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PI3K activation (Langille et al., 1999). However, it should be noted that
ARF1 controls paxillin recruitment to focal adhesions, raising the possibility
that ARF1 may be a target for ARNO-family GEFs at peripheral locations
(Norman et al., 1998). In addition, other studies found that ARNO associates
with the Golgi apparatus, and that this association requires a conserved
amino-terminal coiled-coil motif (Figure 10.1) (Lee and Pohajdak, 2000;
Mansour et al., 2002). In agreement with this result, over-expression of
ARNO GEFs resulted in disruption of the Golgi apparatus and impaired
protein transport through the secretory pathway (Monier et al., 1998; Franco
et al., 1998). The determination of the precise location(s) of ARNO should
help clarify these controversial issues, but awaits the development of specific
antibodies able to detect the endogenous protein.
Based on sequence conservation in the Sec7 domain we identified a new
family of proteins, called EFA6 (Exchange Factor for ARF6), that promote
GDP/GTP exchange specifically on ARF6 in a Bfa-insensitive manner
(Franco et al., 1999; Derrien et al., 2002). The EFA6-GEF family consists
of four members in humans (called EFA6A to D), all of which share a
conserved carboxy-terminal module. In addition to the Sec7 domain, this
module contains a PH domain of unknown ligand specificity required for
membrane association, as well as a putative coiled-coil motif (Figure 10.1). In
contrast to ARNO-family GEFs, over-expressed EFA6A is constitutively
bound to the plasma membrane where it induces the formation of actin-based
membrane ru es (Franco et al., 1999). EFA6-mediated actin cytoskeleton
reorganization requires the carboxy-terminal region including the coiled-coil
motif, and expression of this region alone (PH domain plus coiled-coil)
triggers lengthening of actin-rich microvillus-like structures at the surface of
fibroblastic cells (Franco et al., 1999; Derrien et al., 2002). Finally, and
similarly to the active GTP-bound ARF6Q67L mutant (see below, and
D'Souza-Schorey et al., 1995), EFA6A and EFA6B were found to inhibit
endocytosis of transferrin (Tfn). All together, these results are consistent with
a dual function of EFA6-family GEFs: activation of ARF6 at the plasma
membrane through their Sec7 domain and actin cytoskeleton reorganization.
Function of ARF6 in polarized membrane delivery at the
plasma membrane
Initial studies by D'Souza-Schorey and colleagues showed that Tfn receptors
(Tfn-Rs) accumulated at the surface of ARF6Q67L (GTP-bound)-expressing
CHO cells, while they were trapped intracellularly in cells expressing the
dominant-negative ARF6T27N mutant, indicating that ARF6 activation is
essential for Tfn-R recycling from early endocytic compartments to the cell
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