G proteins couple receptors of extracellular signals to intracellular signaling pathways. The G protein alpha subunit binds guanyl nucleotide and is a weak GTPase. A set of residues that are unique to G-alpha as compared to its ancestor the Arf-like family form a ring of residues centered on the nucleotide binding site [3]. A Ggamma is found fused to an inactive Galpha in the Dictyostelium protein gbqA [3].

1: Structures of active conformations of Gi alpha 1 and the mechanism of GTP hydrolysis. Coleman DE, Berghuis AM, Lee E, Linder ME, Gilman AG, Sprang SR; Science 1994;265:1405-1412. PMID:8073283

2: How G proteins work: a continuing story. Coleman DE, Sprang SR; Trends Biochem Sci 1996;21:41-44. PMID:8851656

3: Comparative genomics uncovers novel structural and functional features of the heterotrimeric GTPase signaling system. Anantharaman V, Abhiman S, de Souza RF, Aravind L; Gene. 2011;475:63-78. PMID:21182906