Professor Pilar Pérez, Instituto de Biología Funcional y Genómica (IBFG) CSIC/USAL, Spain
Wednesday 17th June, 4.00 p.m., Stacey Lecture Theatre 1
Cytokinesis requires assembly of a contractile actomyosin ring adjacent to the membrane which upon closure pulls the cell membrane to form a cleavage furrow. In fungi ring closure is also coordinated with the synthesis of a cell wall septum. Knowledge about the molecules anchoring the contractile ring to the membrane is very limited. Fission yeast paxillin homolog, Pxl1, is localized to the ring and is required for ring integrity and efficient cytokinesis. We will present data demonstrating that cooperation between Pxl1 and Bgs1, the enzyme responsible for primary septum formation, is required for: i) stable anchorage of the CAR to the plasma membrane before septation onset; ii) ring structure integrity; and iii) cleavage furrow ingression and septum deposition. Accordingly, Paxillin is essential when Bgs1 is depleted. In these conditions, contractile rings form but the lateral cell wall overgrows inwards without a defined cleavage furrow, and septa are not completed. During cytokinesis there is an increase of paxillin concentration at the ring that depends on the SH3 domain of the F-BAR protein Cdc15. Consequently, the absence of this domain mimics the phenotype of the lack of paxillin. On the other hand, the absence of paxillin is lethal when Cdc15 function is affected, with cells showing a large sliding of the CAR and deposition of septum wall material along the cell cortex in the absence of ring constriction. This suggests additional functions for both paxillin and Cdc15 proteins that couple ring constriction and septum wall synthesis. In summary, CAR anchorage to the plasma membrane through Cdc15 and Pxl1 together with Bgs1 activity, are necessary for CAR maintenance, and for formation of the cleavage furrow and the septum in fission yeast.