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EMBL News

Heidelberg, 25 February 2013

Yeast’s fusional relationships

Sexual reproduction in yeast and fertilisation in mammals involve similar mechanisms

Two nuclei move closer by pulling on each other’s microtubules, during sexual reproduction in yeast. (Credit: EMBL/F. Nédélec, R. Gibeaux, and A. Politi)


Two nuclei move closer by pulling on each other’s microtubules, during sexual reproduction in yeast. Microtubules can either grow (green arrow head) or shrink (red arrow head) during the process. Each nucleus (blue sphere) is associated with a specific organelle called the Spindle Pole Body (SPB, yellow sphere). Kar3 motors (green dots) are anchored on the SPB, and by pulling on microtubules originating from the other SPB bring along the nuclei together. (
Credit: EMBL/F. Nédélec, R. Gibeaux, and A. Politi)

Click image to play on Youtube.

Yeast get involved in preliminaries too: cells of opposite mating types ‘tickle’ each other with pheromones to trigger attraction and prepare for mating. They then fuse their external membranes to yield one cell with two nuclei, and there is no going back: the two nuclei must come together to become one, so the resulting yeast cell can inherit the genetic information from its two ‘parents’. Researchers at EMBL have elucidated a crucial mechanism of this two-step relationship: how the two nuclei move and get close enough to be able to fuse. Their findings were published in February 2013 in Genes & Development.

As a general rule, the various elements inside a cell don’t just float randomly in the plasma: they hold precise positions; when they need to change position, like the nucleus for example, they may do so by moving along the microtubules – those tube-like elements that make up the skeleton of the cell. In the yeast cell that results from mating, each of the two nuclei has a set of microtubules protruding from one point on its surface, very much like the leaves of a radish. Each nucleus uses the microtubules originating from the other one, rather than its own, to regulate its position.

“We discovered that the point from where microtubules grow, on the surface of the nucleus, also hosts a motor protein called Kar3,” explain François Nédélec and Claude Antony, who co-led the study at EMBL with Michael Knop. “Kar3 binds both to the surface of one of the nuclei and to the microtubules of the other, and it is able to apply a force to attract the second nucleus towards the first one until they touch.”

This mechanism is very similar to what happens in higher types of cells during fertilisation and, as important mechanisms are often conserved across species, this strengthens the scientists’ conclusions. As yeast is one of the most widely used organisms in laboratories, any new knowledge about the various processes regulating its life-cycle can bring valuable understanding to many research teams across the globe.

Researchers were able to elucidate - and even visualise - this mechanism thanks to the collaboration of three EMBL teams with very specific and complementary expertise: physical biology with François Nédélec, electron tomography with Claude Antony (now at the IGBMC, Strasbourg), and molecular biology and live-cell imaging with Michael Knop (now at the University of Heidelberg).

Source article

Spindle pole body-anchored Kar3 drives the nucleus along microtubules from another nucleus in preparation for nuclear fusion during yeast karyogamy - Romain Gibeaux, Antonio Z. Politi, François Nédélec, Claude Antony and Michael Knop – Published in Genes and Development on 1 February 2013.

Watch video on Youtube: Two nuclei moving closer by pulling on each other’s microtubules, during sexual reproduction in yeast.

Two nuclei move closer by pulling on each other’s microtubules, during sexual reproduction in yeast. Microtubules can either grow (green arrow head) or shrink (red arrow head) during the process. Each nucleus (blue sphere) is associated with a specific organelle called the Spindle Pole Body (SPB, yellow sphere). Kar3 motors (green dots) are anchored on the SPB, and by pulling on microtubules originating from the other SPB bring along the nuclei together. (Credit: EMBL/F. Nédélec, R. Gibeaux, and A. Politi)