Author: Sophie Martin

Magdalena Marek’s work on sterols is now published in the Journal of Cell Biology. By using the genetically-encoded sterol sensor D4H to visualize sterol-rich membranes in vivo, she found that Arp2/3 inhibition leads to strong flow of sterol from the plasma membrane to endosomes. These flow depend on a LAM-family protein Ltc1 that transfers sterols across membranes.

Veneta’s paper is now in eLife: Multi-phosphorylation reaction and clustering tune Pom1 gradient mid-cell levels according to cell size. Enjoy the nice quantitative PALM and TIRF imaging, in collaboration with the lab of Prof Suliana Manley at EPFL.

Check out our recent preprint on concentration gradients  of Pom1 kinase on BioRxiv: https://www.biorxiv.org/content/10.1101/546424v1

Cassandre passed her private PhD defense! Congratulations
Come and listen to her public presentation on March 8th, 5pm.

Laura’s work on the feedback inhibition of Ras activity coordinating cell fusion with cell-cell contact now posted on BioRxiv. Check it out!

Check it out here.

Congratulations! And thanks to HFSP for the support! Gaowen will explore the evolutionary diversity of mate pairing.

Congrats to Peter van Gisbergen, who just received an EMBO long-term fellowship to study the interplay between signaling and the cytoskeleton at the fusion site !!!

Our paper on the signal for fusion is out in Genes and Development! We show that the commitment of fission yeast cells to undergoing fusion relies on pheromone signaling and does not necessitate cell-cell fusion. Rather it depends on concentration of the signaling machinery at the fusion focus.

Cellular ‘speed-dating’: new paper in Current Biology, proposing how fission yeast cells pair during mating. We showed that the dynamic exploratory Cdc42-GTP zones present in early mating cells contain pheromone secretion and perception machineries and that these zone lifetimes scale with pheromone concentration. Computational simulations of fluctuating zones stabilized by pheromone lead to efficient pairing, which relies on zone dynamics, local pheromone release, and short decay length. Experimental assays are consistent with this model. We conclude that efficient cell pairing relies on fluctuating local signal emission and perception, which become locked into place through stimulation.

Our study of cell-cell fusion will be funded for the next five years by an ERC Consolidator grant to Sophie Martin!

In this new paper, published in the Journal of Cell Biology, we show that the F-BAR protein Cdc15, a major component of the cytokinetic actomyosin contractile ring, is a direct substrate for Pom1 kinase. Phosphorylation by Pom1 blocks interaction of Cdc15 with several binding partners and destabilizes the ring, allowing it to slide if it assembles at the cell poles. Thus, Pom1 at cell poles protects these regions by blocking ring stabilization.

In this new study, recently published in Nature Communications, we show that Pom1 kinase relocalizes from cell poles around the entire cell cortex upon glucose starvation. This is the first instance shown where Pom1 localization naturally changes and overlaps with its substrate Cdr2 to control mitotic entry.

Sophie Martin published a review in Bioessays, in which she discusses the mechanisms by which the Cdc42 GTPase, a central organizer eukaryotic cell polarity, spontaneously polarizes.

Our recent study on the mechanism of concentration gradient robustness just came out in Molecular Systems Biology. This is a collaborative experimental-theoretical work showing that inter-molecular phosphorylation of Pom1 contributes a buffering system to the Pom1 concentration gradient.

Check out our new paper in PLoS Biology, in which we create a first functional Cdc42 GFP-tagged allele. A fast-folding variant of GFP, or mCherry, is inserted in a poorly conserved internal loop, yielding a functional sandwich fusion. We then use this allele to study the localization and dynamics of Cdc42. One of several findings is that Cdc42-GTP is significantly less mobile than Cdc42-GDP, leading to its accumulation at sites of activity.

Omaya Dudin et al show in this publication how the actin cytoskeleton is focalized in a tight “fusion focus” in preparation for cell-cell fusion during the sexual life cycle. This permits the focalized delivery of enzymes that degrade the cell wall. This paper is also discussed in a comment in the same JCB issue.

Congratulations to Aleks for being awarded an EMBO long-term research fellowship!

Here is a link to a recent interview of Sophie Martin in the radio program CQFD on RSR1.

Sophie Martin will be awarded the EMBO Gold Medal 2014 at the EMBO conference in Paris. The press release from the EMBO can be accessed here. The press release from the University of Lausanne can be found here. An article from Lab Times can be found here.

Check our latest publication in the Journal of Cell Science. We show that ectopic localization of Tea4 on cell sides is sufficient to activate Cdc42 and promote growth at this location, yielding cells with a medial bulge. This activity depends on Tea4 binding a catalytic phosphatase 1 subunit, and indeed ectopic targeting of a catalytic subunit to the same location produces the same phenotype. This activity is further dependent on Gef1, a Cdc42 GEF, and Rga4, a Cdc42 GAP.

Sophie Martin receives the Friedrich Miescher award at the LS2 meeting 2014. The press release from the University of Lausanne can be found here.

Check our latest publication at Cell Cycle. We show that two distinct levels of Pom1 control cell cycle timing and positioning: partial inhibition of Pom1 promotes faster mitotic entry, but has no effect on the position of the division plane at the cell middle. We further show that Pom1 phosphorylates the C-terminal tail of Cdr2 to negatively regulate its activity, independently of Cdr2 localization. We also show in this publication that Pom1 levels at the medial cortex do not substantially vary in cells of increasing sizes. This raises questions about where Pom1 inhibits Cdr2 activity, and whether Pom1 acts as cell size sensor.

Congratulations to Libera Lo Presti who receives the Faculty Prize for her PhD thesis!

Congratulations to Kyriakos Kokkoris for successfully defending his PhD thesis!

See our latest publication at Current Biology. We show that zones of active Cdc42 dynamically localize to the cell periphery when cells enter the sexual differentiation process and search for a mate. Dynamic Cdc42 exploration depends on pheromone signaling and is required for the orientation of growth towards a mate.

Sophie Martin receives the Women in Cell Biology Junior Award at the 2012 ASCB meeting. The press release from the University of Lausanne can be found here.

Congratulations to Libera Lo Presti for successfully defending her PhD thesis!

Congratulations to Omaya Dudin for winning the poster prize at the EMBO conference “Experimental Approaches to Evolution and Ecology using Yeast”.

See our latest publication at Current Biology. By constructing a chimeric motor protein bearing a kinesin motor domain and a myosin cargo-binding domain, we show that retargeting of growth vesicles to cell poles along microtubules compensates for the absence of actin cables and restores a normal rod shape. A press release from the University of Lausanne can be found here.

Check our latest publication at Cell. We describe a complete molecular mechanism by which Pom1 forms concentration gradients at the plasma membrane. This occurs through local dephosphorylation at cell poles, which reveals a Pom1 lipid-binding region allowing plasma membrane binding. At the plasma membrane Pom1 then moves laterally and auto-phosphorylates, weakening lipid binding and promoting plasma membrane detachment. A press release from the University of Lausanne can be found here.

Sophie Martin receives an ERC Junior Grant, entitled “Geometric control of the cell cycle in the fission yeast“. Press release from the University of Lausanne.

Sophie Martin is selected on the EMBO YIP (Young Investigator Program). Press release here.

Check our new publication in Nature. We show that the polarity factor Pom1 acts as an inhibitor of mitotic entry, upstream of Wee1 kinase. Pom1 negatively regulates the kinase Cdr2, itself an inhibitor of Wee1. As Pom1 forms concentration gradients from cell poles, and Cdr2 localizes at the medial cortex, this regulatory network may allow cells to measure their own length and delay division until they reach sufficient size. The press release from the University of Lausanne can be found here.

Press release of the University of Lausanne here.