Cooperation between Paxillin-like Protein Pxl1 and Glucan Synthase Bgs1 Is Essential for Actomyosin Ring Stability and Septum Formation in Fission Yeast

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.

The Cell Biology of Fission Yeast Septation

“…Despite these observations, it is still unknown whether it is Bgs1 itself or its product that is responsible for the stable AMR maintenance in the cell middle and how this attachment is achieved. Recently, it has been reported that absence of paxillin Pxl1, a conserved ring protein that is required for AMR integrity and whose localization depends on the SH3 domain of Cdc15 (89)(90)(91)(92), induces simultaneous Bgs1 sliding and AMR sliding from the cell middle, indicating that the mere presence of Bgs1 alone is not enough to stably maintain the ring location (43). In agreement, the cps1-191 mutant, in which the function of Bgs1 is compromised, also displays AMR sliding (43,46,87,88).…”

“…However, since there are no biochemical data supporting the proposed reduction of the septum ␤(1,3)-D-glucan or the in vitro GS activity, it is unknown how these mutations compromise the biosynthetic activity of Bgs1. It has recently been reported that cps1-191 allows the slow formation of a partial septum which is heterogeneously stained with the fluorochrome calcofluor white (CW), so it is possible that this mutated version of Bgs1 still maintains some residual activity at high temperatures (43,71). The finding that Bgs1 was localized at the AMR, and that it was essential for cell survival, suggested that it was required for PS formation (44,45).…”

“…Recently, it has been reported that absence of paxillin Pxl1, a conserved ring protein that is required for AMR integrity and whose localization depends on the SH3 domain of Cdc15 (89)(90)(91)(92), induces simultaneous Bgs1 sliding and AMR sliding from the cell middle, indicating that the mere presence of Bgs1 alone is not enough to stably maintain the ring location (43). In agreement, the cps1-191 mutant, in which the function of Bgs1 is compromised, also displays AMR sliding (43,46,87,88). The precise coincidence between the emergence of a PS structure and the arrest of AMR sliding suggests that the cleavage furrow of PS and membrane ingression might be required for the stable AMR maintenance in the cell middle (43).…”

“…The precise coincidence between the emergence of a PS structure and the arrest of AMR sliding suggests that the cleavage furrow of PS and membrane ingression might be required for the stable AMR maintenance in the cell middle (43). Interestingly, the combined reduction of Pxl1 function and Cdc15 function induces Bgs1 and AMR sliding even after activation of synthesis of L-BG material, which is deposited along the longitudinal axis of the cell without cleavage furrow formation, suggesting that cooperation between the two proteins is needed to coordinate simultaneous activation of Bgs1 GS function and AMR constriction (43).…”

“…A reduction of Bgs1 function through the use of the cps1-191 mutant induced the formation of disorganized rings, showing that the septum L-BG or Bgs1 is required to maintain a stable AMR structure during septum progression (43). Bgs1 also collaborates with Pxl1 in maintaining the AMR and allowing septum ingression.…”