Introduction to Fungi, Third Edition

646 USTILAGINOMYCETES: SMUT FUNGI AND THEIR ALLIES
functions at subsequent developmental stages
such as gall formation and teliospore production
(Gold et al., 1997). For instance, the fuz7 gene
product which encodes a MAP kinase kinase
involved in the signalling chain shown in
Fig. 23.8 is also involved in the process of
hyphal fragmentation leading to teliospore production
(Banuett & Herskowitz, 1996). Further,
there is extensive cross-talk between the cAMP/
protein kinase A and MAP kinase chains at
different time-points in development.
The cytoskeleton
Much valuable work has been performed on
U. maydis to assign various cellular transport
phenomena to particular elements of the cytoskeleton
and their associated motor proteins.
Thus, microtubules have been implicated in
the transport of nuclei, mitochondria, vacuoles
and the endoplasmic reticulum, as well as
secretory vesicles in exocytosis and endosomes
in endocytosis (Steinberg, 2000; Basse &
Steinberg, 2004; Steinberg & Fuchs, 2004).
Bidirectional movement can be achieved by
kinesin motors which move towards the polymerizing
end (plus end) of microtubules, and
dynein which moves towards the minus end.
Actin cables and their myosin motors are also
involved in morphogenetic events and transport
processes. Since the genome of U. maydis has been
completely sequenced, the number of genes
encoding myosin (3), dynein (1) and kinesin (10)
is known (Basse & Steinberg, 2004) and further
rapid progress on the role of the cytoskeleton
in morphogenesis and cellular transport can be
expected with U. maydis as an experimental
organism.
Mycoviruses and killer toxins in U. maydis
Although mycoviruses are not uncommon in
fungi, few of them have been investigated
in detail. Two examples we have encountered in
earlier chapters of this book are the virus-like
particles in S. cerevisiae (p. 273) and related yeasts
which contain double-stranded RNA (dsRNA)
encoding killer toxins, and the hypovirulencecausing
dsRNA viruses of Cryphonectria parasitica
(p. 375) and Ophiostoma novo-ulmi (p. 366).
Mycoviruses infecting Ustilago maydis are similar
to those of S. cerevisiae in that they encode killer
toxins. The first evidence of them was found
when certain U. maydis strains killed sexually
compatible strains upon anastomosis in mating
assays. The cytoplasmic inheritance of the killing
trait, the proteinaceous nature of the toxin and
the presence of virus particles in killer strains
were quickly established (Hankin & Puhalla, 1971;
Wood & Bozarth, 1973). There are three types
of virus (P1, P4 and P6) each encoding its own
killer protein (KP) toxin. Day (1981) showed that
virus-infected U. maydis strains are common in
field populations.
A great deal is now known about viruses of
U. maydis (see Magliani et al., 1997; Martínez-
Espinoza et al., 2002). Their genomes are fragmented
into three size classes of dsRNA, whereby
each size class can have several members.
In total, there are six dsRNA fragments in P1,
seven in P4 and five in P6, with one capsid able
to accommodate either one H or one to several
M chains (Bozarth et al., 1981). In all three
viruses, a heavy (H) segment encodes the capsid
protein and the replication machinery, and
H segments are also essential for the maintenance
of the medium-sized (M) and light (L)
segments. The toxins are encoded by the
M fragments, and their synthesis as prepropolypeptide
chains followed by proteolytic
cleavage and secretion of the mature toxins
is similar to that of the S. cerevisiae killer
toxins (p. 273). The function of the L segments
is unknown at present.
The modes of action of the three toxins seem
to be diverse. The best-characterized is KP4 which
is active as a monomer blocking certain types
of Ca 2þ uptake channel. This activity can be
observed in susceptible U. maydis strains as well
as in mammalian cells, where it acts in a similar
way to the black mamba snake venom, calciseptine
(Gage et al., 2001, 2002). The KP1 and KP6
toxins are released as two separate polypeptides
after proteolytic cleavage, but in contrast to the
yeast killer toxins these do not re-associate with
each other by covalent (disulphide) bonds. Whilst
the mode of action of KP1 is unknown, KP6 may
form membrane pores which disrupt the ionic
balance of the target cells (N. Li et al., 1999). The
toxin-producing cell must obviously be resistant