210 A. Rose
The nuclear envelope and nuclear pore complexes subsequently gained
complexity and functionality through diversification of existing protein fam-
ilies, such as the karyopherins, and the incorporation of novel proteins in
the different eukaryotic lineages. Some of the domains in these proteins can
be traced back to bacterial origins (endosymbiotic horizontal gene trans-
fer), while others are paralogs of eukaryotic proteins that have gained new
functions. The nuclear lamina, a prominent feature of the metazoan nu-
clear envelope, evolved only in that lineage, possibly as an adaptation to the
greater mechanical stress, in the form of contractile forces, in animal cells as
compared to fungi and plants (Mans et al. 2004). As a result of this diver-
gent evolution, the nuclear envelopes found in the three kingdoms of higher
eukaryotes show distinctive differences in composition as well as function,
despite their common origin.
The evolutionarily late acquisition of novel functions in nuclear envelope
proteins is also supported by the finding of distinct targeting mechanisms for
Ran GTPase-activating protein (RanGAP). RanGAP is targeted to the nuclear
pore in animal as well as plant cells, but not in yeast. This subcellular lo-
calization is achieved through additional nuclear envelope targeting domains
present in the plant and animal versions of the protein. However, these do-
mains differ from each other, are attached to opposite termini of the catalytic
core of the protein, and are not functionally interchangeable (Jeong et al.
2005; Rose and Meier 2001). This suggests that they evolved after the diver-
gence of the major eukaryotic kingdoms and utilize kingdom-specific nuclear
envelope components as anchors. The extension of animal and plant RanGAP
with a specific targeting domain changing its subcellular localization is an
example for the common principle of “tinkering together” existing protein
domains to gain functionality during the evolution of the nuclear envelope
and nuclear pore complex (Mans et al. 2004). It has been postulated that the
targeting of RanGAP to the nuclear envelope constitutes an adaptation to
the development of open mitosis in which the nuclear envelope disassembles
(Rose and Meier 2001).
2.2
From Closed Mitosis to Open Mitosis
In most unicellular eukaryotes, the nuclear envelope stays intact during mito-
sis. In this closed mitosis, an intranuclear spindle separates the chromosomes
before karyokinesis occurs. In fenestral mitosis, the spindle forms outside the
nucleus and penetrates the nuclear envelope; however, the nuclear envelope
does not disassemble completely. During the evolution of multicellular organ-
isms, open mitosis evolved where the nuclear envelope disintegrates before or
during spindle formation and reforms around the two daughter nuclei after
the separation of the chromosomes. Open mitosis occurs in the plant and an-
imal kingdoms and possibly some fungi of the phylumbasidiomycota.Asitis