PROSITE documentation PDOC51882

G-alpha domain profile

Signaling via heterotrimeric G proteins, composed of α, β, and γ subunits, is highly conserved in eukaryotes. Classically, G protein-mediated signaling is initiated by ligand binding to a cell surface G protein-coupled receptor, which has seven transmembrane domains. Ligand binding activates receptor-mediated GDP/GTP exchange on the α subunit. Resultant GTP binding causes the dissociation of G-α from the G-β/γ dimer, and the activated G-α and G-β/γ interact with downstream target proteins, leading to numerous cellular responses. Although at least 23 G-α subunits have been identified in mammalian systems, the genome of Arabidopsis encodes only one G protein α subunit. However, the Arabidopsis genome also encodes several extra-large G (XLG) proteins besides the prototypical G protein α subunit. Each XLG protein has a C-terminal G-α domain and a ~400 amino acid N-terminal extension, which includes a nuclear localization sequence (NLS) and a cysteine-rich region. XLG proteins appear to be unique to the plant kingdom and, despite their unusual structure, are bona fide G proteins, that specifically bind GTP and possess GTPase activity. However, the XLGs are distinct from canonical G-α subunits in their dependence on Ca(2+), rather than Mg(2+), as a cofactor [1,2,3,4].

The G-α domain is composed of a nucleotide-binding subdomain common to members of the GTPase superfamily, into which an α-helical subdomain, unique to the highly homologous family of heterotrimeric G-proteins, is inserted (see <PDB:1TAG>). The GTPase subdomain consists of five helices surrounding a six-stranded β-sheet with five strands running parallel and one running antiparallel to the others. Within the GTPase subdomain, five regions, designated G-1 to G-5, are implicated in guanine nucleotide binding and hydrolysis. The G-1, G-2, and G-3 regions interact with the phosphate groups of the bound guanine nucleotide and coordinate Mg(2+) or Ca(2+) to stabilize the guanine nucleotide binding structure. The G-4 region interacts with the guanine ring and the G-5 interaction with the guanine nucleotide is indirect. The helical subdomain, which is inserted between G-1 and G-2, has an entirely α-helical secondary structure with one long central helix surrounded by five shorter helices and is linked to the GTPase subdomain by two extended strands. Between these two subdomains lies a deep cleft within which the nucleotide is tightly bound. The helical subdomain is postulated to be involved in activation of GTPase activity and in inhibition of guanine nucleotide dissociation [1,2].

The profile we developed covers the entire G-α domain.