245">Vaccinia virus</taxon>.</p> <p>Proteins containing one or more copies of the DHR domain, an SH3 domain as well as a C-terminal GK-like domain, are collectively termed MAGUKs (membrane-associated guanylate kinase homologs) [<cite idref="PUB00003556"/>], andinclude Drosophila lethal(1)discs large-1 tumor suppressor protein (gene dlg1); mammalian tight junction protein Zo-1; a family of mammalian synaptic proteins that seem to interact with the cytoplasmic tail of NMDA receptor subunits (SAP90/PSD-95, CHAPSYN-110/PSD-93, SAP97/DLG1 and SAP102); vertebrate 55kDa erythrocyte membrane protein (p55); <taxon tax_id="6239">Caenorhabditis elegans</taxon> protein lin-2; rat protein CASK; and human proteins DLG2 and DLG3. There is an ATP-binding site (P-loop) in the N-terminal section of GK, which is not conserved in the GK-like domain of the above proteins. However these proteins retain the residues known, in GK, to be involved in the binding of GMP.</p>