Small G Proteins: Arf Family GTPases in Vesicular Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 183

five distinguishable domains: (1) a -barrel domain, (2) a zinc finger domain, (3) a

trunk domain, (4) an all-helical region, and (5) a carboxy-terminal gelsolin-like
domain. The trunk domains of Sec23 and Sec24 form the interface of the
heterodimer (Bi et al. 2002 ).
The Sec23/24 complex is recruited to ER membranes by direct interaction of
Sar1 with the Sec23 subunit (Bi et al. 2002 ), which is the Sar1-specific GTPase
activating protein (Bi et al. 2002 ; Yoshihisa et al. 1993 ). Mammals possess two
isoforms Sec23A and Sec23B, with ~85 % sequence identity. Interestingly,
although these proteins are highly related, mutations in individual Sec23 isoforms
are linked to various human genetic diseases. Mutations in Sec23A result in cranio-
lenticulo-sutural dysplasia (CLSD) (Boyadjiev et al. 2006 ), while congenital
dyserythropoietic anaemia type II (CDAII) is caused by mutations in the Sec23B
isoform (Bianchi et al. 2009 ; Schwarz et al. 2009 ).
On the other hand, the Sec24 subunit is responsible for binding and thus
concentration of cargo proteins into nascent COPII vesicles (Miller et al. 2002 ,
2003 ; Mossessova et al. 2003 ). In S. cerevisiae two non-essential Sec24 homo-
logues: Iss1p/Sfb2p (Kurihara et al. 2000 ) and Lst1p/Sfb3p (Roberg et al. 1999 )
exist, which are ~55 % and ~23 % identical with Sec24p, respectively. Database
search and cloning of the corresponding mammalian cDNAs revealed the existence
of four homologues Sec24A, Sec24B, Sec24C, and Sec24D. The mammalian
homologues can be grouped into two classes: Sec24A/B and Sec24C/D. Within a
class, the isoforms are ~50 % identical, whereas Sec24A/B and Sec24C/D share
~20 % identity (Pagano et al. 1999 ; Tang et al. 1999 ).
The Sec13/31 subcomplex is a heterotetramer, consisting of two Sec13 and two
Sec31 subunits. Single-particle electron microscopy analysis revealed a structure
with an elongated shape of 28-30 nm length, comprising five distinguishable
globular domains connected by flexible joints (Lederkremer et al. 2001 ). The
molecular architecture of the complex was solved by X-ray crystallography. In
the complex the C-terminal

ʲ

-solenoid structures of two Sec31 subunits dimerise
tail to tail to form a rod-like structure, whereas the N-terminus of Sec31 forms a
seven-bladed ʲ -propeller. One Sec13 subunit each is wedged between the
ʲ -propeller and α -solenoid domain of one Sec31 subunit and forms a second
7-bladed

α

-propeller structure, in which one blade is contributed by the Sec31
subunit (Fath et al. 2007 ). The Sec13/31 subcomplex together with the Sec23/24
subcomplex (Antonny et al. 2003 ; Stagg et al. 2008 ), as well as the Sec13/31
subcomplex alone (Stagg et al. 2006 ) can self-assemble into COPII cage-like
particles similar to 'empty cages' of clathrin triskelia. Cryo-electron microscopic
analysis of self-assembled cages revealed that the COPII coat can adopt various
geometries (Stagg et al. 2006 , 2008 ). Fitting of the Sec13/31 crystal structures into
an electron microscopy density map revealed that the N-terminal

ʲ

-propeller
domains of Sec31 subunits form the contact points of the vertices. A hinge in the
central part formed by interlocked

ʲ

-solenoid structures might allow the coat to
adapt to various sizes (Fath et al. 2007 ).
Taken together the structural features and their location in cage structures
explain how the coat confers stability and at the same time functional flexibility

α