Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 187

modes of binding of dilysine-containing proteins and p24 proteins are further
highlighted by the fact that binding of p24 proteins to coatomer is dependent on
the diphenylalanine motif and is modulated by the di-basic motif (Dominguez
et al. 1998 ; Fiedler et al. 1996 ; Sohn et al. 1996 ). Cycling of the p24 proteins
between Golgi and ER (Nickel et al. 1997 ) and their binding sites on coatomer
different from cargo binding sites, as well as induction of polymerisation of
coatomer upon binding of p23 or p24 dimers (Langer et al. 2008 ; Reinhard
et al. 1999 ), qualifies these type-I transmembrane proteins as membrane machinery
of COPI vesicles. This view is further supported by studies in yeast (Aguilera-
Romero et al. 2008 ; Stamnes et al. 1995 ).
Arginine-based motifs are another class of ER-retrieval signals with the consen-
sus sequence

/RRxR. These sorting signals are involved in the quality control
of multimeric membrane proteins by maintaining unassembled subunits in the ER,
and become inactivated upon correct assembly to allow export from the ER
(Margeta-Mitrovic et al. 2000 ; Zerangue et al. 1999 ). Binding of coatomer to
different arginine-based sorting signals implies that ER localisation can be
achieved due to COPI-dependent

ʦ / ʨ

retrieval (Brock et al. 2005 ; O'Kelly

et al. 2002 ; Yuan et al. 2003 ). More recently -COP were identified

as coatomer subunits involved in the recognition of arginine-based motifs
(Michelsen et al. 2007 ).
Additional less well-characterised sorting signals include the

ʲ -COP and ʴ

ʴ L-motif, which

was identified to bind to -COP (Cosson et al. 1998 ), and a motif FxxxFxxxFxxLL

found in Dopamine 1 receptor, which interacts with

ʴ

-COP (Bermak et al. 2002 ).
More recent studies imply that correct localisation of Golgi glycosyl-transferases
(lacking known COPI binding motifs) is achieved by binding to cytosolic Vps74p
in yeast and its mammalian homologue GOLPH3, which links Golgi enzymes via a
conserved arginine motif to coatomer (Schmitz et al. 2008 ; Tu et al. 2008 , 2012 ).
Soluble ER-resident proteins contain the C-terminal motif KDEL (HDEL in
yeast), which prevents their secretion (Munro and Pelham 1987 ) by an ER-retrieval
mechanism from post-ER compartments (Dean and Pelham 1990 ; Pelham 1988 ).
The integral membrane protein Erd2p was initially identified as receptor mediating
retrieval of proteins with the motif HDEL (Semenza et al. 1990 ). Vertebrates have
three homologues of Erd2p (Lewis and Pelham 1990 , 1992b ; Raykhel et al. 2007 ).
At steady state the KDEL receptors (KDELr) are localised to the ERGIC and cis -
Golgi (Griffiths et al. 1994 ). Binding to KDEL signal-containing protein induces its
redistribution to the ER (Lewis and Pelham 1992a ). A higher pH in the ER is
believed to dissociate the cargo and to allow recycling of the free KDELr (Wilson
et al. 1993 ). Colocalisation of KDELr with coatomer and its presence in purified
COPI vesicles (Griffiths et al. 1994 ; Sonnichsen et al. 1996 ), a requirement for Arf
and coatomer in an in vitro retrieval assay (Spang and Schekman 1998 ), and
inhibition of ER targeting of endocytosed cholera toxin by antibodies against
ʲ

ʴ

-COP and p23 (Majoul et al. 1998 ) together imply a role of COPI in the retrograde
transport of the KDELr.
Efficient incorporation of vectorially transported proteins into COPI vesicles
was proposed to require GTP hydrolysis (Lanoix et al. 1999 ; Malsam et al. 1999 ;