Clustering-mediated activation of Cdc42 GTPase antagonized by GAPs in fission yeast

Abstract

The small GTPase Cdc42 is critical for cell polarization in eukaryotic cells. In rod-shaped fission yeast Schizosaccharomyces pombe cells, active GTP-bound Cdc42 promotes polarized growth at cell poles, while inactive Cdc42-GDP localizes ubiquitously also along cell sides. Zones of Cdc42 activity are maintained by positive feedback amplification involving the formation of a complex between Cdc42-GTP, the scaffold Scd2 and the guanine nucleotide exchange factor (GEF) Scd1, which promotes the activation of more Cdc42. Here, we use the CRY2-CIB1 optogenetic system to recruit and cluster a cytosolic Cdc42 allele at the plasma membrane and show that this leads to its moderate activation also on cell sides. Surprisingly, activation of CRY2-Cdc42 does not individually depend on Scd1 or the GEF Gef1. We show that activated Cdc42 clusters at cell sides are able to recruit Scd1, dependent on the scaffold Scd2. However, Cdc42 activity is not amplified by positive feedback and does not lead to morphogenetic changes, due to antagonistic activity of the GTPase activating protein Rga4 on cell sides. Thus, the cell architecture is robust to moderate activation of Cdc42 at cell sides.

Cdc42 reactivation at growth sites is regulated by local cell-cycle-dependent loss of its GAP Rga4

“…However, after division, Cdc42 activation at the cell ends requires both removal of Rga4 from the cell ends and MOR activation. A recent preprint has shown that presence of the GAPs at the cortex prevents Cdc42 activation at those sites even when the scaffold Scd2, which recruits Scd1 to the cortex, localizes to those sites (Lamas et al, 2020b). Another preprint suggests that Cdc42 GAP levels in daughter cells determine the pattern of polarized growth in those cells (Pino et al, 2020).…”

“…Polarized Cdc42 activation depends on Ras1 GTPase, which promotes polarized localization of the Cdc42 GEF Scd1 (Chang et al, 1994;Chen et al, 2019;Lamas et al, 2020b). However, ras1Δ mutants continue to display the PrESS phenotype after recovery from LatA treatment (Supplementary figure S3B).…”