Ran in Mitosis - Ras Superfamily Small G Proteins: Biology and Mechanisms 2 - page 133

of proteins in mitosis. The specific kinetochore localisation of RanBP2/RanGAP-
SUMO-Ubc9 requires RanGTP and exportin 1, which form an export-like complex
with RanBP2/RanGAP-SUMO-Ubc9. The specific localisation mediates key
mitotic functions of the complex in proper MT to kinetochore attachment (Joseph
et al. 2002 , 2004 ; Salina et al. 2003 ; Swaminathan et al. 2004 ; Arnaoutov
et al. 2005 ).
Interestingly, regulation by Cdk1 overlays the mitotic function of exportin 1:
Cdk1 phosphorylates exportin 1 on Serine 391 to strengthen association with the
RanGAP-RanBP2 complex and thus integrates RanGTP and Cdk1 regulation of
spindle formation (Wu et al. 2013 ). Recent data suggest that kinetochore localiza-
tion of RanGAP1 also depends on RanGTP through release of factors from importin
ʲ

(Roscioli et al. 2012 ), indicating that both principal mechanisms to regulate Ran
functions in mitosis cooperate on the same protein complex. The Nup107 complex
that in human cells consists of seven proteins also localises to kinetochores in
human cells (Belgareh et al. 2001 ; Loiodice et al. 2004 ). Immunodepletion of this
complex from Xenopus egg extracts leads to almost complete loss of MTs around
sperm nuclei (Orjalo et al. 2006 ) and siRNA knock-down to deplete the complex
from mitotic cells causes chromosome alignment failure (Zuccolo et al. 2007 ).
Impaired localisation of the chromosome passenger complex including Aurora B
kinase after depletion of SEH1, a critical component of the complex, contributes to
alignment defects (Platani et al. 2009 ) as well as the Nup107-dependent kineto-
chore targeting of the -tubulin ring complex for proper nucleation of kinetochore

MTs (Mishra et al. 2010 ). Consistently, previous data had shown that RanGTP
accumulation supports local MT nucleation at kinetochores (Torosantucci
et al. 2008 ). The Nup107 complex in general and its recently identified nuclear-
associated protein ELYS/Mel28 in particular pioneer the re-assembly of NPCs after
mitosis (Rasala et al. 2006 ; Franz et al. 2007 ; Doucet et al. 2010 ). ELYS/Mel28 acts
as a Ran-regulated scaffolding factor for further NPC protein association
(Fernandez and Piano 2006 ; Franz et al. 2007 ). This opens up an intriguing
licensing mechanism, which couples interphase and mitosis. At the onset of mitosis,
the Nup107 complex needs to be released from NPCs to allow spindle formation. In
turn, dissociation of the complex from kinetochores after mitosis is a prerequisite
for postmitotic NPC assembly. Very recently, we could show that ELYS/Mel28
also carries out an essential function in spindle formation as a Ran-regulated MAP.
Specific immunodepletion of ELYS/Mel28 leaves most of the Nup107 complex in
Xenopus egg extracts but completely abolishes any Ran-driven MT assembly
around chromatin and Ran-dependent stimulation of centrosomal nucleation
(Yokoyama et al., in revision). ELYS/Mel28 therefore not only is the key factor
to control timely, apparently mutually exclusive spindle formation and NPC
assembly, but also puts both processes under the strict control of the Ran system.
Experimental evidence from Xenopus cell-free extracts documents that nuclear
lamins, which stabilise the nuclear structure during interphase, may support mitotic
functions by wrapping a shell-like matrix around the spindle under the control of
RanGTP (Tsai et al. 2006 ).

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