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Figure 1.
FIG. 1. Atomic structures of the LARG DH/PH domains and
their complex with RhoA. a, the uncomplexed LARG DH/PH domains.
In all figures, the DH domain is colored orange and the PH
domain is blue. The six helical segments of the DH domain (9)
are labeled, as are the  N1 and N2
helices of the novel N1/ N2 extension at the N
terminus of the domain. The side chains of Trp-769, which packs
in the hydrophobic core of the N1/ N2 extension, and
Glu-1023, which appears to contribute to PH domain-assisted
nucleotide exchange, are shown as stick models. Like other
RhoGEF PH domains, the LARG PH domain has an N-terminal
extension that begins with an -helix ( N),
followed by a  -strand ( N), and
then a 3[10] helix. The loop connecting N and the 3[10] helix is
highly variable and can contain long inserts (disordered in the
uncomplexed LARG DH/PH structure). The 3 and 4 strands
of RhoGEF PH domains are also longer, allowing them to form an
additional small -sheet with the N strand.
b, the LARG DH/PH-RhoA complex. With respect to panel a, The PH
domain has swung  30° downward to
engage RhoA (green). The side chains of residues that form a
conserved, solvent-exposed hydrophobic patch on the PH domain
are drawn as stick models in yellow. This patch forms similar
2-fold dimer interfaces in both the DH/PH and DH/PH-RhoA crystal
structures. Based on the position of the C terminus of RhoA
(which is geranylgeranylated in vivo), the putative phospholipid
binding surface of the PH domain and the flatness and positive
charge of the top surface of the complex, the plasma membrane is
predicted to run along the top of the panel. The LARG N-3[10]
loop becomes ordered upon the binding of RhoA and forms an
additional helix ( Nb). The
inset shows the tetramer observed in the asymmetric unit of the
LARG DH/PH-RhoA crystals, with the DH/PH domains rendered as
space-filling models and the RhoA chains as green tubes. c,
comparison of the DH/PH domains of LARG, intersectin (PDB code
1KI1 [PDB]
) and Dbs (1LB1 [PDB]
). The structures were aligned by superposition of their GTPase
substrates (not shown). The orientation of the LARG PH domain
with respect to the DH domain is most similar to that of Dbs,
whereas that of intersectin is rotated 18° away from the DH
domain and does not contact the GTPase substrate (10). The 6/ N helix
of Dbs is shorter than that of LARG, allowing its extended 3- 4 loop to
engage its GTPase substrate (not shown) (10). The conformation
of the N-terminal DH/PH domains of Trio is essentially the same
as that of Dbs (6). LARG, Dbs, and the N-terminal DH/PH domains
of Trio exhibit PH domain-assisted nucleotide exchange in vitro,
whereas intersectin does not.
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