Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 193

et al. 1995 ) and mammalian cells (Takida et al. 2008 ). Recognition of GPI-AP by
p24 requires GPI-anchor remodelling (Castillon et al. 2011 ; Fujita et al. 2011 ).
Release of GPI-APs from its adaptor is triggered by the more acidic pH in post-ER
compartments (Fujita et al. 2011 ). Individual Erv family proteins seem to be
involved in transport of the soluble cargo protein glycosylated pro-alpha-factor
(Belden and Barlowe 2001 ; Otte and Barlowe 2004 ), the plasma membrane glyco-
protein Axl2p (Powers and Barlowe 1998 , 2002 ), vacuolar alkaline phosphatase,
and the cis -Golgi localised manosyl transferase Ktr3p (Bue and Barlowe 2009 ; Bue
et al. 2006 ).
Secretion of collagen, which is too large to fit into a typical COPII vesicle, relies
on the cargo adaptors TANGO1 and its homologue cTAGE5. These accessory
proteins concomitantly promote the formation and loading of collagen carriers,
but do not enter the nascent vesicle (Saito et al. 2009 , 2011 ).
Although an overwhelming number of studies support a primary role of the
Sec24 subunit in cargo recognition, other coat subunits might also contribute to this
process (Giraudo and Maccioni 2003 ; Quintero et al. 2010 ).
More comprehensive reviews on cargo sorting into COPII vesicles can be found
elsewhere (Dancourt and Barlowe 2010 ; Gillon et al. 2012 ; Kinoshita et al. 2013 ;
Malhotra and Erlmann 2011 ; Venditti et al. 2014 ).
9.4.3 Scission of COPII Vesicles
Like Arf1, Sar1 exerts membrane curvature potentiating activity by shallow inser-
tion of its N-terminal amphipathic helix. Addition of purified Sar1 to artificial
membranes in the absence of the inner and outer COPII coat caused conversion
of liposomes into long tubules (Bielli et al. 2005 ; Lee et al. 2005 ). The extent of
tubulation is dramatically reduced when bulky hydrophobic residues of the
amphiphatic helix are mutated. Concomitantly, formation of free COPII vesicles
from native membranes is reduced, although the COPII coat proteins are recruited
to membranes. Likewise, a his-tagged N ʔ 23-Sar1 variant did not exhibit membrane
curvature potentiating activity when recruited to liposomes via nickel-chelating
lipids. Addition of Sec23/24, Sec13/31, and GMP-PNP to this system caused the
formation of COPII-coated buds, which fail to detach from the donor membrane.
Thus, like in the COPI system, a lack of membrane curvature potentiating activity
causes a scission arrest (Lee et al. 2005 ), and vesicle scission does not require GTP
hydrolysis (Adolf et al. 2013 ).
9.4.4 COPII Coat Stabilisation and Vesicle Uncoating
In the prevailing view GTP hydrolysis leads to a conformational change of the
small GTPase and thus retraction of its N-terminal amphipathic helix, dissociation