Toroid-shaped homohexamer that has a central cavity of about 38 Angstroms diameter. Is organized into three distinct structural domains with interconnecting topological connectivities. The two NIF3 domains at the N- and C-terminus of the protein have the same overall fold as canonical NIF3-like proteins. The middle region of the polypeptide (residues 126-236) bulges out between the two NIF3 domains and is structured as a classical PII-like fold. The two entries to the central hollow space are capped by the two PII-like domain trimers. The trimeric PII domain may play a ligand induced signaling role and probably regulates the function of the NIF3-like domains. Belongs to the GTP cyclohydrolase I type 2/NIF3 family.