214 A. Rose
3.1
Spindle and Kinetochore Assembly
Most prominently, components of the nuclear envelope are tightly linked
with mitotic spindle assembly. In yeast and some protists, the spindle pole
bodies are embedded into the nuclear envelope. Additional nuclear envelope
proteins are involved in the regulation of spindle pole body formation. For
example, the yeast nuclear pore protein MLP2 has been found to promote
spindle pole body assembly (Niepel et al. 2005). Proteins of the nuclear pore
complexes and the nucleocytoplasmic transport machinery also play crucial
roles during mitotic spindle assembly in animals. InC. elegans, several nucle-
oporins are required for proper spindle orientation (Schetter et al. 2006).
In mammalian cells, the nuclear pore proteins RanBP2/Nup358 and Ran-
GAP redistribute to the kinetochores during mitosis where they are essential
for kinetochore assembly and function (Joseph et al. 2004; Salina et al. 2003).
The depletion of RanBP2/Nup358 from mammalian cells causes mitotic ar-
rest, suggesting a function in mitotic checkpoint control (Salina et al. 2003).
Another protein shuttling between the nuclear envelope and mitotic kine-
tochores, MEL-28/ELYS, is required for structural and functional integrity
of the nuclear envelope as well as chromosome condensation and kineto-
chore and spindle assembly inC. elegansegg cells (Fernandez and Piano
2006; Galy et al. 2006). MEL-28/ELYS has been implicated in postmitotic nu-
clear pore complex assembly, mirroring the mitotic roles of RanBP2/Nup358
(Franz et al. 2007; Rasala et al. 2006).
3.2
Mitotic Functions of the Ran Cycle
In animal cells, the Ran cycle and karyopherins, facilitators of nucleocyto-
plasmic transport during interphase, play important roles in spindle assembly
as well as vesicle fusion and nuclear envelope reassembly (Askajer et al. 2002;
Di Fiore et al. 2004). Disturbances of the proper ratios and localization of
proteins within the Ran cycle interfere with mitotic check point control (Ar-
naoutov and Dasso 2003; Li et al. 2003; Quimby et al. 2000). In plant cells, the
nuclear envelope protein and Ran cycle component RanGAP is redistributed
to the mitotic spindle and subsequently to the growing cell plate of the phrag-
moplast, linking nuclear envelope components to cytokinesis (Jeong et al.
2005; Pay et al. 2002). The functional significance of this localization is un-
clear, but it is likely that the plant Ran cycle plays roles in mitotic microtubule
assembly and vesicle fusion in analogy to its functions found in animal cells.
In the fungusAspergillus nidulans, the nuclear pore complexes disassemble
partially and the nuclear envelope becomes “leaky”, allowing cytoplasmic
RanGAP access to the nucleus during a short time window preceding nuclear
division (De Souza et al. 2004). Yeast RanGAPs contain nuclear export sig-