Cell-cell fusion is central for fertilization during sexual reproduction. It also underlies the formation of some tissues, such as muscles, during development. We study how yeast gametes fuse to produce the zygote during sexual reproduction.
The pairing of fission yeast cells of opposite mating types in the initial stages of the mating process culminates in fusion of these two haploid cells to form a diploid zygote. For this, the cell wall that surrounds each of the cells has to be locally remodeled, so as to form a single shell allowing local wall dissolution at the point of contact between the two cells. This must be achieved to permit plasma membrane contact for fusion and formation of a joint cytoplasm. Because yeast cells are under strong turgor pressure, dissolution of the cell wall must be very precise, to avoid lysis of the cells in the hypotonic environment.
We have show that fission yeast cells organize a specific actin structure – the actin fusion focus – to achieve this. The actin fusion focus is an aster-like organization of actin filaments nucleated by the formin Fus1, whose barbed ends are focused close to the plasma membrane and allow the focalized accumulation of type V myosins. One likely important cargo for the myosins are vesicles containing glucanases, enzymes that digest the cell wall and whose delivery is focalized at the actin fusion focus. By contrast, cell wall synthases, which catalyze the formation of bonds between glucan polymers, are more broadly distributed at the cell-cell contact site. We propose that this geometric separation of cell wall hydrolases and synthases achieves the local cell wall digestion required for successful cell-cell fusion.
We are very interested in working out the communication required for each of the two partner cells to organize an actin fusion focus at precisely facing locations, and defining how this event is temporally coordinated with cell-cell contact.