222 A. Rose
et al. 2001) and inAspergillus nidulans, a fungus exhibiting an intermediate
between closed and open mitosis (De Souza et al. 2004).
4.2.2
Reassembly of the Nuclear Envelope
Reformation of the nuclear envelope occurs during late anaphase and
telophase and involves association of membranes containing nuclear envelope
proteins with the separated chromosomes. Microscopic studies in the charo-
phyte algaSpirogyrashow that vesicles remain associated with the spindle
and chromosomes until they fuse to form the envelopes of the daughter nu-
clei during early anaphase (Ueda et al. 1986). In higher plant cells, the tubular
ER/nuclear envelope network interweaving the spindle apparatus gives rise
to the newly forming nuclear envelopes as well as the cell plate during for-
mation of the phragmoplast. Intense staining of the phragmoplast region has
been observed using an ER marker (Gupton et al. 2006). During the reassem-
bly of the daughter nuclei, the condensed chromosomes are surrounded by
ER/nuclear envelope material. Chromosomes lagging in the transition from
metaphase to anaphase can cause the trapping of such material within the nu-
cleus, leading to the formation of nuclear grooves and invaginations observed
in plant cells (Gupton et al. 2006).
What drives nuclear envelope reassembly? In vitro reconstitution experi-
ments suggest that the principles of plant and animal nuclear envelope as-
sembly are roughly the same.Xenopussperm chromatin, when presented
with carrot or tobacco cell extracts, is capable of recruiting plant components
for nuclear envelope formation, suggesting that the underlying mechanisms
are ubiquitous to animals and plants (Lu and Zhai 2001; Zhao et al. 2000).
Conversely, plant sperm chromatin triggered the nuclear reconstitution from
animal cell extracts, including the nuclear lamina (Lu et al. 2001).
Since animal and plant systems seem compatible in terms of nuclear en-
velope assembly, it is feasible to postulate a common regulatory mechanism
for the initiation of nuclear envelope assembly around chromatin. How is this
process regulated on a molecular level?
Increasing evidence has been accumulated to suggest a crucial role for the
Ran cycle and karyopherins in nuclear envelope reassembly. Beads coated
with Ran have been shown to induce the formation of nuclear envelopes com-
plete with nuclear pore complexes capable of transport (Zhang and Clarke
2000, 2001). Depletion of Ran by RNAi results in failure of nuclear envelope
assembly inC. elegans(Askajer et al. 2002; Bamba et al. 2002). This pro-
cess requires both the exchange of RanGDP to RanGTP by the nucleotide
exchange factor RCC1 as well as hydrolysis of RanGTP to RanGDP (Hetzer
et al. 2000; Zhang and Clarke 2000). The correct balance of RCC1 and Ran
binding protein 1 is critical for nuclear envelope formation (Nicolas et al.
1997; Pu and Dasso 1997). The Ran GTPase cycle is also involved in nuclear