Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 192

the inner coat before Sec13/31 recruitment (Stagg et al. 2008 ). This idea is
supported by the observation that pre-budding complexes can oligomerise in
artificial planar bilayers (Tabata et al. 2009 ).
9.4.2 Cargo Sorting into COPII Vesicles
Efficient ER export of cycling machinery proteins and many secretory proteins
relies on signal-mediated sorting into COPII vesicles. Initially a complex of Sar1
and Sec23/24 was identified to bind selectively to secretory cargo and SNARE
proteins. This complex is an elementary functional unit of the COPII coat, also
referred to as pre-budding complex (Aridor et al. 1998 ; Kuehn et al. 1998 ; Springer
and Schekman 1998 ).
A variety of genetic, biochemical, and structural studies revealed multiple
independent cargo binding sites on Sec24 (Buchanan et al. 2010 ; Miller
et al. 2002 , 2003 ; Mossessova et al. 2003 ). The capacity to bind to diverse ER
export motifs is further expanded by Sec24 isoforms, with overlapping but also
distinct functions (Kurihara et al. 2000 ; Pagano et al. 1999 ; Peng et al. 2000 ;
Roberg et al. 1999 ; Tang et al. 1999 ).
The motif DxE was identified to be required for efficient ER export of VSV-G
(Nishimura and Balch 1997 ; Nishimura et al. 1999 ). Although this motif is in
general referred to as diacidic motif (D/ExD/E), additional neighbouring residues
are required for efficient ER export of VSV-G and other proteins (Sevier
et al. 2000 ). A series of structural studies led to the identification of several distinct
binding sites on Sec24, also referred to as A-, B-, and C-site. The motif YxxxNPF
found in the yeast SNARE Sed5p binds to the A-site, Sys1p containing an ExE
motif, as well as Bet1p containing an LxxLE motif, utilises the B-site, and the
SNARE Sec22 binds to a third independent binding site, also referred to as C-site,
(Mossessova et al. 2003 ). Crystallographic analysis and biochemical characterisa-
tion of the mammalian proteins revealed various Sec24 isoform-specific interac-
tions with ER-to-Golgi SNARE proteins. The SNAREs syntaxin5 and membrin/
GS27 bind to Sec24C and Sec24D via the motif IxM, whereas the SNARE Sec22b
binds to Sec24A and Sec24B via a conformational epitope (Mancias and Goldberg
2007 , 2008 ).
Another type of ER export signals are dihydrophobic motifs (FF, YY, LL, or FY)
found in the cytoplasmic tails of the transmembrane cargo adaptors ERGIC53/
Emp47p (Kappeler et al. 1997 ; Sato and Nakano 2002 ), p24 family proteins
(Dominguez et al. 1998 ; Fiedler et al. 1996 ), and the Erv41/46p complex (Otte
and Barlowe 2002 ). ERGIC53/Emp47p is described as a cargo adaptor for multiple
high mannose containing glycoproteins (Appenzeller et al. 1999 ; Moussalli
et al. 1999 ; Nichols et al. 1998 ; Vollenweider et al. 1998 ). Members of the p24
family are directly required for COPII-mediated export of glycosyl-
phosphatidylinositol-anchored proteins (GPI-APs) in both S. cerevisiae (Belden
and Barlowe 1996 ; Marzioch et al. 1999 ; Muniz et al. 2000 ; Schimmoller