Open Mitosis: Nuclear Envelope Dynamics 215
nals, suggesting that they are capable of shuttling between the nucleus and the
cytoplasm. InSchizosaccharomyces pombe, RanGAP appears to be involved
in heterochromatin assembly, centromeric silencing and mitotic chromosome
segregation (Kusano et al. 2004; Nishijima et al. 2006). Together, these data
emphasize the dual role of components of the nucleocytoplasmic transport
machinery during mitotic events.
3.3
The Plant Nuclear Envelope as Microtubule Organizing Center
One key difference between higher plants and other organisms is the absence
of centrosomes or spindle pole bodies as microtubule organizing centers.
With the development from single cell, motile algae to multicellular, sessile
plants, plant cells have lost their centrioles and therefore depend on alterna-
tive means for microtubule nucleation during spindle formation. This func-
tion is taken over by the nuclear envelope serving as a site for microtubule
nucleation at the onset of mitosis (Bakhuizen et al. 1985; Stoppin et al. 1994;
reviewed by Canaday et al. 2000). Microtubule-associated proteins (MAPs)
modulate the microtubule nucleating activity of the plant nuclear envelope
(Stoppin et al. 1996).
The minimal core of the microtubule nucleation machinery consists of the
proteins Spc98/Spc97 andγ-tubulin. Together, these form theγ-tubulin ring
complex (γ-TuRC) to initiate microtubule nucleation (reviewed by Job et al.
2003). Homologs of all three components of theγ-TuRC can be identified in
plant genomes. Consistent with its function and the position of microtubule
nucleation sites in plant cells, the plant homolog of Spc98 has been found to
co-localize withγ-tubulin at the plant nuclear envelope (Erhardt et al. 2002).
It is unclear how the attachment of theγ-TuRC to the nuclear envelope is
achieved in plant cells. In yeast, the spindle pole body components Spc72 and
Spc110 serve as anchoring sites on the cytoplasmic and nucleoplasmic sur-
faces of the spindle pole body, respectively (Knob and Schiebel 1997, 1998).
No homologs of these proteins exist in plants. However, they share the com-
mon feature of long coiled-coil domains, a common protein interaction motif.
Long coiled-coil proteins have also been identified at the plant nuclear en-
velope, for example the carrot nuclear matrix constituent protein NMCP1
(Masuda et al. 1997). It is possible that these proteins might be involved in an-
choring the microtubule nucleation machinery to the plant nuclear envelope.
3.4
Meiotic Telomere Tethering at the Nuclear Envelope
Another function of the nuclear envelope becomes evident in meiosis. Ho-
mologous chromosome synapsis during meiotic prophase is accompanied by
the clustering of telomeres at the nuclear envelope in a so-called bouquet