Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 182

9.2.2 COPII Vesicles and Their Machinery
The most gene products required for the formation of COPII-coated vesicles were
initially identified by a screen in S. cerevisiae for temperature-sensitive secretion
mutants ( SEC ) with a defect in protein secretion and cell growth (Novick
et al. 1980 ). Subsequent studies revealed striking similarity of the secretory path-
ways in budding yeast and mammalian cell (Novick et al. 1981 ). Morphological
analysis classified a set of mutants causing accumulation of excess ER membranes
(class I mutants) ( SEC12 , SEC13 , SEC16 , and SEC23 ), and another class that led to
the accumulation of ER-derived vesicles (class II mutants) ( SEC17 , SEC18 , and
SEC22 ) (Kaiser and Schekman 1990 ).
Combination of mutant cell lysates deficient in ER-to-Golgi transport with
in vitro ER-to-Golgi transport assays allowed characterisation of proteins involved.
Hereby, a new class of vesicular carriers was identified as ER-to-Golgi transport
intermediates (Baker et al. 1988 ; Groesch et al. 1990 ; Rexach and Schekman 1991 ;
Ruohola et al. 1988 ). Biochemical characterisation revealed that Sec23p (Hicke and
Schekman 1989 ) together with Sec24p (Hicke et al. 1992 ) operates in a
heterodimeric complex required for the formation of ER-derived transport vesicles
(Hicke et al. 1992 ). Subsequently, Sec13p was found to operate in a
heterooligomeric complex with Sec31p (Pryer et al. 1993 ; Salama et al. 1993 ).
Eventually, it became clear that the five cytosolic proteins Sar1p, the Sec23/24p,
and the Sec13/31p subcomplexes are required and sufficient for the formation of
ER-derived vesicles in vitro (Barlowe et al. 1994 ; Salama et al. 1993 ).
The presence of proteins involved in vesicle fusion (Sec22p and Bet1p) and the
absence of ER-resident proteins (Kar2p/BiP) in vesicles generated in vitro (Rexach
et al. 1994 ; Salama et al. 1993 ), combined with the competence of these vesicles to
fuse with Golgi-membranes (Barlowe et al. 1994 ; Rexach et al. 1994 ) established
that this minimal set of proteins is capable to form functional ER-to-Golgi transport
intermediates. Electron microscopy revealed 60-65 nm vesicles with an electron-
dense coat, morphologically distinct from Clathrin-coated and Golgi-derived 'non-
clathrin-coated' vesicles. Consequently, the latter were renamed to COPI vesicles
and the vesicles formed by the five cytosolic proteins Sar1p, Se23/24p, and Sec13/
31p were termed COPII vesicles (Barlowe et al. 1994 ).
9.2.3 The COPII Coat Subcomplexes Sec23/24 and Sec13/31
The Sec23/24 complex is a heterodimeric complex. Single-particle EM analysis
revealed a bow-tie-shaped complex, of 17 nm in length, comprising two globular
domains of similar type (Lederkremer et al. 2001 ). X-ray crystallographic analysis
disclosed a positively charged concave membrane-proximal surface of the Sec23/
24 complex, which seems to match the curvature of a COPII vesicle with a typical
diameter of 60 nm. The subunits Sec23 and Sec24 have striking similar folds with