Growth, Differentiation and Sexuality

Goldstein and Drubin 2003). Proteins that are
required for endocytosis, including actin, Arp2/3
complex and its activators, as well as endocytic
adapters and scaffolds, localize to actin patches.
Elegant studies have revealed a pathway for the association
of receptors, adaptors, and actin patches
during endocytic internalization (Kaksonen et al.
2003). Consistent with this, recent studies showed
that the lipophilic endosomal marker, FM4-64, colocalizes
with actin patches during their assembly,
internalization, and movement from the bud into
the mother cell (Huckaba et al. 2004). Thus, there
is direct evidence that actin patches of budding
yeast are early endosomes.
Actin cables exist in many fungi, and are best
characterized in budding yeast. These structures
have been implicated as tracks for the movement
of mitochondria, secretory vesicles, mRNA, and
spindle alignment elements from mother cells to
developing daughter cells during yeast cell division
(Simon et al. 1997; Takizawa et al. 1997, 2000;
Pruyne et al. 1998; Schott et al. 1999, 2002; Beach
et al. 2000; Yin et al. 2000; Fehrenbacher et al. 2003).
Moreover, actin cables are required for normal inheritance
of Golgi and vacuoles (Hill et al. 1996;
Rossanese et al. 2001).
Fig. 2.3. Model for retrograde flow of actin cables during
their assembly and elongation in budding yeast. ABPs:
Actin-binding proteins. Please refer to text for description
Organelle Inheritance in Fungi 23
Recent studies indicate that actin cables
undergo retrograde movement, i.e., movement in
the direction opposite that of cargo movement,
as they assemble and elongate (Yang and Pon
2002). This process is mediated by the formins
(Bni1p and Bnr1p), proteins that stimulate actin
polymerization at sites of actin cable assembly, and
resident actin cable proteins including the actin
bundling proteins, fimbrin (Sac6p) and Abp140p,
and tropomyosin proteins (Tpm1p and Tpm2p;
Adams et al. 1991; Drees et al. 1995; Asakura et al.
1998; Pruyne et al. 1998, 2002; Evangelista et al.
2002; Sagot et al. 2002a,b). Actin cable assembly
and elongation occurs at two sites in budding
yeast: the bud tip and bud neck (Yang and Pon
2002). During elongation of these structures, new
material is incorporated into an existing actin
cable at its assembly site, and the growing actin
cable moves toward the distal tip of the mother
cell with an average velocity of ∼0.29 ± 0.11 μm/s
(Fig. 2.3). This “retrograde flow” of actin cables
hasanimpactonboththevelocityanddirectionof
organelle and vesicle movement in budding yeast
(Fehrenbacher et al. 2004; Huckaba et al. 2004).
B. Organization of the Microtubule
Cytoskeleton in Fungi
The microtubule cytoskeleton of fungi, like that of
other cell types, is highly dynamic and undergoes
ordered changes throughout the cell cycle (Hagan
1998; Sato and Toda 2004). Fungi typically differ regarding
how their microtubules are nucleated. For
example, the spindle pole body (SPB), a structure
that is embedded in the nuclear envelope, nucleates
both cytoplasmic and nuclear microtubules in
non-dividing yeast, and nucleates microtubules in
the spindle apparatus in dividing yeast (Fig. 2.4). By
contrast, microtubules in U. maydis and S. pombe
are nucleated by other, cytoplasmically located nucleation
centers (Straube et al. 2003).
In the fission yeast, S. pombe, bundlesofmicrotubules
extend along the major axis of the cell
during interphase. The microtubules in these bundles
are arranged in an antiparallel manner, with
theplusendsorientedtowardthecelltips(Tran
et al. 2001). Microtubule nucleation during interphaseoccursfromtheSPBandfromtheinterphase
microtubule organizing center (iMTOC; Tran et al.
2001). In mitotic yeast, the SPB is duplicated during
late G2 phase. During prophase, microtubules
nucleated from the SPB form a short, bipolar spin-