Growth, Differentiation and Sexuality

is tightly regulated and confined to S phase and
G2 (Osmani et al. 1987). In addition, like cyclins,
NimA is degraded by proteolysis during mitosis,
and expression of a non-degradable version prevents
mitotic exit (Pu and Osmani 1995). Moreover,
NimA and the NimX-NimE CDK complex coregulate
each other within an apparent feedback
loop (Fig. 3.2). As part of this loop, NimA is hyperphosphorylated
by activated NimX, which boosts
NimA activity at the point of mitotic entry (Ye et al.
1995). In parallel, NimA promotes the localization
of the NimX-NimE CDK module to chromatin, the
nucleolus,andSPBs duringG2 (Wu etal.1998).This
mutual regulation presumably coordinates the activity
of the two kinases and ensures timely entry
into mitosis (Osmani and Ye 1996).
What is the mitosis promoting function of
NimA? In A. nidulans, aswellasS. pombe, NimA
can promote chromosome condensation (Osmani
et al. 1988a; O’Connell et al. 1994). NimA possesses
the ability to phosphorylate histone H3 at the
conserved serine-10 residue, and localizes to
chromatin at the point of mitotic entry when this
phosphorylation event occurs (de Souza et al.
2000). Notably, histone H3 kinase activity and
nuclear localization appear to occur after the
NimX-NimE CDK module hyper-phosphorylates
NimA. Although this activity may account for
the effect of NimA on chromatin condensation,
it does not fully explain how NimA promotes
mitotic entry. For example, NimA also localizes
to mitotic spindles and SPBs after mitotic entry
(de Souza et al. 2000). How NimA may regulate
spindle organization and/or SPB function during
mitosis remains to be determined. However,
one mechanism may be via interaction with
the NimA-interacting protein TinA, which localizes
to SPBs during mitosis and regulates
microtubule-nucleating capacity (Osmani et al.
2003).
Recent observations have provided further insight
into the role of NimA in promoting mitotic
entry in A. nidulans. In particular, mitotic entry is
coupled to the rapid influx of tubulin from the cytoplasm
into the nucleus, thereby enabling assembly
of the mitotic spindle (Ovechkina et al. 2003). This
influx occurs downstream of NimX activation, and
presumably depends upon a sudden increase in the
permeability of the nuclear envelope. Strikingly,
concomitant genetic analyses suggest that NimA
may regulate nuclear transport during mitotic entry.
Mutations affecting two different components
of the nuclear pore complex, SonA and SonB, can
Fungal Mitosis 41
suppress the partially active nimA1 allele (Wu et al.
1998; de Souza et al. 2003), apparently by restoring
transport of both NimA1 and the NimX-NimE CDK
module. Although it remains to be tested, bioinformatic
analyses suggest that both SonA and SonB
are potential phosphorylation substrates of NimA.
Taken together, these results set up an attractive
model whereby NimA activity directly modifies nuclear
pore complexes to permit import of mitotic
regulators, tubulin, and other factors required for
mitosis.
IV. Regulation of Mitotic Exit
A. Anaphase-Promoting Complex
The anaphase-promoting complex (APC) is
a multi-protein complex that functions as an E3
ubiquitin ligase that targets specific proteins for
proteolytic degradation (Murray 2004). Key targets
defined in yeast include securin, which blocks
the dissolution of sister chromatid cohesion, and
B-type cyclins (Thornton and Toczyski 2003).
Accordingly, by eliminating sister chromatid
cohesion, the APC promotes mitotic progression,
and by destroying mitotic cyclins, also triggers
exit from mitosis (Fig. 3.3). Components of the
APC have been characterized in A. nidulans,
where Ts mutations in bimA and bimE arrest
cells in mitosis (Morris 1976). BimA (=Apc3)
and BimE (=Apc1) associate within a complex
that is slightly larger than the typical APC (Lies
et al. 1998), and phenotypic characterization of
mutations affecting either gene shows that mitotic
exit requires APC function (Osmani et al. 1988b;
O’Donnell et al. 1991). How does the APC trigger
mitotic exit in A. nidulans? Likely targets include
NimA and the cyclin NimE (Fig. 3.2; Lies et al.
1998; Ye et al. 1998), both of which must be
degraded to permit exit from mitosis. Notably,
genetic and biochemical evidence suggests that
BimA may have a specific role in targeting the
APC to NimA (Ye et al. 1998). The fungal APC
hasalsobeenimplicatedinapotentiallynovel
checkpoint function that may prohibit mitotic
entry in response to specific interphase perturbations
(Ye et al. 1996; Lies et al. 1998). Although it
remains unclear how this checkpoint may operate,
it presumably involves APC-mediated destruction
of a key mitotic regulator such as NimE.
The mechanisms underlying the temporal regulation
of APC function have been partially charac-