Ran in Nucleocytoplasmic Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 114

6.6 Karyopherin-Based Nuclear Export Pathways

The nuclear export of both proteins and small RNAs (such a tRNA and microRNA)
is mediated by karyopherin-based export factors such as CRM1 (Xpo1 in yeast),
CAS (Cse1 in yeast), Exportin-t (Los1 in yeast), and Exportin-5. In contrast to the
nuclear import cycle, these transport factors bind their cargoes in the nucleus in
conjunction with RanGTP. The cargo:carrier:RanGTP complex then diffuses back
and forth through the pore transport channel until it is dissociated following
RanGAP-stimulated GTP hydrolysis in the cytoplasm. CRM1 serves as a general
protein nuclear export factor, whereas CAS is a specific export factor for importin-

α
(Kutay et al. 1997 ). Exportin-t mediates the export of tRNAs (Kutay et al. 1998 )
and Exportin-5 that of microRNAs (Lund et al. 2004 ). Crystal structures of these
nuclear export factors bound to representative cargos have established the basis for
their molecular recognition (reviewed by Conti et al. 2006 ; Stewart 2003 , 2009 ;
Cook and Conti 2010 ; Chook and S¨el 2011 ). Although most cargoes bind on the
inner concave face of the transport factors, CRM1 is an exception where specific
hydrophobic nuclear export sequences (NESs) instead bind on the exterior, convex,
surface in a groove formed between two of the

α

-helical HEAT repeats from which
the molecule is constructed (Dong et al. 2009a , b ; Monecke et al. 2009 ; Okada
et al. 2009 ; Cook and Conti 2010 ;G¨ ttler et al. 2010 ; Koyama and Matsuura 2010 ;
Saito and Matsuura 2013 ).

6.7 RanGTP Binding Alters Karyopherin Conformation
and Thus Affinity for Cargoes

The crystal structures of a considerable number of karyopherins alone or bound to
cargoes and/or RanGTP have shown that the conformation of their HEAT repeat
helicoid is sensitive to the presence of RanGTP and that these conformational states
alter the affinity of the karyopherin for its cargo (reviewed by Conti et al. 2006 ). In
the case of import transport factors such as importin- and transportin, RanGTP

binding appears to increase the helicoid pitch (Chook and Blobel 1999 , 2001 ; Lee
et al. 2005 ), whereas the converse appears to be true for export transport factors
such as Xpo1 (Monecke et al. 2009 ; Dong et al. 2009a ;G¨ ttler et al. 2010 ; Koyama
and Matsuura 2010 ), Cse1 (Matsuura and Stewart 2005 ; Cook et al. 2005 ), and Los1
(Cook et al. 2009 ), where RanGTP binding makes the helicoid more compact. In the
case of CRM1, these conformational changes appear to be augmented by the
binding of the tip of a long

β

-helix to a region on the convex underside of the
helicoid that lies under the NES-binding site (Dong et al. 2009b ; Fox et al. 2011 ). It
has been proposed that in all of these transport factors energy stored by distorting
the helicoid in the presence of RanGTP generated a “spring-loaded” molecule that
facilitates dissociation of complexes in the cytoplasm following GTP hydrolysis on
Ran (Matsuura and Stewart 2004 ; Lee et al. 2005 ; Conti et al. 2006 ).

α