Growth, Differentiation and Sexuality

Fig. 2.5.A–C Disposition of microtubules and nuclei in an
interphase germling of Aspergillus nidulans. A Immunofluorescence
micrographs of microtubules. The arrow points
to a site where several microtubules converge, known from
other studies to correspond to the spindle pole body. B Nuclei
stained with the DNA-binding dye DAPI. The unstained
regions correspond to nucleoli (arrows). C Phase contrast
image of the same field. Preprinted from Jung et al. (1998)
and Oakley (2004), with permission. Bar = 5 μm
have implicated microtubules and microtubuledependent
motors in nuclear positioning in
hyphae. Specifically, fungi bearing mutations in
dynein subunits, dynactin, the num1p-related
protein apsA, kinesin, or in three proteins required
for dynein function, NUDF/LIS1, NUDE/RO11
and NUDC, display defects in nuclear position
(Graia et al. 2000; Xiang et al. 2000; Alberti-
Segui et al. 2001; Han et al. 2001; Requena et al.
2001).
Currently, the mechanism underlying dynein
function in nuclear migration is not well understood.
Since dynein, dynactin, NUDF/LIS1, and
NUDE/RO11 all form comet-like structures on
the plus ends of microtubules, and microtubule
plus ends are less dynamic in dynein mutants
(Han et al. 2001; Zhang et al. 2002, 2003; Efimov
2003), it is possible that dynein-mediated
microtubule dynamics control nuclear position
during interphase in filamentous fungi. This
may occur by a pushing mechanism similar to
that described in fission yeast (Tran et al. 2000).
Alternatively, dynein may pull microtubules and
their associated nuclei toward the cell cortex by
a mechanism similar to that which contributes to
nuclear migration during anaphase in budding
yeast. During this process, dynein is found on
the plus ends of astral microtubules emerging
from the spindle pole body. Moreover, evidence
from two groups supports a model whereby
dynein that is associated with microtubule
plus ends is delivered to, and activated at, the
cell cortex. Once activated, the cortical dynein
can exert a pulling force on nuclear-associated
microtubules, effectively pulling them toward
the cell cortex (Lee et al. 2003; Sheeman et al.
2003).
Organelle Inheritance in Fungi 25
IV. Organelle-Specific Inheritance
in Fungi
A. Mitochondria
Mitochondria are essential for their role in aerobic
energy mobilization and the biosynthesis of amino
acids, fatty acids, pyrimidines, heme, and steroid
hormones. Although mitochondria contain DNA
(mtDNA) and the machinery to express mtDNA,
over 95% of the proteins in mitochondria are
encoded in the nucleus and synthesized in the
cytoplasm prior to import into the organelle.
Since the machinery for the transport of proteins
into mitochondria is encoded in the nucleus and
must be imported into mitochondria, mitochondrial
membranes can be produced only from
preexisting, import-competent mitochondrial
membranes. Thus, mitochondrial membranes and
mtDNA cannot be produced de novo, and both
mustbetransferredfrommothertodaughtercells
to ensure that the cells contain fully functional
mitochondria.
Many yeasts are not obligate aerobes and can
survive under conditions which produce lethality
in other eukaryotic cells. As a result, mitochondrial
biogenesis and inheritance have been studied
extensively in fungal systems. Visualization of mitochondrial
motility in budding yeast has revealed
the “mitochondrial inheritance cycle”, a series of
cell cycle-linked motility events that result in an
equal distribution of the organelle during cell division
(Fig. 2.6). Mitochondria are tubular structures
that form an extended cytoplasmic network and
align along the axis of growth in budding yeast
and other fungi (Stevens 1977; Lazzarino et al.
1994; Prokisch et al. 2000; Suelmann and Fischer
Fig. 2.6. The mitochondrial inheritance cycle. Please refer
to text for description