Crystal Structure of the ATPPase Subunit and Its Substrate-Dependent Association with the GATase Subunit: A Novel Regulatory Mechanism for a Two-Subunit-Type GMP Synthetase from Pyrococcus horikoshii OT3

Guanosine 5′-monophosphate synthetase(s) (GMPS) catalyzes the final step of the de novo synthetic pathway of purine nucleotides. GMPS consists of two functional units that are present as domains or subunits: glutamine amidotransferase (GATase) and ATP pyrophosphatase (ATPPase). GATase hydrolyzes glutamine to yield glutamate and ammonia, while ATPPase utilizes ammonia to convert adenyl xanthosine 5′-monophosphate (adenyl-XMP) into guanosine 5′-monophosphate. Here we report the crystal structure of PH-ATPPase (the ATPPase subunit of the two-subunit-type GMPS from the hyperthermophilic archaeon Pyrococcus horikoshii OT3). PH-ATPPase consists of two domains (N-domain and C-domain) and exists as a homodimer in the crystal and in solution. The N-domain contains an ATP-binding platform called P-loop, whereas the C-domain contains the xanthosine 5'-monophosphate (XMP)-binding site and also contributes to homodimerization. We have also demonstrated that PH-GATase (the glutamine amidotransferase subunit of the two-subunit-type GMPS from the hyperthermophilic archaeon P. horikoshii OT3) alone is inactive, and that all substrates of PH-ATPPase except for ammonia (Mg²⁺, ATP and XMP) are required to stabilize the active complex of PH-ATPPase and PH-GATase subunits.