Growth, Differentiation and Sexuality

iod in early mitosis when CDK and NimA activity
are presumably at their highest (Fox et al. 2002).
The potential targets of BimG remain unknown,
but its association with duplicated SPBs suggests
that it might regulate the activity and/or assembly
of a mitotic exit complex. BimG also localizes to
nucleoli, which appear to divide surprisingly late
in fungal mitosis (Fox et al. 2002). In yeast, another
protein phosphatase, Cdc14, is required for
mitotic exit (Seshan and Amon 2004), and is normally
retained in the nucleolus prior to anaphase.
However, it is possible that Cdc14 may not regulate
mitotic exit in filamentous fungi. Although it
is conserved in several sequenced fungal genomes,
the network of proteins that link its function to
anaphase progression appears to be absent (S. Harris,
unpublished data). In this context, it is tempting
to speculate that BimG may in part fulfill the Cdc14
mitotic exit function.
V. Mitotic and Post-Mitotic Functions
of Motor Proteins
A. Motor Proteins and Mitosis
The microtubule-based motor proteins kinesin
and dynein are required for hyphal morphogenesis,
which has been attributed to their ability to
transport vesicles and organelles in a directed
manner to and from hyphal tips (Seiler et al.
1999). However, they have also been implicated in
multiple aspects of mitosis, including the assembly
of a bipolar spindle, chromosome segregation
during anaphase A, and spindle elongation during
anaphase B (reviewed by Aist 2002). Kinesins are
plus-end directed motor proteins, whereas dyneins
are minus-end directed motors. Both proteins
are ATPases that convert chemical energy into
mechanical force that permits progressive movement
along microtubules. Kinesins and dyneins
are conserved in all sequenced filamentous fungal
genomes (Schoch et al. 2003; Xiang and Plamann
2003). In fact, the diverse roles of microtubules in
mitosis and morphogenesis are reflected by the
presence of up to 11 kinesins in fungi such as A.
nidulans (Rischitor et al. 2004).
BimC is the prototype of a family of kinesins involved
in mitosis. In A. nidulans, bimC mutants undergo
SPB duplication, but the SPBs fail to separate
(Enos and Morris 1990). As a result, the mutant arrests
with short monopolar spindles. BimC acts as
Fungal Mitosis 43
a cross-bridge between anti-parallel microtubules
and, because it is a plus-end directed motor, it effectively
pushes the SPBs apart to generate a bipolar
spindle. Another kinesin, KlpA, possesses an opposing
force that appears to resist BimC and modulate
the kinetics of SPB separation (O’Connell et al.
1993). Kinesins are also involved in anaphase B,
where an inward-directed force within the mitotic
spindle counters the astral microtubule-mediated
pulling apart of the spindle (Aist 2002). In Fusarium
solani, the kinesin NhKRP1 is apparently responsible
for the inward-directed force (Aist 2002). In
A. nidulans, another kinesin, KipB, contributes to
mitosis, where it may promote the disassembly of
the spindle (Rischitor et al. 2004).
In filamentous fungi, the primary function of
cytoplasmic dynein is to control nuclear distribution
(Xiang and Fischer 2004). However, in Nectria
haematococca, it has been shown that dynein
also plays a role in spindle elongation during
anaphase B (Inoue et al. 1998a). In particular,
the astral microtubule-based pulling force is
eliminated in Nhdhc1 mutants. Similarly, in A.
nidulans, a genetic screen for synthetic lethal
mutants uncovered a previously unexpected role
for dynein in mitosis (Efimov and Morris 1998). As
in N. haematococca, the rate of spindle elongation
during anaphase B was dramatically reduced in
nudA mutants (Efimov and Morris 1998). It has
been proposed that dynein may generate force
through the destabilization of astral microtubules,
which may in turn allow the cortical attachment
needed for proper spindle elongation and nuclear
migration (Efimov and Morris 1998).
B. Post-Mitotic Nuclear Movement
Post-mitotic nuclear movement occurs during
the period between completion of anaphase B
and septum formation (Aist and Morris 1999).
During this time, the movement of daughter
nuclei relative to each other establishes the normal
pattern of nuclear distribution in hyphal cells.
In basidiomycete hyphae, this involves nuclear
migration through the clamp connection (Iwasa
et al. 1998). Less is known about this process
in filamentous ascomycetes, although the nonrandom
distribution of labeled nuclei observed
in A. nidulans (Rosenberger and Kessel 1967)
suggests that nuclei may slide past one another.
The mechanisms underlying post-mitotic nuclear
movement remain unclear, although SPBs and