Fission yeast growth polarity decisions depend on integration of multiple internal cues
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Date
07/07/2017Item status
Restricted AccessEmbargo end date
31/12/2100Author
Ashraf, Sanju
Metadata
Abstract
The establishment of cell polarity is a vital requirement for cellular processes
such as proper cell division, growth and movement. Cell polarization relies on
different internal and external cues in order to reorient the cell growth machinery
along the axis of polarity. The core mechanisms involved in establishment of
polarized growth are highly conserved from yeast to humans. Cells of the fission
yeast Schizosaccharomyces pombe grow in a highly polarized fashion, with cell
growth restricted to the cell tips, making fission yeast an excellent model system to
study polarized growth.
Here I describe a system for long-term live-cell imaging of fission yeast
polarized growth that is stress free, physiological and accessible to media change
and drug addition. I use this improved imaging system along with yeast genetics and
drug perturbations to address how cell polarity is established and maintained in
fission yeast. I have shown that fission yeast growth polarity depends on competition
and cooperation among three distinct internal polarity cues: 1) A microtubule-based
cue involving Tea1/Tea4 polarity proteins positively regulates polarized growth,
initially at the “old” cell end (i.e., the end that pre-existed in the mother cell) and later
at the “new” cell end (i.e. the end that is generated by septation), in order to initiate
the transition from monopolar to bipolar growth (also known as New End Take-Off, or
“NETO”). 2) An actin cable-based cue “clears” polarity proteins from the new end
immediately after cytokinesis thereby reinforcing old-end growth. As a result
perturbation of actin cable-based transport by either deleting actin cable nucleator
For3 or cable-based transporter Myo52 results in premature bipolar growth. 3) A
novel “memory-based” growth polarity cue helps to establish polarized growth in the
absence of the microtubule-based cue. This memory-based cue is dependent on the
predicted transmembrane proteins Rax1/Rax2. In the absence of both Tea1/Tea4
cue and Rax1/Rax2 cue, cells depend on septation cue and grow exclusively from
the cell ends generated by septation. Furthermore, both Tea1/Tea4 and Rax1/Rax2
cue are important to maintain polarized growth under various environmental
stresses.
In fission yeast, during interphase, nucleus is positioned at the centre of the
cell and this precise positioning of nucleus, which is important for defining the
position of cytokinetic ring is thought to be exclusively MT-dependent. Here I show
that MT-independent nuclear movement exists in fission yeast and this nuclear
movement is mediated by actin cables and type myosin myo52. Furthermore, I show
that actin cable might be important for buffering the pushing forces generated by MTs
on the nucleus. In this way both microtubules and actin cables are involved in
nuclear movement in fission yeast.