Global Analysis of Fission Yeast Mating Genes Reveals New Autophagy Factors

Abstract

Macroautophagy (autophagy) is crucial for cell survival during starvation and plays important roles in animal development and human diseases. Molecular understanding of autophagy has mainly come from the budding yeast Saccharomyces cerevisiae, and it remains unclear to what extent the mechanisms are the same in other organisms. Here, through screening the mating phenotype of a genome-wide deletion collection of the fission yeast Schizosaccharomyces pombe, we obtained a comprehensive catalog of autophagy genes in this highly tractable organism, including genes encoding three heretofore unidentified core Atg proteins, Atg10, Atg14, and Atg16, and two novel factors, Ctl1 and Fsc1. We systematically examined the subcellular localization of fission yeast autophagy factors for the first time and characterized the phenotypes of their mutants, thereby uncovering both similarities and differences between the two yeasts. Unlike budding yeast, all three Atg18/WIPI proteins in fission yeast are essential for autophagy, and we found that they play different roles, with Atg18a uniquely required for the targeting of the Atg12–Atg5·Atg16 complex. Our investigation of the two novel factors revealed unforeseen autophagy mechanisms. The choline transporter-like protein Ctl1 interacts with Atg9 and is required for autophagosome formation. The fasciclin domain protein Fsc1 localizes to the vacuole membrane and is required for autophagosome-vacuole fusion but not other vacuolar fusion events. Our study sheds new light on the evolutionary diversity of the autophagy machinery and establishes the fission yeast as a useful model for dissecting the mechanisms of autophagy.

Molecular interactions of theSaccharomyces cerevisiaeAtg1 complex provide insights into assembly and regulatory mechanisms

“…2,[10][11][12][13] Of these, 20 core proteins are essential for autophagosome formation. These proteins can be classified into 5 functional groups: (i) the Atg1 complex, (ii) the autophagy-specific phosphatidylinositol 3-kinase (PtdIns3K) complex, (iii) Atg9 and the Atg2-Atg18 cycling system, (iv) the Atg12 conjugation system, and (v) the Atg8 conjugation system.…”

Atg20- and Atg24-family proteins promote organelle autophagy in fission yeast

“…S1B,C). Thus, similar to the situation in starvation-induced bulk autophagy (Sun et al, 2013), starvation-induced organelle autophagy requires all five of these Atg proteins in fission yeast.…”

“…To examine whether organelle autophagy occurs in wild-type cells, we applied nitrogen starvation treatment, which is a trigger of general autophagy in S. pombe (Kohda et al, 2007;Mukaiyama et al, 2009;Sun et al, 2013), and monitored the subcellular localization of two organelle markers, an ER integral membrane protein Ost4 and a mitochondrial matrix protein Sdh2. We used Cpy1 as a vacuole lumen marker (Tabuchi et al, 1997).…”

“…Within the Atg24 family, S. pombe Atg24 is a closer homolog of S. cerevisiae Atg24 than is S. pombe Atg24b. Autophagy proteins in both budding yeast and fission yeast usually localize to a subcellular site called the pre-autophagosomal structure or phagophore assembly site (PAS) when autophagy occurs (Sun et al, 2013;Xie and Klionsky, 2007). Budding yeast Atg20 and Atg24 are PASlocalizing proteins (Nice et al, 2002).…”

“…We have previously comprehensively identified and characterized the fission yeast genes that are required for starvation-induced autophagy, and unveiled interesting differences in autophagic mechanisms between the two model yeasts (Sun et al, 2013). For example, unlike its budding yeast counterpart, fission yeast Atg11 is required for general autophagy.…”