220 A. Rose
tive model of absorption into the ER is favored by strong evidence from in
vivo experiments. While the outer nuclear envelope membrane is continu-
ous with the ER even in interphase, the inner nuclear envelope membrane,
visualized through immunostaining or with fluorescent markers, also shows
continuity with mitotic ER membranes (Ellenberg et al. 1997; Irons et al. 2003;
Yang et al. 1997). No vesiculation could be observed, suggesting that the nu-
clear envelope integrates with the endoplasmic reticulum in both animal and
plant mitosis.
What triggers the disassembly of the nuclear envelope? Since the process of
open mitosis has evolved independently in plants, animals, and certain fungi,
it would be reasonable to assume that the mechanisms of nuclear envelope
breakdown and reassembly differ. However, a common theme is emerging
in the form of mechanical forces exerted onto the nuclear membrane by the
cytoskeleton. Alternatively, nuclear pore complex disassembly has been im-
plicated in creating a “leaky” envelope and might constitute the first step in
nuclear envelope disassembly.
In animal cells, phosphorylation of the proteins of the nuclear lamina has
been suggested as the starting point for nuclear envelope breakdown (Foisner
and Gerace 1993). More recently, a model has emerged that suggests a crucial
role for the physical tearing of the nuclear envelope by microtubules as the
first step in nuclear envelope breakdown (Beaudouin et al. 2002; Lenart and
Ellenberg 2003). Early spindle microtubules cause folds and invaginations in
the nuclear envelope as early as an hour prior to the onset of mitosis. The nu-
clear envelope finally ruptures under increasing tension, allowing kinases to
enter the nuclear interior and in turn release the nuclear membrane from the
lamina and chromatin by phosphorylating inner nuclear membrane proteins
and their ligands (Gerace and Foisner 1994; Goldberg et al. 1999; Takano et al.
2002).
This process of physical tearing is facilitated by dynein and dynactin con-
centrated at the nuclear envelope in animal cells (Salina et al. 2002). A similar
mechanism apparently exists in fungi. In the basidiomyceteUstilago maydis,
the nuclear envelope is stripped off the chromosomes via dynein-mediated
nuclear migration (Straube et al. 2005).
Is this mechanical disruption of the nuclear envelope applicable to plant
cells as well? Plants lack both dyneins and nuclear lamins, therefore other
components must be involved in nuclear envelope disassembly in plants.
While the breakdown process appears similar in plant and animal open mito-
sis on a microscopic level, some features are unique to plants. As mentioned
earlier, the plant nuclear envelope serves as a microtubule organizing center
for spindle formation at the end of the G2 phase of the cell cycle. The result-
ing microtubule array around the plant nuclear envelope has been suggested
to play a role in moving and anchoring the nucleus in the division plane
(Bakhuizen et al. 1985). The microtubules align outside the nuclear mem-
brane, forming a bipolar spindle before the nuclear envelope breaks down.