Abstract
In fungal cells cytokinesis requires coordinated closure of a contractile actomyosin ring (CAR) and synthesis of a special cell wall structure known as the division septum. Many CAR proteins have been identified and characterized, but how these molecules interact with the septum synthesis enzymes to form the septum remains unclear. Our genetic study using fission yeast shows that cooperation between the paxillin homolog Pxl1, required for ring integrity, and Bgs1, the enzyme responsible for linear β(1,3)glucan synthesis and primary septum formation, is required for stable anchorage of the CAR to the plasma membrane before septation onset, and for cleavage furrow formation. Thus, lack of Pxl1 in combination with Bgs1 depletion, causes failure of ring contraction and lateral cell wall overgrowth towards the cell lumen without septum formation. We also describe here that Pxl1 concentration at the CAR increases during cytokinesis and that this increase depends on the SH3 domain of the F-BAR protein Cdc15. In consequence, Bgs1 depletion in cells carrying a cdc15ΔSH3 allele causes ring disassembly and septation blockage, as it does in cells lacking Pxl1. On the other hand, the absence of Pxl1 is lethal when Cdc15 function is affected, generating a large sliding of the CAR with deposition of septum wall material along the cell cortex, and suggesting additional functions for both Pxl1 and Cdc15 proteins. In conclusion, our findings indicate that CAR anchorage to the plasma membrane through Cdc15 and Pxl1, and concomitant Bgs1 activity, are necessary for CAR maintenance and septum formation in fission yeast.
Highlights
Cytokinesis is the final stage of the eukaryotic cell cycle, when a mother cell separates into two daughter cells
In fungi ring closure is coordinated with the synthesis of a cell wall septum
We have found that fission yeast paxillin, located at the ring, and Bgs1, the enzyme responsible for primary septum formation, located at the membrane, cooperate during cytokinesis
Summary
Cytokinesis is the final stage of the eukaryotic cell cycle, when a mother cell separates into two daughter cells. The septum of fission yeast is a three-layered polysaccharide structure made of a middle primary septum (PS) flanked by two secondary septa (SS), one on each side Both the PS and SS are formed by essential β-glucans. Bgs is responsible for the linear β(1,3)glucan necessary for PS formation [11]; Bgs builds branched β(1,3)glucan [12], which is the most abundant polymer in the septum and cell wall. During cytokinesis this polymer is required for connecting the CAR to the extracellular cell wall, for SS formation, for the correct PS structure, and to maintain the cell integrity during cell separation [4]. During cytokinesis α-glucan is essential for the PS adhesion strength needed to withstand the internal turgor pressure during cell abscission, for the SS structure, and for cell integrity [15]
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