The longer chain: A link between polyprenols and outer spore wall formation in Saccharomyces cerevisia

Polyprenols are isoprene polymers present in all three domains of life. They are best characterized as sugar carriers and anchors during N-glycosylation and O-mannosylation, but other roles for polyprenols have been speculated and there are types of polyprenols present in cells with yet-unknown function. My work suggests an alternative role for polyprenols during sporulation in Saccharomyces cerevisiae, and may imply similar roles in various forms of life. In S. cerevisiae, two protein complexes act as cis-prenyltransferases to synthesize polyprenols; these are dimers of Nus1 and either Rer2 or Srt1. Rer2 is important to N-glycosylation and its loss results in cells with growth and trafficking defects. In contrast, srtDelta cells grow normally. The deletion of either both SRT1 and RER2 or NUS1 alone is lethal. While it shows no strong vegetative phenotype, srtDelta was identified as causing sporulation defects by multiple studies. My work uses transmission electron microscopy and fluorescent staining to characterize the phenotype of srtDelta cells as a failure to produce chitin during outer spore wall formation, resulting in the failure to produce the protective outer spore wall layers that are unique to spores in yeast. Further, I demonstrate that outer spore wall formation requires the cis-prenyltransferase activity of Srt1, and thus requires its product polyprenols. I show that Srt1 regulates the activity of Chs3, the chitin synthase necessary for chitin formation in sporulation, without affecting the localization of Chs3. Together, this strongly suggests a regulatory function involving polyprenols that is required for the activity of Chs3 during sporulation -- novel in yeast and, due to the ubiquity of polyprenols, potentially universal.