Open Mitosis: Nuclear Envelope Dynamics 223
pore complex assembly in yeasts and animals (Ryan et al. 2003; Walther et al.
2003). Both nuclear envelope and nuclear pore assembly mediated by the
Ran cycle are inhibited by importinβin a RanGTP-reversible manner (Harel
et al. 2003).
While none of these mechanisms have been confirmed in plants yet, the
major components of the Ran cycle have been identified (Ach and Gruissem
1994; Merkle et al. 1994), and the localization of plant RanGAP to the mitotic
spindle and the growing cell plate are suggestive of similar roles for the plant
Ran cycle to its mammalian and yeast counterparts in regulating spindle as-
sembly and vesicle fusion events (Jeong et al. 2005; Pay et al. 2002).
Mammalian nuclear envelope components are recruited sequentially to
the newly forming daughter nuclei with the nucleoporin Nup153, RanBP2/
Nup358, LBR, and emerin being among the first proteins to associate with
chromatin (Bodoor et al. 1999; Chaudhary and Courvalin 1993; Haraguchi
et al. 2000; Schwartz 2005). InDrosophila, membrane vesicles containing nu-
clear envelope proteins associate with chromatin depending on the presence
of lamins (Ulitzur et al. 1997). The association of A-type lamins to chromatin
does not require the presence of membranes (Burke 1990; Glass and Gerace
1990). The exact order of recruiting events leading to the reassembly of the
nuclear envelope and pores in plant cells is not known, partly due to a lack of
knowledge about the corresponding plant cell components. None of the above
mentioned animal proteins has a homolog in plants.
5
Outlook
While the plant nuclear envelope shares similar functions and mitotic be-
havior with the animal nuclear envelope, distinct differences exist in protein
compositions and specialized functions, such as microtubule nucleation in
plants. Comparatively few nuclear envelope proteins have been identified so
far in plants, and many are still functionally uncharacterized. While a role
during mitosis can be inferred for conserved components of the Ran cycle
and it is tempting to speculate that plant and animal cells might use the same
underlying molecular signals for nuclear envelope assembly, experimental
evidence from plant systems corroborating this hypothesis is still lacking.
Further studies need to be done to determine the exact events of nuclear en-
velope disassembly and reassembly and their regulation in plant cells. With
increasing knowledge about plant nuclear envelope proteins and the avail-
ability of specific plant nuclear envelope markers, it is becoming possible to
address these questions in more detail. Future studies may be directed to-
wards the identification of integral membrane proteins of the plant nuclear
envelope and towards protein and membrane trafficking during the plant
cell cycle.