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PDBsum entry 1i4t
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Signaling protein
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PDB id
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1i4t
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Contents |
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* Residue conservation analysis
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References listed in PDB file
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Key reference
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Title
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The structural basis of arfaptin-Mediated cross-Talk between rac and arf signalling pathways.
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Authors
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C.Tarricone,
B.Xiao,
N.Justin,
P.A.Walker,
K.Rittinger,
S.J.Gamblin,
S.J.Smerdon.
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Ref.
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Nature, 2001,
411,
215-219.
[DOI no: ]
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PubMed id
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Abstract
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Small G proteins are GTP-dependent molecular switches that regulate numerous
cellular functions. They can be classified into homologous subfamilies that are
broadly associated with specific biological processes. Cross-talk between small
G-protein families has an important role in signalling, but the mechanism by
which it occurs is poorly understood. The coordinated action of Arf and Rho
family GTPases is required to regulate many cellular processes including lipid
signalling, cell motility and Golgi function. Arfaptin is a ubiquitously
expressed protein implicated in mediating cross-talk between Rac (a member of
the Rho family) and Arf small GTPases. Here we show that Arfaptin binds
specifically to GTP-bound Arf1 and Arf6, but binds to Rac.GTP and Rac.GDP with
similar affinities. The X-ray structure of Arfaptin reveals an elongated,
crescent-shaped dimer of three-helix coiled-coils. Structures of Arfaptin with
Rac bound to either GDP or the slowly hydrolysable analogue GMPPNP show that the
switch regions adopt similar conformations in both complexes. Our data highlight
fundamental differences between the molecular mechanisms of Rho and Ras family
signalling, and suggest a model of Arfaptin-mediated synergy between the Arf and
Rho family signalling pathways.
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Figure 1.
Figure 1: Sequence homology and structure of Arfaptin. a,
Sequence homology between five Arfaptin homologues. The  -helical
elements derived from the crystal structure are indicated and
coloured as in b. The N terminus of the Arfaptin fragment used
in this study, which encompasses the entire predicted
coiled-coil region of these molecules, is indicated by the black
triangle. Residues absolutely conserved between the six Arfaptin
homologues are indicated by blue circles. b, Three orthogonal
views of the Arfaptin dimer in ribbons representation26. Top,
Arfaptin dimer viewed along its dyad axis. Helices A, B and C of
each monomer are red, green and blue, respectively, and the
dimer-related helices are labelled A', B' and C'. Middle,
Arfaptin dimer viewed along the long axis, illustrating the
cavity created by the five-helix barrel at the dimer interface.
Bottom, Arfaptin viewed with its long axis horizontal and the
dyad axis vertical, showing the crescent-like shape of the dimer.
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Figure 3.
Figure 3: Structure of the Rac -Arfaptin complex. a, Rac
-Arfaptin complex shown in ribbons representation, with Arfaptin
in the same orientation as shown in Fig. 1b, bottom. Helices of
the Rac are red,  -strands
are green and the nucleotide is in yellow ball-and-stick
representation. One monomer of the Arfaptin dimer is shown in
blue, the other in pink. b, Arfaptin -Rac interface shown as an
'open book' representation. The C positions
of residues that interact are indicated by white spheres.
Residue type and number are shown in black type with interacting
residues from the other protein indicated in red
(hydrogen-bonding interactions) or green (non-polar/van der
Waals interactions). Asterisk denotes His 57 from Arfaptin
molecule 'B' of the dimer (blue); all other Arfaptin residues
are contributed from molecule 'A' (yellow). The two 'switch'
regions of Rac are highlighted in red.
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The above figures are
reprinted
by permission from Macmillan Publishers Ltd:
Nature
(2001,
411,
215-219)
copyright 2001.
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