{PDOC51882}
{PS51882; G_ALPHA}
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* G-alpha domain profile *
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Signaling via  heterotrimeric  G  proteins, composed of alpha, beta, and gamma
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 alpha subunit. Resultant GTP binding
causes the  dissociation  of  G-alpha  from  the  G-beta/gamma  dimer, and the
activated G-alpha  and  G-beta/gamma interact with downstream target proteins,
leading to  numerous cellular responses. Although at least 23 G-alpha subunits
have been  identified  in mammalian systems, the genome of Arabidopsis encodes
only one G protein alpha subunit. However, the Arabidopsis genome also encodes
several extra-large  G (XLG) proteins besides the prototypical G protein alpha
subunit. Each  XLG  protein  has  a C-terminal G-alpha 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-alpha subunits in their dependence on Ca(2+), rather
than Mg(2+), as a cofactor [1,2,3,4].

The G-alpha  domain  is  composed  of a nucleotide-binding subdomain common to
members of  the  GTPase  superfamily,  into  which an alpha-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  beta-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 alpha-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-alpha domain.

-Sequences known to belong to this class detected by the profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.
-Last update: January 2019 / First entry.

[ 1] Lambright D.G., Noel J.P., Hamm H.E., Sigler P.B.
     "Structural determinants for activation of the alpha-subunit of a
     heterotrimeric G protein."
     Nature 369:621-628(1994).
     PubMed=8208289; DOI=10.1038/369621a0
[ 2] Lee Y.-R., Assmann S.M.
     "Arabidopsis thaliana 'extra-large GTP-binding protein' (AtXLG1): a
     new class of G-protein."
     Plant Mol. Biol. 40:55-64(1999).
     PubMed=10394945
[ 3] Ding L., Pandey S., Assmann S.M.
     "Arabidopsis extra-large G proteins (XLGs) regulate root
     morphogenesis."
     Plant J. 53:248-263(2008).
     PubMed=17999646; DOI=10.1111/j.1365-313X.2007.03335.x
[ 4] Heo J.B., Sung S., Assmann S.M.
     "Ca2+-dependent GTPase, extra-large G protein 2 (XLG2), promotes
     activation of DNA-binding protein related to vernalization 1 (RTV1),
     leading to activation of floral integrator genes and early flowering
     in Arabidopsis."
     J. Biol. Chem. 287:8242-8253(2012).
     PubMed=22232549; DOI=10.1074/jbc.M111.317412

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