Growth, Differentiation and Sexuality

116 J. Wendland and A. Walther
analysis of the regulation and activity of these
enzymes was carried out recently, and in fact
demonstratedtheindependenceofbothactivities
in S. cerevisiae and C. albicans (Selvaggini et al.
2004).
V. Coordination of the End of Mitosis
with Cytokinesis
In uninucleate yeasts, cytokinesis must be strictly
coordinated with mitosis to ensure that nuclear division
and migration into the daughter cell occur
prior to septum formation. This requires monitoring
the state of mitosis, and coupling mitotic exit
withcontrolonthecellcycletobeabletodelaycytokinesis
until mitosis has been completed. Similar
molecular mechanisms have evolved, using highly
conserved protein networks that are called SIN in
S. pombe and MEN in S. cerevisiae. Researchin
this field is vigorously pursued, and has resulted
in a large advance of our knowledge that has been
covered by excellent recent reviews (Balasubramanian
et al. 2000; McCollum and Gould 2001; Jensen
et al. 2002; Simanis 2003; Barral 2004; Stegmeier
and Amon 2004). Therefore, we will present only
a short overview to outline the general picture. To
elucidate how mitotic exit in multinucleate compartments
of filamentous fungi is coupled to septation
will be a task of the future, since only one
component of these networks has been analyzed
in a filamentous fungus, this being the Cdc15p homolog
SepH of A. nidulans. Since SepH functions
upstream of actin ring formation in A. nidulans,the
mitotic exit network seems to control septation at
a more upstream level than in S. cerevisiae (Bruno
et al. 2001; Harris 2001).
Generally, progression through the cell cycle is
controlled by the activity of a cyclin-dependent kinase
(CDKs), which in S. cerevisiae is encoded by
CDC28, inS. pombe by CDC2, andinA. nidulans
by NIMX cdc2 (Nurse 1990; Osmani and Ye 1996).
Mitotic CDK activity is eliminated by degradation
via the anaphase promoting complex (APC), and
CDK targets are dephosphorylated by the Cdc14
protein phosphatase (Visintin et al. 1998; Jaspersen
et al. 1999; Peters 2002). Cdc14p is bound to Net1p
and sequestered in the nucleolus (Visintin et al.
1999). The FEAR network (cf. Cdc fourteen early
anaphase release) helps in releasing Cdc14p to the
nucleus, while MEN triggers release into the cytoplasm.
MEN signaling resembles a GTPase module
signaling cascade. It consists of a GTPase module
using the Tem1p-GTPase and its regulators (Lte1p
as a putative GTP exchange factor; Bub2p-Bfa1p
dimer as a GTPase activating factor; Li 2000; Lippincott
et al. 2001). Nud1p acts as a scaffold protein
to convey the signal from the GTPase module to
the protein kinases Cdc5, Cdc15, and Dbf2p/Mob1p
(Gruneberg et al. 2000). Actually, Cdc5p is also part
of FEAR, since it inactivates the Bub2p-Bfa1p complex
(Geymonat et al. 2002). To integrate positional
information of nuclear migration into the daughter
cell with mitotic exit, subcellular localization of the
GTPase module components is used: Lte1p localizes
to the daughter cell cortex and is kept there
by the diffusion barrier set up by the septin ring,
while Tem1p is bound to the spindle pole body
that is directed to the daughter cell (Bardin et al.
2000; Pereira et al. 2000, 2002). This ensures that
mitoticexitisactivatedonlywhenthedaughter
nucleus has found its way into the bud. Once S.
cerevisiae Cdc14p enters the cytoplasm, it dephosphorylates
and activates Cdh1p and Sic1p. Sic1p
is a CDK inhibitor while Cdh1p is an activator of
the APC. This indicates that the main function of
MEN in S. cerevisiae is to control CDK activity at
the end of mitosis, whereas the S. pombe SIN actually
initiates the contraction of the actin ring and
septum formation (Simanis 2003). In S. cerevisiae,
ring constriction depends on the degradation of
Hof1/Cyk2. This is achieved upon the activation of
MEN and the Skp1-Cullin-Fbox protein complex
(SCF), which depends on Grr1p localization to the
bud neck (Blondel et al. 2005).
Exitofmitosiswasshowntobelinkedtothe
spindle pole body (SPB), since several proteins required
for this process are associated with the SPB
(Pereira and Schiebel 2001). Particularly, Nud1p
links astral microtubule organization with the control
of exit from mitosis (Gruneberg et al. 2000).
Taken together, although key components of
both machineries are conserved in fungi, differences
may exist to which extent they contribute to
both mitotic exit and septum formation.
VI. Conclusions
Recent genome sequences have provided a wealth
of information on a variety of filamentous fungi
that enable comparisons with yeast-like fungi. It
became evident that key elements of acto-myosin
ring formation, septum formation, and the poorly