Fig. 1. Distribution of G protein components among eukaryotes.

(A) The indicated taxa are representative genomes. The presence of G protein elements in the indicated species or lineages is represented by red, blue, green, and yellow dots for genes encoding Gα, Gβγ, Opisthokont GPCRs, and RGS proteins, respectively. Lack of a dot signifies that those genes were not found. We organized the eukaryotes into six supergroups: Opisthokonta (containing C. owczarzaki and H. sapiens), Amoebozoa (containing D. discoideum), Archaeplastida (containing A. thaliana), Excavata (containing T. vaginalis), Chromalveolata (containing E. siliculosus), and Rhizaria. (B) Regulation of G protein activation in animals. Ligand-bound GPCR accelerates the dissociation of GDP from the G protein α subunit by changing the orientation of its helical domain. Gα hydrolyzes GTP, thereby inactivating itself. GTP hydrolysis is promoted by an RGS or other GAP protein. Nonreceptor GEFs, such as the protein Ric8 (resistance to inhibitors of cholinesterase), act as noncanonical and cytosolic GEFs. (C) Regulatory model of G protein signaling in Arabidopsis. The Arabidopsis Gα protein AtGPA1 rapidly releases its GDP as a result of spontaneous fluctuations between its Ras domain and helical domain. AtGPA1 slowly hydrolyzes its bound GTP; however, the membrane-localized 7TM-RGS protein AtRGS1 constitutively promotes GTP hydrolysis or acts as a GDI. (D) Frequency of TM helices in RGS domain–containing sequences among the 5169 sequences queried (see Materials and Methods). The TM helices were predicted with the membrane prediction software program SOSUI. (E) Distribution of 7TM-RGS proteins in Eukaryotes by a maximum-likelihood (ML) tree of 7TM-RGS proteins. Individual trees of the 7TM and RGS domains are shown in fig. S4 and were generated as described in Materials and Methods. The single genus Naegleria has 50 7TM-RGS proteins.