Guanylate kinase catalyzes the ATP-dependent phosphorylation of GMP into GDP. It is essential for recycling GMP and indirectly, cGMP. In prokaryotes (such as Escherichia coli), lower eukaryotes (such as yeast) and in vertebrates, GK is a highly conserved monomeric protein of about 200 amino acids. GK has been shown to be structurally similar to protein A57R (or SalG2R) from various strains of Vaccinia virus.
^Bryant PJ, Woods DF (February 1992). "A major palmitoylated membrane protein of human erythrocytes shows homology to yeast guanylate kinase and to the product of a Drosophila tumor suppressor gene". Cell68 (4): 621–2. doi:10.1016/0092-8674(92)90136-Z. PMID1310897.