ayout 1 - EMBL Grenoble

Cell Biology and Biophysics Unit
Cell morphogenesis and spatial microtubule
organisation
Previous and current research
To create a defined morphology, cells need to polarise and correctly orient their polarity axis. Both
processes require a defined intracellular order based on the specific sub-cellular arrangement of
actin and microtubule filaments. Our investigations focus on the contribution of microtubules,
whose organisation varies tremendously between different cell types and also in individual cells
during different developmental or cell cycle stages. Little is known about how this variability is
achieved and how cells switch from one organisational state to another. We address these questions
in two model organisms, the unicellular fission yeast Schizosaccharomyces pombe and the fruit fly
Drosophila melanogaster.
In the cylindrical fission yeast cells we describe the cell autonomous machinery that organises and
maintains a defined interphase microtubule distribution. In these cells, approximately 30 anti-parallel
microtubules form 3-6 bundles that are arranged parallel to the long cell axis. Microtubule
minus-ends overlap in the cell centre. From there the plus-ends grow to the cell poles where they
switch to shrinkage, an event termed catastrophe (figure 1). This localised catastrophe is fundamental
to proper cell morphology. Our findings suggest a model where conserved proteins at the
growing microtubule plus-ends (+TIPs) mediate cell-pole targeting in two steps. First, the yeast
EB1 homolog Mal3p promotes growth until cortical regions are encountered. In central regions of the cell cortex, the CLIP-170 homolog
Tip1p then prevents premature catastrophes by suppressing Mal3p removal, which keeps microtubules growing below the cortex until the cell
poles are reached.
A central question concerning +TIP function is how these proteins can accumulate at growing microtubule plus-ends. We discovered that
Tip1p is transported there by the Tea2p motor protein. Mal3p in contrast, seems to ‘treadmill’, preferentially binding to plus-ends followed
by rapid unbinding. In an attempt to further describe Mal3p plus-end binding (collaboration with Andreas Hoenger, University of Colorado
at Boulder), we discovered that Mal3p also binds and stabilises the microtubule lattice seam, explaining its weak localisation all along the microtubules.
This provides a new twist to the model of how microtubule dynamics are controlled
and shows that microtubules have two different surfaces for molecular interactions.
In fruit flies we explore to what extent the basic machinery found in fission yeast is used to maintain
microtubule organisation in a multi-cellular organism and how non-autonomous cells achieve
and coordinate changes in microtubule distribution. We have shown how microtubules become reorganised
during embryonic dorsal closure (DC), a wound healing-related process. Thereby, antiparallel
microtubules transiently form bundles in the epidermal cells that move dorsally to close
a cavity (figure 2). Surprisingly, these microtubules are essential exclusively for the final step, the
fusion of the epithelium.
Future projects and goals
In fission yeast we now focus on two topics. First, we want to understand how catastrophes are induced
at cell poles and second we want to identify the critical molecules/processes for switching
between the seven different microtubule arrangements found in S. pombe cells.
In flies, we are trying to identify the signals triggering microtubule reorganisation during DC and
we want to uncover the molecular mechanisms driving the process.
Damian Brunner
PhD 1995, University of
Zürich.
Postdoctoral research at the
Imperial Cancer Research
Fund, London.
Group leader at EMBL since
2000. Joint appointment with
the Developmental Biology
Unit.
Figure 1 (top): Interphase microtubule bundles in fission yeast cells. Figure 2 (bottom): Microtubule
bundles in the epidermal cells during dorsal closure in Drosophila melanogaster.
Selected references
Foethke, D., Makushok, T., Brunner, D. & Nédélec, F. (2009). Force
and length-dependent catastrophe activities explain interphase
microtubule organization in fission yeast. Molecular Systems Biology,
5, 21
Bieling, P., Laan, L., Schek, H., Munteanu, E.L., Sandblad, L.,
Dogterom, M., Brunner, D. & Surrey, T. (2007). Reconstitution of a
microtubule plus-end tracking system in vitro. Nature 5, 1100-115
Jankovics, F. & Brunner, D. (2006) Transiently reorganised
microtubules are essential for zippering during dorsal closure in
Drosophila melanogaster. Cell, 11, 375-385
Sandblad, L., Busch, K.E., Tittmann, P., Gross, H., Brunner, D. &
Hoenger, A. (2006). The Schizosaccharomyces pombe EB1 homolog
Mal3p binds and stabilizes the microtubule lattice seam. Cell, 127,
115-12
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