Catalyzes the phosphorylation of GMP to GDP. Essential enzyme for recycling GMP and indirectly, cyclic GMP (cGMP) (PubMed:31201273). Involved in the cGMP metabolism in photoreceptors (By similarity). It may also have a role in the survival and growth progression of some tumors (PubMed:31201273). In addition to its physiological role, GUK1 is essential for convert prodrugs used for the treatment of cancers and viral infections into their pharmacologically active metabolites, most notably acyclovir, ganciclovir, and 6-thioguanine and its closely related analog 6-mercaptopurine (PubMed:197968, PubMed:6248551, PubMed:6306664). ATP + GMP = ADP + GDP Up-regulated by RD3. kcat is 58.5 sec(-1) with GMP as substrate. Monomer (PubMed:31201273). Interacts with RD3 (PubMed:29515371). Colocalizes with RD3 in photoreceptor inner segments and to a lesser extent in the outer plexiform layer. Widely expressed. Belongs to the guanylate kinase family.