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Figure 2.19
Cell polarity in budding yeast is established by a polarized actin cytoskeleton
throughout the cell cycle. A cap (blue) of regulatory and cytoskeletal proteins establishes
the polarity of actin cables (beige) and cortical patches (brown). Tight localization of the
cap orients actin cables. Actin cables then guide secretory vesicles to the cap, where they
accumulate (also shown in blue) and fuse, thus polarizing growth (arrows). During isotropic
growth, the proteins of the cap are more diffusely distributed, cortical patches are isotropically
distributed, and actin cables form a meshwork. A fourth cytoskeletal structure, a cytokinetic
ring, mediates cell division (bright pink) (
Pruyne and Bretscher, 2000
).
Asexual Reproduction in Fungi
Reproduction by Budding in Yeasts
Reproduction by budding in yeasts is known in greater detail than in any other fungi.
The three major phases of the yeast life cycle are cell division by budding, mating
between haploid cells, and sporulation of diploids (
Chant, 1999
). Formation of buds
in yeasts is an epigenetically controlled process. It starts with the establishment of
cell growth polarity, and the centerpiece of the regulation of this polarization for
bud formation is the yeast's actin cytoskeleton (
Chant, 1999
), as opposed to animal
cells, where the establishment of the cell's asymmetry depends on microtubules.
Actin polymerization helps protein compounds to move and actin cables (actin long
fibers) direct the vectorial transport of organelles to the bud (
Catlett and Weisman,
2000; Moseley and Goode, 2007
) (
Figure 2.19
). Actin cortical patches, small actin-
containing structures, initially determine the budding site and later reorganize to
form two rings in the neck of the bud, where they are believed to help in septation
and cytokinesis. They may not only facilitate the anchoring of cables to the cortex
but also be the sites of cable formation (
Amberg, 1998
).
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