Growth, Differentiation and Sexuality

114 J. Wendland and A. Walther
ments (approx. 40 m in A. nidulans and A. gossypii;
McIntyre et al. 2001; Knechtle et al. 2003). By contrast,
the hyphal tip compartments, e.g., in A. nidulans
and A. gossypii, can exceed this standard size
several-fold. Interestingly, in the tip, compartment
sites of future septation will be defined by the presence
of actin rings, such that the tip compartment
canharboruptonineactinrings.InA. gossypii,
dynamic constriction of actin rings in the tip cell is
activated successively, starting with the most subapical
one. Ring constriction is accompanied by the
generation of the chitin-rich septum (Wendland
and Philippsen 2002; see also Sietsma and Wessels,
Chap. 4, and Latgé and Calderone, Chap. 5, this
volume). By contrast, in A. nidulans septation follows
the mitotic wave of nuclear division starting at
the hyphal tip (Clutterbuck 1970; Westfall and Momany
2002). This allows one to use different model
systems to analyze the coupling of mitotic exit with
septation.
Assembly of the protein complexes at division
sites is a complex and, for some proteins, dynamic
process. Recently, it was shown by fluorescence recovery
techniques in S. cerevisiae that GFP-tagged
septin proteins within the septin ring form a highly
stable structure with low turnover rates (Dobbelaere
and Barral 2004). This is in accord with the
function of the septin ring as a rigid scaffold and
diffusion barrier (Schmidt and Nichols 2004). The
single septin ring splits into two rings late in mitosis,
producing a corral that maintains the localization
of proteins (e.g., Spa2p, Sec3p, Chs2p) at
the septal site by generating a specific compartment
at the cell cortex (Dobbelaere and Barral
2004). Splitting of the septin ring is dependent on
the activity of the mitotic exit network (MEN, see
Sect. V.). Bud3p exhibits similar dynamics as that
of the septin ring, as it undergoes a ring splitting
as well. The role of Bud3p for septation/cytokinesis
in S. cerevisiae, however, is unclear. Loss of BUD3
in yeast results in a change of budding pattern in
haploid cells, but does not produce a phenotype in
diploid cells (Chant et al. 1995). Since ScBUD3 is
expressed in diploid cells, it may still serve a general,
but redundant role. Deletion of the BUD3 homolog
in A. gossypii resulted in a pronounced septation
defect. AgBud3p was found to be involved
in localizing Cyk1p. Deletion of AgBUD3 resulted
in mislocalization of Cyk1p, which formed linear
filaments attached to the cortex, rather than rings
at septal sites. Subsequently, this resulted also in
the formation of linear actin rings, indicating that
AgBud3pplaysaroleinconveyingpositionalin-
formation not directly coming from the septin filaments
(Wendland 2003). Contributing additional
positional information used not only for bud site
selection could, therefore, be a yet uncharacterized
function of Bud3p in S. cerevisiae. Lossofeither
bud3 or bud4 in S. cerevisiae has an impact on
the budding pattern of haploid cells. In S. pombe,
Mid2,aproteinrelatedtotheS. cerevisiae Bud4p,
was found to stabilize the septin ring and inhibit
turnover of septin proteins (Berlin et al. 2003). The
turnover of Mid2p may, therefore, be required to
trigger septin ring splitting or disassembly (Tasto
et al. 2003). Acto-myosin rings are rather immobile
in yeast, even in the absence of septin rings, which
suggests that another scaffold protein or protein
complex may act upon them, and once released, the
acto-myosin rings may undergo constriction. In S.
pombe the contractile acto-myosin ring is kept in
its position during cytokinesis by a postanaphase
array of microtubules (Pardo and Nurse 2003). In
fission yeast as in S. cerevisiae, it was also shown
that, despite being a stable structure, the actin ring
is highly dynamic. Actin and other ring components
(e.g., in S. pombe, tropomyosin, Cdc8p, and
a myosin light chain, Cdc4p) assemble into, and
disassemble from the ring structure (Pelham and
Chang 2002; Tolliday et al. 2002).
Two main questions are the timing of events,
and the coupling of acto-myosin constriction with
septum formation. The timing of ring constriction
may be species specific, and either coupled or uncoupled
from mitotic events. In A. nidulans,theS.
cerevisiae Cdc15p homolog SepH is required early
in septin and actin ring assembly, whereas in yeast
Cdc15p regulates mitotic exit and is largely not involved
in these early steps (Bruno et al. 2001; Westfall
and Momany 2002). An intriguing example of
the regulation of ring constriction is presented by
the hyphal tip compartments of filamentous fungi.
Multiple sites of septation within a single compartment
are activated differently, which implies
that diffusible factors are involved to finally trigger
septum completion (Westfall and Momany 2002;
Wendland and Philippsen 2002). However, such
signals, and any coordination with MEN and/or
the cell cycle kinase Cdc28/Cdc2 have yet to be
identified.
To coordinate acto-myosin ring constriction
with septum formation, there has to be a link between
the generation of the forces that result in ring
closure and the delivery of vesicles that transport
membrane and cell wall material to the relative position
of the ring once it becomes dynamic. Recent