Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 189

9.3.4 Uncoating of COPI Vesicles

Uncoating of a vesicle is a prerequisite for membrane fusion. In the prevailing
model GTP hydrolysis by Arf, triggered by ArfGAPs, results in the release of Arf
and coatomer from the vesicle membrane (Reinhard et al. 2003 ; Tanigawa
et al. 1993 ). ArfGAP1 and its yeast homologue Gcs1p, as well as ArfGAP2/3 and
their yeast homologue Glo3p, were proposed to participate in COPI vesicle trans-
port with partially redundant functions. Yeast is viable after deletion of either
Gsc1p or Glo3p; however, efficient retrograde Golgi-to-ER transport requires
both proteins (Poon et al. 1999 ). In mammalian cells only a triple knock-down of
ArfGAP1-3 had a phenotypic effect, an increased level of membrane-bound Arf1,
and accumulation of ERGIC53, GM130, and coatomer in the ER-Golgi intermedi-
ate compartment. As a consequence retrograde Golgi-to-ER transport was blocked,
similar to experiments where

ʲ

-COP was knocked down (Saitoh et al. 2009 ).
Surprisingly, the non-catalytical domain controls the recruitment of ArfGAP1
and ArfGAP2/3 by different mechanisms. ArfGAP1 is recruited to the membrane in
a curvature-sensitive manner (Bigay et al. 2003 ), based on the presence of an
ArfGAP1 lipid packing sensor (ALPS) motif (Bigay et al. 2005 ). ArfGAP2/3 lack
this ALPS motif and consistently do not display curvature sensitivity. In contrast,
ArfGAP2/3 recruitment seems to be strictly dependent on a direct interaction with
coatomer (Frigerio et al. 2007 ; Kliouchnikov et al. 2009 ; Pevzner et al. 2012 ;
Weimer et al. 2008 ).
The exact functions of the different ArfGAP proteins are still elusive. Based on
the data described above it was proposed that the coatomer-independent ArfGAP1
might be involved in GTP hydrolysis-dependent uptake of cargo proteins into COPI
vesicles, whereas the coatomer-dependent ArfGAP2/3 might be the GAPs involved
in COPI vesicle uncoating (Popoff et al. 2011a ). Likewise, based on their properties
a role of ArfGAP1 in vesicle uncoating, and of ArfGAP2/3 in cargo sorting, was
proposed (Antonny 2011 ).
The tethering complex Dsl1, comprising Dsl1p, Dsl3p, and Tip20, is required for
Golgi-to-ER transport in yeast (Andag et al. 2001 ). Dsl1p directly interacts with
α -COP (Andag et al. 2001 ; Reilly et al. 2001 ). The finding that Dsl1p

utilises binding sites identical to those involved in interactions between coatomer
subunits, and the observation that cells defective in the Dsl1 complex accumulate
COPI-coated vesicles, led to the suggestion that this tethering complex plays an
additional role in vesicle uncoating (Zink et al. 2009 ).
Individual steps in the formation and uncoating of COPI-coated vesicles are
depicted in Fig. 9.1 .

-COP and ʴ