Ran in Nucleocytoplasmic Transport - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 116

Å

structures of the CRM1-NES-RanGTP complexes with a 2.1 resolution crystal

structure of unliganded yeast CRM1 (Xpo1p) shows how RanGTP binding alters
the conformation of CRM1 to facilitate NES binding (Saito and Matsuura 2013 ).
An internal loop of CRM1 (referred to as HEAT9 loop) is primarily responsible for
maintaining the NES-binding cleft in a closed conformation, preventing NES
binding in the absence of RanGTP and also shows that the C-terminal tail of
CRM1 stabilizes the autoinhibitory conformation of the HEAT9 loop, reinforcing
this auto-inhibition (Saito and Matsuura 2013 ).

6.8 Ran Binding to Other Components
of the Nucleocytoplasmic Transport Machinery

In addition to its interactions with karyopherins that are crucial for establishing the
direction of transport between the nuclear and cytoplasmic compartments, Ran also
interacts with Ran-binding protein 1 (RanBP1) and several nucleoporins that have
putative Ran-binding motifs, including Nup358 (RanBP2), Nup153, Nup1, and
Nup2. RanBP1 functions in the GTP hydrolysis step associated with the release
of Ran from karyopherins in the cytoplasm and it is possible that the Ran-binding
domains of Nup358 have an analogous function (Yaseen and Blobel 1999 ). The
Ran-binding domains of the nucleoporins located on the nucleoplasmic face of
NPCs (Nup153, Nup1, and Nup2) may function in accelerating the binding to Ran
to karyopherins as they reach the nucleus, albeit there does not appear to be direct
experiments evidence to support this hypothesis.
RanBP1 (yeast Yrb1) is a small, 23 kDa protein that binds Ran in the GTP-bound
state but not in the GDP-bound state. RanBP1 by itself does not activate
RanGTPase, but functions to increase the GTP hydrolysis induced by RanGAP1
by an order of magnitude (Bischoff et al. 1995 ). However, although RanBP1
functions to increase the efficiency of GTP hydrolysis on Ran, the structure of the
Ran-RanBP1-RanGAP complex (Seewald et al. 2002 ) shows that RanBP1 is
located away from the active site of Ran and detailed kinetic analysis shows that
RanBP1 does not influence the rate-limiting step of the reaction, which is the
cleavage of GTP and/or inorganic phosphate release. Instead RanBP1 influences
the dynamics of the Ran-RanGAP interaction, resulting in a 20-fold stimulation of
the already very fast association rate (Seewald et al. 2003 ). In yeast, RanBP1 forms
a complex with the nuclear protein export factor CRM1, RanGTP, and
NES-containing cargoes that is exported to the cytoplasm (K¨nzler et al. 2000 ).
In addition to promoting the dissociation of Ran from karyopherins in the cyto-
plasm, RanBP1 also promotes the dissociation of NES-containing cargos from
CRM1 in the cytoplasm (Kehlenbach et al. 1999 ). The 2.0 Å resolution crystal
structure of yeast CRM1:RanBP1:RanGTP complex, which is an intermediate in
the disassembly of the CRM1 nuclear export complex, shows that the Ran-binding
domain of RanBP1 in conjunction with the acidic C-terminal tail of Ran generates a