Fss1, a CTNS/PQ domain protein, has dual roles in promoting fertility in Schizosaccharomyces pombe

In the study of Schizosaccharomyces pombe, a single protein known as Fss1 emerges as a dual player in the realm of fertility. Fss1, originally identified through a broad genetic survey, has been found to possess a structural similarity to a human protein, the PQ-loop repeat family PQLC2 transceptor. However, unlike its human counterpart, Fss1 lacks the capability to transport amino acids due to the absence of a crucial segment known as the PQ motif.

Interestingly, the fss1 mutation does not adversely affect meiosis, the process that leads to the formation of gametes. However, it does cause significant issues in the packaging of the meiosis products into spores, especially when the environment is nutrient-deprived. The spore formation phenotypes in fss1{Delta} mutants are recessive, meaning that spores produced by a heterozygote (an organism with one normal and one mutated gene) develop normally.

Further examination under an immuno-gold electron microscope reveals that Fss1 localizes to vesicles that subsequently fuse to the ephemeral outer spore membrane as the spore wall matures below. It has also been discovered that Fss1 protein is synthesized within spores, and this population is crucial for maintaining normal vacuole morphology and ensuring timely germination.

The researchers propose a model where Fss1 functions as a vesicle receptor, facilitating the formation of the spore wall from proteins expressed during the diploid stage. Meanwhile, a spore-expressed version of the protein operates within the vacuole to foster an internal cellular environment that promotes germination. This discovery not only broadens the list of genes contributing to sexual reproduction but also identifies two previously unknown roles of Fss1 in spore biology.