Differential gene expression drives cell-cycle-dependent transition from monopolar to bipolar growth in...

Cell polarity is important for maintaining cell structure and function. In Schizosaccharomyces pombe, after division, cells grow monopolarly from the old end, then transition to bipolar growth at a certain size when the new end activates. However, G1-arrested cells do not become bipolar despite continued growth, suggesting a role for the cell cycle in this process. To identify how the cell cycle impacts monopolar to bipolar transition, we performed high-throughput mRNA sequencing to detect differentially expressed genes in G1-arrested and G2-phase cells of the cell-cycle mutant cdc10-129. DESeq2 analysis identified 65 unique genes upregulated in G1 phase and 35 in G2 phase. Enrichment analysis shows that G1 phase cells upregulated the MAPK pheromone-response pathway, protein folding, rRNA processing, and heat-shock protein binding. G2 phase cells showed upregulation of plasma membrane maintenance and cell wall organization. In G2 phase cells, protein-protein interaction networks identified the cdc15-hob3-rho1-bgs1 hub, known to promote bipolar growth and regulate cell wall biogenesis. In G1 phase cells, the spk1-byr2-ste11 hub involved in pheromone-response and nutritional stress-dependent G1-arrest was identified. In agreement with these findings, we find that spk1Δ cells are precociously bipolar, indicating a role for this kinase in preventing bipolar growth. We hypothesize that bipolar growth in G2 cells requires a combination of factors that favor cell growth. In G2, stress response pathways are downregulated while anabolic pathways are upregulated, enabling transition from monopolar to bipolar growth.