Involved in maintaining the homeostasis of cellular nucleotides by catalyzing the interconversion of nucleoside phosphates. Has GTP:AMP phosphotransferase and ITP:AMP phosphotransferase activities. Does not accept ATP as phosphate donor. a ribonucleoside 5'-triphosphate + AMP = a ribonucleoside 5'-diphosphate + ADP Optimum pH is 8.5. Optimum temperature is 30 degrees Celsius. Active from 30 to 55 degrees Celsius. Monomer. Consists of three domains, a large central CORE domain and two small peripheral domains, NMPbind and LID, which undergo movements during catalysis. The LID domain closes over the site of phosphoryl transfer upon GTP binding. Assembling and disassembling the active center during each catalytic cycle provides an effective means to prevent GTP hydrolysis. Depending on the yeast strain, the GTP:AMP phosphotransferase is encoded by ADK2 with or without a single base pair deletion/insertion near the 3' end of the open reading frame, resulting in a long or a short form. The ADK2 long form (this entry) is also referred to as AKY3, while the short form has been named PAK3. A sequence of the short form can be found in strain S288c (AC P26364). The modified C-terminus in the long form contributes to protein folding and is critical for protein stability. Belongs to the adenylate kinase family. AK3 subfamily.