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Yeast Colony Group |
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Our Research |
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Our group studies development and behaviour of yeast colonies. Yeast
cells, when growing on solid surfaces form multicellular structures,
colonies, with a characteristic organised morphologies,
formed during the division of non-motile yeast cells. Organisation of yeast
colonies should, therefore, be ensured by signals transmitted and received by
dividing cells within a colony (13). In this regard,
the yeast colony exhibits an analogy to embryogenesis of higher organisms,
where the determination of the polarity of cell division plays a key role in
the ontogenesis (1).
Colony morphology can be influenced by cell-cell adhesion and budding pattern
of the cells (2). Ammonia Signaling and
Its Role in the Long-Term Development of Yeast Colonies · One of the
characteristic attributes of multicellular organisms is their ability to emit
and receive signals over long distances. Yeast colonies use for the
long-range inter-colony signalling a simple volatile compound, ammonia,
produced by colonies in pulses (3). Ammonia
action results in synchronisation of the development in neighbouring colonies
and in Candida mogii colonies is accompanied by expressive cell and
colony morphology changes (4). Amino acid
uptake and metabolism seems to be important for pulse ammonia production (5). The
transition of colonies from acidic phase to the phase of intense ammonia
production (alkali phase) is connected with decrease of mitochondrial
oxidative catabolism and by peroxisome activation, which in parallel with
activation of biosynthetic pathways contribute to decrease of the general
stress level in colonies (6).
These metabolic features characterise a novel survival strategy used by yeast
under starvation conditions prevalent in nature. In this regard volatile
ammonia acts as starvation signal between colonies. · Ammonia signaling and the
related changes appear to be important for long-term colony survival, as
indicated by studies performed on colonies of a strain defective in the Sok2p
transcription factor (7)
and strains defective in stress defense enzymes catalase Ctt1p and superoxide
dismutases Sod1p and Sod2p (8). sok2
colonies are not able to produce or to accept the ammonia signal, they cannot
effectively switch on the genes of adaptive metabolism and they exhibit
defects in long-term survival. Absence of mitochondrial Sod2p (or cytosolic
Ctt1p) brings colonies serious developmental problems. sod2 and ctt1 colonies
fail in ammonia production and sufficient activation of the alternative
metabolism and are incapable of centre-margin differentiation (see below),
but they do not increase ROS. Colonies defective in cytosolic Sod1p, however,
develop the same way as wt colonies; they produce comparable levels of
ammonia and undergo similar developmental changes. These data indicate that
long-term survival of yeast colony population depends on metabolic adaptation
rather than on stress defense. Colony disorders are not accompanied by ROS
burst, but are a consequence of metabolic defects, which, however, could be
elicited by imbalance in ROS production at early developmental phases. Sod2p
protein and homeostasis of ROS may participate in regulatory events leading
to ammonia signaling (8,
9). • Three membrane proteins Ato1p, Ato2p and
Ato3p are involved in ammonia production in S. cerevisiae colonies. (5, www)
(7, review).
Production of all the Ato proteins is controlled by ammonia. Ato proteins
associate with detergent-resistant membranes (similar to those described in
mammalian cells as membrane rafts) and two of them (Ato1p, Ato3p) localize to
larger patches. While Ato3p forms stable patches, formation of those by Ato1p
is pH dependent (14). Cell Differentiation within
Yeast Colonies · There are
several indications that cells in distinct areas of multicellular colonies
differentiate. A regulated dying exhibiting features of apoptosis-like
programmed cell death localizes to specific areas of aging Saccharomyces
cerevisiae colonies. The ammonia signal and related metabolic changes
appear to be important for cell differentiation and location of cells
exhibiting apoptotic-like dying features only to the colony centre. The cell
population located at more propitious areas (i.e., at the colony margin)
activates adaptive metabolism (15), exploits
nutrients released from dying cells and survives (9). In non-signalling
sok2 colonies, cell dying spreads throughout the whole colony
population, which is doomed to an untimely death. The absence of Mca1p yeast
metacaspase or Aif1p (yeast homologue of mammalian apoptosis-inducing factor
Aif) does not prevent regulated cell death in yeast colonies. (9,
10) · To
explore colony third dimension, new two-photon confocal microscopy technique enabled us
monitoring of the presence and spatial localisation of fluorescently labelled
proteins as well as of structures stained with specific fluorescent dyes within
the whole Saccharomyces cerevisiae
microcolony. Viewing the microcolony from different angles allowed us to
reconstruct a three-dimensional profile of the cells producing Ato1p-GFP. It
enabled us also to uncover skin-like protective cell layer covering the whole
microcolony. This “skin” is formed by living cells tightly joined
via thick cell walls, probably connected by surface proteins. (11) Distinct Lifestyle of
Wild Yeast and Its Domesticated Variants · In contrast
to colonies formed by laboratory strains of S. cerevisiae, which
are usually smooth, wild S. cerevisiae strains isolated from nature
exhibit structure fluffy colony morphology. Cells within wild S.
cerevisiae are covered and connected by abundant extracellular matrix
material as revealed by environmental scanning electron microscopy. Under
laboratory conditions fluffy morphology is efficiently switched to the smooth
morphology indistinguishable from that of laboratory strains. This
domestication is accompanied by specific changes in gene expression. (6) (review
11) · Studying individual phases of structured
microcolony development, we found that early after microcolony origination,
some cells undergo dimorphic transition, which is induced by ammonia
independently of nutrients. It results in oriented pseudohyphal cell
expansion in the direction of ammonia source, which consequently leads to
unification of adjacent microcolonies to one more numerous entity. The
subsequent colony development is accompanied by another dimorphic switch,
which is strictly dependent on Flo11p adhesin and is indispensable for proper
formation of biofilm-like aerial 3-D colony architecture. In this, Flo11p is
required for both elongation of cells organized to radial clusters and their
subsequent pseudohyphal expansion. Just before this expansion, Flo11p
relocalises from the bud-neck of cells in radial clusters also to the tip of
elongated cells. (12) Funding
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