Growth, Differentiation and Sexuality

40 S.D. Harris
2001; Borkovich et al. 2004). However, with the
exception of A. nidulans PclA, which associates
with NimX and likely regulates its activity during
asexual development (Schier and Fischer 2002),
the function of these cyclins remains unknown.
Nevertheless, the paucity of fungal cyclins predicted
to regulate Cdk2 (i.e., three vs. nine in
S. cerevisiae) is somewhat surprising. Perhaps
filamentous fungi resort to a similar mechanism
as has been proposed for S. pombe (Stern and
Nurse 1996), whereby threshold levels of a single
B-type cyclin are sufficient to regulate all cell cycle
transitions.
It has been firmly established that mitotic entry
is regulated by reversible phosphorylation of
the conserved CDK tyrosine-15 residue (Murray
and Hunt 1993). Phosphorylation of this site by
the Wee1 kinase inhibits CDK activity and blocks
mitotic entry, whereas de-phosphorylation by the
Cdc25 phosphatase stimulates CDK activity and
promotes mitotic entry (Fig. 3.2). Accordingly, the
relative balance of Wee1 and Cdc25 activities determines
the timing of mitotic entry in response
to growth and checkpoint signals. A. nidulans is
the only filamentous fungus in which the roles of
Wee1 and Cdc25 have been extensively characterized.
As predicted, mutational inactivation of the
Wee1 homologue, AnkA, largely abolishes NimX
tyrosine-15 phosphorylation and triggers premature
mitotic entry (Ye et al. 1996; Kraus and Harris
2001), whereas mutations in the Cdc25 homologue,
NimT, cause a mitotic block with elevated NimX
tyrosine-15 phosphorylation (Osmani et al. 1991).
Unexpectedly, AnkA- and NimT-mediated regulation
of NimX also exerts a post-mitotic effect on
the timing of septum formation in pre-divisional
hyphae (Harris and Kraus 1998; Kraus and Harris
2001), and is also modulated by developmental signals
to control cell patterning during conidiation
(Ye et al. 1999). These observations place NimX
tyrosine-15 phosphorylation at the interface between
mitotic regulation and morphogenesis in A.
nidulans, though the precise mechanisms by which
this occurs await investigation.
CDK inhibitors play a crucial role in the regulation
of mitotic entry by interfering with the
phosphorylation of substrates. In yeast, the CDK
inhibitor Sic1 binds to mitotic cyclins and must
be degraded to permit their accumulation (Cross
2003). The lack of obvious homology has precluded
the straightforward identification of fungal CDK
inhibitors by sequence annotation (Borkovich et al.
2004). However, in light of the finding that filamen-
tous fungi possess fewer cyclins despite their increased
morphological complexity, CDK inhibitors
may provide one mechanism for imposing developmental
constraints upon mitotic entry.
B. NimA Kinase
In A. nidulans, mitotic entry requires the parallel
activity of both the CDK module and the NimA
kinase (Fig. 3.3; Ye et al. 1995). NimA was first
identified via temperature sensitive (Ts) mutations
that caused a reversible arrest in late G2 (Morris
1976). Notably, at the arrest point, the CDK NimX
is in an active, tyrosine-15 de-phosphorylated
state (Osmani et al. 1991), such that mitosis
rapidly ensues when the block is released. NimA
is a conserved protein kinase, with functional
homologues present in other filamentous fungi
(Pu et al. 1995). Additional homologues with
mitotic functions have been characterized in S.
pombe and animals (O’Connell et al. 2003; Grallert
et al. 2004). Indeed, NimA is the founding member
of a family of protein kinases involved in diverse
aspects of mitosis.
NimA is subject to multiple modes of regulationthatsharesomefeaturesincommonwith
cyclins (Fig. 3.2). For example, nimA expression
Fig. 3.3. Overview of mitotic regulatory circuits in Aspergillus
nidulans.Mitoticentry(G2→M) and mitotic exit
(M→G1) are depicted. The NimX CDK module and the
NimA kinase act in a coordinate manner to regulate mitotic
entry. Mitotic exit requires the APC, the BimG phosphatase,
and γ-tubulin. BimG is also involved in septum formation.
Unlike yeast, the SIN/MEN has no apparent role in mitotic
exit, and is required only for septation. See text for additional
details