Cell fusion requires the reorganization of cellular envelopes, which is a challenging process that can produce cell lysis. The involvement of Schizosaccharomyces pombe Prm1p in cell fusion and its relationship with other fusion-promoting proteins has been addressed. prm1Δ cells fail to fuse due to an abnormal organization of plasma membrane and cell wall at the area of cell-cell contact. Although this phenotype is similar to that described for cells devoid of the claudin-like Dni proteins, prm1+ and the dni+ genes act in different pathways. A significant number of prm1Δ, dni2Δ, and fus1Δ zygotes lyse during mating. For prm1Δ and fus1Δ, lysis is maximal in the presence of high calcium concentrations at low temperature. In prm1Δ, zygote lysis does not correlate with an abnormal actin distribution at the shmoo tip, and Prm1p is normally localized in fus1Δ zygotes, suggesting that prm1+ and fus1+ act in different pathways. Reducing calcium concentration or deleting dni2+ does not enhance zygote lysis in prm1Δ, showing that the roles of budding and fission yeast Prm1 proteins are not equivalent. Our results show that multiple functional pathways and environmental conditions are required to ensure cell fusion and cell integrity during mating, and that the role of each of those cues varies with the biology of different organisms.

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