Nh. Keep et al., A MODULATOR OF RHO-FAMILY G-PROTEINS, RHOGDI, BINDS THESE G-PROTEINS VIA AN IMMUNOGLOBULIN-LIKE DOMAIN AND A FLEXIBLE N-TERMINAL ARM, Structure, 5(5), 1997, pp. 623-633
Background: The rho family of small G proteins, including rho, rac and
cdc42, are involved in many cellular processes, including cell transf
ormation by ras and the organization of the actin cytoskeleton. Additi
onally, rac has a role in the regulation of phagocyte NADPH oxidase. G
uanine nucleotide dissociation inhibitors (GDIs) of the rhoGDI family
bind to these G proteins and regulate their activity by preventing nuc
leotide dissociation and by controlling their interaction with membran
es. Results: We report the structure of rhoGDI, determined by a combin
ation of X-ray crystallography and NMR spectroscopy. NMR spectroscopy
and selective proteolysis show that the N-terminal 50-60 residues of r
hoGDI are flexible and unstructured in solution. The 2.5 Angstrom crys
tal structure of the folded core of rhoGDI, comprising residues 59-204
, shows it to have an immunoglobulin-like fold, with an unprecedented
insertion of two short beta strands and a 3(10) helix. There is an unu
sual pocket between the beta sheets of the immunoglobulin fold which m
ay bind the C-terminal isoprenyl group of rac. NMR spectroscopy shows
that the N-terminal arm is necessary for binding rac, although it rema
ins largely flexible even in the complex. Conclusions: The rhoGDI stru
cture is notable for the existence of both a structured and a highly f
lexible domain, both of which appear to be required for the interactio
n with rac. The immunoglobulin-like fold of the structured domain is u
nusual for a cytoplasmic protein. The presence of equivalent cleavage
sites in rhoGDI and the closely related D4/Ly-GDI (rhoGDI-2) suggest t
hat proteolytic cleavage between the flexible and structured regions o
f rhoGDI may have a role in the regulation of the activity of members
of this family. There is no detectable similarity between the structur
e of rhoGDI and the recently reported structure of rabGDI, which perfo
rms the same function as rhoGDI for the rab family of small G proteins
.