Growth, Differentiation and Sexuality

Latrunculin A (Lat-A) decreases ER motility, it is
clearthatthe actincytoskeletonplays arole incortical
ER dynamics and morphology in budding yeast
(Prinz et al. 2000; Fehrenbacher et al. 2002). However,
the precise function of the actin cytoskeleton
in ER dynamics is not well understood.
The dimorphic heterobasidiomycete U. maydis
serves as an example of how the ER is organized
in mycelial fungi. In U. maydis the ER forms
a polygonal network of tubules that is associated
with the cell cortex and makes contact with the
nuclear envelope. As in S. cerevisiae (Prinz et al.
2000), the ER in U. maydis is a highly dynamic
structure, with ER tubules undergoing continuous
extension, sliding, and fusion. Analysis of the effect
of the destabilization of microtubules or mutations
in microtubule-dependent motors revealed that ER
motility in U. maydis requires microtubules and cytoplasmic
dynein (Wedlich-Söldner et al. 2002a,b).
Although microtubules support ER movement in U.
maydis, as in vertebrate cells (Dabora and Sheetz
1988), they are not required for peripheral organization
of the network. However, in contrast to
animal systems, motility events are not essential
for ER inheritance in U. maydis; ERtubulesare
present in the growing bud in cells with conditional
mutations in tubulin at permissive and restrictive
conditions (Wedlich-Söldner et al. 2002a).
In S. cerevisiae, nuclear ER undergoes spindledriven
inheritance in conjunction with the nucleus.
By contrast, inheritance of cortical ER occurs by
a fundamentally different, actin-dependent mechanism.
Cortical ER is the first organelle inherited
during the cell cycle (Preuss et al. 1991; Koning
et al. 1996; Du et al. 2001). Moreover, cortical ER
morphology is sensitive to treatment with Lat-A
and to mutations in the actin-encoding ACT1 gene
(Fehrenbacher et al. 2002). Using Sec63p-GFP to
visualize cortical ER, Fehrenbacher et al. (2002)
found that cortical ER is anchored to sites of bud
emergence and apical bud growth during the S and
G2 phases of the cell cycle, and that this anchorage
allows cortical ER to be drawn into and maintained
in the bud as it develops. These observations support
a mechanism for cortical ER inheritance that
is actin cytoskeleton-dependent but relies on anchorage,
not directed, organelle movement.
In support of this model, a protein that localizes
to the site of ER anchorage has been implicated
in cortical ER inheritance. The exocyst componentSec3plocalizestothebudtipwhereitmediates
post-Golgi membrane traffic (Walch-Solimena
et al. 1997; Grote et al. 2000). Since sec3Δ cells ex-
Organelle Inheritance in Fungi 29
hibit a defect in cortical ER inheritance but not in
Golgi and mitochondrial inheritance, it has been
proposed that Sec3p contributes to anchoring cortical
ER at the bud tip (Wiederkehr et al. 2003). In
addition deletion of Aux1p/Swa2p, a protein that
localizes to ER membranes but has no obvious role
in membrane traffic, produces a delay in the transferofcorticalERtubulestodaughtercells(Galletal.
2000; Pishvaee et al. 2000; Du et al. 2001). Therefore,
it is possible that this protein also contributes
to anchorage of ER at the bud tip.
OtherstudiessupportaroleforatypeVmyosin
in cortical ER inheritance. Deletion of the MYO4
gene results in defects in actin cable-dependent
movement of mRNAs from mother to daughter cells
during cell division (Bertrand et al. 1998; Bohl et al.
2000). The She2p/She3p protein complex serves as
an adaptor to bind mRNA to a region in the Myo4p
C-terminal tail (Bohl et al. 2000). Recent studies indicate
that a point mutation in the ATP-binding region
of the motor domain of Myo4p or a mutation of
She3p inhibit ER inheritance (Estrada et al. 2003).
Moreover, both She3p and Myo4p are recovered in
fractions enriched in ER-derived membranes after
subcellular fractionation. These findings raise
the possibility that myosin may drive transport of
cortical ER from mother to daughter cells in budding
yeast. Thus, it is possible that two distinct
processes, anchorage of ER in the bud tip and active
transport of ER into the bud, may contribute to
the inheritance of cortical ER. Alternatively, Myo4p
and She3p may mediate the transport of ER anchoring
proteins or mRNAs that encode ER anchoring
proteins from mother to the bud tip.
C. Vacuoles, the Lysosomes of Yeast
Vacuoles are evenly distributed among mother
and daughter cells in S. cerevisiae. Early studies
indicated that the yeast vacuole fragments into
small vesicles that are then distributed between
the mother cell and the bud (Wiemken et al. 1970;
Severs et al. 1976). However, more recent work
indicates that vacuoles remain relatively constant
in size during cell division, and that the primary
event during vacuole inheritance is the formation
of a tubular, vacuole-derived “segregation structure”
(Weisman et al. 1987; Weisman and Wickner
1988). The segregation structure forms near the
bud and rapidly extends from the mother cell to
the bud before the nucleus enters into the neck.
Thereafter, the segregation structure disappears,