Growth, Differentiation and Sexuality

transport vesicles are formed and that are morphologically
and functionally distinct from the rest of
the ER (Soderholm et al. 2004). In these studies, apparent
formation of tER and late Golgi were monitored
using Sec7p-DsRed and Sec13p-GFP, respectively.
Consistent with this, tER localizes to regions
in close proximity to Golgi stacks in P. pastoris.
These observations lend further support to a refined
model of Golgi inheritance in which fusion of
the COPII vesicles results in the de novo synthesis
of Golgi cisternae (Bevis et al. 2002).
There is no detectable tER in S. cerevisiae.
Indeed, the Golgi of budding yeast does not
appear to be organized in stacks. Rather, they are
dispersed throughout the cytoplasm (Rossanese
et al. 1999). Nonetheless, Golgi inheritance is
believed to be a cell-cycle-dependent, non-random
process (Rossanese et al. 2001). Using Sec7p-GFP
as a marker, late Golgi membranes are detected
near the incipient budding site and dispersed
throughout the bud. Moreover, movement of
Sec7p-GFP from the bud neck to bud tip has been
documented. Since mutation of the Myo2p motor
domain (myo2-66) results in defects in late Golgi
localization, it is possible that late Golgi movement
during inheritance is mediated by Myo2p-driven
processes (Rossanese et al. 2001).
A capture mechanism, similar to that first observed
during mitochondrial inheritance, has been
detected during late Golgi inheritance in budding
yeast – that is, late Golgi elements accumulate in the
bud tip and are released from their retention site in
the bud tip prior to cytokinesis. Finally, during cytokinesis,
Golgi cisternae appear grouped together
with secretory vesicles at sites of cell wall synthesis
in order to deposit cell surface material (Preuss
et al. 1992). Since destabilization of F-actin reduced
the amount of Sec7p-GFP that accumulates in the
bud tip, retention of late Golgi in the bud tip, like
retention of mitochondria at that site, appears to be
actin-dependent. Consistent with this, a mutation
in Cdc1p (cdc1-304), which results in depolarization
of the actin cytoskeleton in budding yeast, also
results in defects in retention of late Golgi elements
in the bud tip, but has no effect on the inheritance
of early Golgi (Rossanese et al. 2001).
E. Peroxisomes
Peroxisomes are small, lipid bilayer-bound organelles
that perform diverse functions including
fatty acid β-oxidation or H2O2 metabolism (van
Organelle Inheritance in Fungi 31
den Bosch et al. 1992). Peroxisome abundance
depends on a balance between biogenesis, division,
and degradation. In budding yeast, peroxisome
biogenesis is induced by growth conditions (e.g.,
fatty acid- or methanol-based growth media)
thatrequireperoxisomeactivity(seereviewby
Veenhuis and Harder 1988). Under inducing
conditions, peroxisomes can occupy up to 80% of
the cytoplasmic volume in Hansenula polymorpha
(Veenhuis et al. 1979). Conversely, peroxiphagy
(rapid autophagy of peroxisomes) occurs after removal
of peroxisome inducers. The protein Pex14p
contributes to peroxiphagy in H. polymorpha
(Bellu et al. 2001).
According to the classical model of peroxisome
biogenesis, peroxisomes arise by fission from preexisting
peroxisomes (Lazarow and Fujiki 1985).
This view has been challenged recently by evidence
from several groups supporting the de novo synthesis
of early and immature peroxisomes (Eckert
and Erdmann 2003; Lazarow 2003). The proteins
that contribute to peroxisome fission and movement
during inheritance are described below.
Several “peroxins”, proteins required for
normal peroxisome development and function,
have been identified and characterized in budding
yeast (Distel et al. 1996). Peroxins contribute to
the maintenance of the peroxisomal membrane,
import of proteins into the peroxisome matrix,
and the control of peroxisome abundance or
morphology. Pex11p is important for peroxisome
proliferation in budding yeast. PEX11 overexpression
produces proliferation of very small
peroxisomes. On the other hand, pex11 deletion
strains contain a small number of abnormally
large peroxisomes (Erdmann and Bolbel 1995).
These results suggest that Pex11p functions mainly
in dividing the peroxisomal compartment. An
additional role of Pex11p in fatty acid oxidation
has also been proposed (van Roermund et al.
2000). Another protein required for peroxisome
division in S. cerevisiae is Vps1p (one of the three
dynamin-like proteins in budding yeast). Mutants
lacking VPS1 show single, big peroxisomes or
clusters of small peroxisomes that failed to separate
during the fission process (Hoepfner et al. 2001).
In Y. lipolytica, coordination of peroxisome
maturation and division occurs by redistribution
of the peroxisomal protein acyl-CoA oxidase (Aox)
from the matrix to the membrane, and its interaction
with the membrane-associated protein Pex16p
(Guo et al. 2003). Peroxisome maturation in this
cell type requires budding of a COP-vesicle from