Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 181

subunits of the tetramer show weak but significant sequence homology to subunits
of the clathrin adaptor protein complexes (APs). The two large subunits ʲ -COP and

ʳ -COP are built of an N-terminal trunk domain and a C-terminal appendage domain

connected by an unstructured linker region, and share homology with the large
subunits of clathrin adaptor protein complexes (e.g.,

α ʲ

-adaptin and -adaptin)

ʴ ʶ

(Duden et al. 1991 ; Serafini et al. 1991b ). Likewise -COP and -COP share

ʼ ˃

homology with the medium ( -adaptins) subunits,

respectively (Cosson et al. 1996 ; Faulstich et al. 1996 ). Identification of a common
ancestor of subunits of the terameric subcomplex and APs reaffirms this view
(Schledzewski et al. 1999 ).
A related domain organisation between the subunits of the

-adaptins) and the small (

-COP
subcomplex and APs was revealed by yeast two-hybrid analysis (Eugster
et al. 2004 ; Faulstich et al. 1996 ). X-ray crystallisation analysis of the

ʲ / ʳ / ʴ / ʶ

-COP
appendage domain showed a striking structural similarity with the corresponding
domains of

ʳ

2-adaptin, the large subunits of the AP2 complex
(Hoffman et al. 2003 ; Watson et al. 2004 ). In addition,

α -adaptin and ʲ

ʶ -COP shows striking

resemblance to 2-adaptin, the small subunit of AP2 (Yu et al. 2009 ). Taken

together, the overall structure and the mode of membrane binding of the

˃

ʲ / ʳ /

ʴ -COP subcomplex and APs are related. Similar interactions between Arf1 and

subunits of coatomer and APs revealed by site-specific cross-linking (Austin
et al. 2000 , 2002 ; Sun et al. 2007 ; Zhao et al. 1997 , 1999 ), and by a most recently
solved X-ray crystal structure of Arf1 bound to the

/ ʶ

-COP subcomplex of
coatomer, together with biochemical characterisation of the Arf1 binding site in
the

ʳ / ʶ

-COP subcomplex (Yu et al. 2012 ), corroborate this view.
Two isoforms, each, were identified for the

ʲ / ʴ

ʳ -COP ( ʳ 1-COP and ʳ 2-COP) and

ʶ -COP ( ʶ 1-COP and ʶ 2-COP) subunit, which allow the assembly of four

heptameric coatomer isoforms ( ʳ 1 ʶ 1, ʳ 1 ʶ 2, ʳ 2 ʶ 1, and ʳ 2 ʶ 2). ʳ 1-COP and ʳ 2-COP

ʶ ʶ

are ~80 % identical, whereas 2-COP share ~75 % identity, and differ

largely by a 30 amino acids extension at the N-terminus of

1-COP and

ʶ

2-COP (Futatsumori
et al. 2000 ; Wegmann et al. 2004 ). Quantitative analysis revealed different amounts
of coatomer isoforms in mammalian cells. Furthermore, significant differences in
the localisation of coatomer isoforms were observed. Whereas ʳ 1 ʶ 1 and ʳ 1 ʶ 2 were
preferentially localised at the ERGIC and the cis -Golgi, the majority of 1 was

localised to the trans -Golgi. Taken together, existence and differential localisation
within a cell point to different roles of coatomer isoforms, similar to the various
roles of distinct APs in the secretory and endocytic pathways (Moelleken
et al. 2007 ; Wegmann et al. 2004 ). This view is further supported by identification
of different subpopulations of COPI vesicles containing distinct sets of cargo
proteins in living cells (Orci et al. 1997 ) and in vitro (Lanoix et al. 2001 ; Malsam
et al. 2005 ).

ʳ 2 ʶ