PAK-family kinases promote cell fusion irreversibility by preventing cell wall repair

Cell-cell fusion is critical for the sexual life cycle, as it drives the unidirectional transition between haploid and diploid phases. In eukaryotes where fusion occurs between partners with cell wall, fusion requires not only plasma membrane merging but also local removal of the intervening cell wall. 1-3. However, the integrity of the cell wall, which resists the strong internal turgor pressure and is essential for survival 4, is normally monitored by signaling pathways that promote repair upon damage 5,6. How cell wall integrity pathways allow localized cell wall degradation is not known. In the fission yeast Schizosaccharomyces pombe, we previously identified pak2{Delta} mutants that exhibit transient fusion: cells briefly fuse and exchange cytoplasmic contents that signal meiosis but then reseal their fusion pore. This reversal of fusion directionality induces meiosis in haploid cells, with catastrophic consequences 7. Here, we show that PAK activity is essential for cell-cell fusion, with major contribution for Pak2 and a more minor role for the essential Pak1. Pak2 accumulates at the fusion site, where its kinase activity is controlled to promote both fusion pore opening and expansion, thereby preventing post-fusion pore closure. Strikingly, osmotic stabilization largely suppresses all PAK-associated fusion defects, pointing to a role in cell-wall remodeling rather than plasma-membrane merging. Indeed, correlative light and cryo-soft-X-ray tomography reveals that resealed pores in PAK mutants rebuilt an intact cell wall. Thus, PAKs enforce fusion directionality by antagonizing cell-wall repair mechanisms that otherwise restore separation between mating partners.