SCS3 and YFT2 Link Transcription of Phospholipid Biosynthetic Genes to ER Stress and the UPR

Robyn D Moir,David A Gross,Ian M Willis,David L Silver,Kaveh Ashrafi

doi:10.1371/journal.pgen.1002890

Robyn D Moir, David A Gross + Show 3 more

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https://doi.org/10.1371/journal.pgen.1002890

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Abstract

The ability to store nutrients in lipid droplets (LDs) is an ancient function that provides the primary source of metabolic energy during periods of nutrient insufficiency and between meals. The Fat storage-Inducing Transmembrane (FIT) proteins are conserved ER–resident proteins that facilitate fat storage by partitioning energy-rich triglycerides into LDs. FIT2, the ancient ortholog of the FIT gene family first identified in mammals has two homologs in Saccharomyces cerevisiae (SCS3 and YFT2) and other fungi of the Saccharomycotina lineage. Despite the coevolution of these genes for more than 170 million years and their divergence from higher eukaryotes, SCS3, YFT2, and the human FIT2 gene retain some common functions: expression of the yeast genes in a human embryonic kidney cell line promotes LD formation, and expression of human FIT2 in yeast rescues the inositol auxotrophy and chemical and genetic phenotypes of strains lacking SCS3. To better understand the function of SCS3 and YFT2, we investigated the chemical sensitivities of strains deleted for either or both genes and identified synthetic genetic interactions against the viable yeast gene-deletion collection. We show that SCS3 and YFT2 have shared and unique functions that connect major biosynthetic processes critical for cell growth. These include lipid metabolism, vesicular trafficking, transcription of phospholipid biosynthetic genes, and protein synthesis. The genetic data indicate that optimal strain fitness requires a balance between phospholipid synthesis and protein synthesis and that deletion of SCS3 and YFT2 impacts a regulatory mechanism that coordinates these processes. Part of this mechanism involves a role for SCS3 in communicating changes in the ER (e.g. due to low inositol) to Opi1-regulated transcription of phospholipid biosynthetic genes. We conclude that SCS3 and YFT2 are required for normal ER membrane biosynthesis in response to perturbations in lipid metabolism and ER stress.

Highlights

Eukaryotic cells store neutral lipids in cytoplasmic lipid droplets (LDs) surrounded by a monolayer of phospholipids and associated proteins [1]
In this work we show that specific functions of the Fat storage-Inducing Transmembrane (FIT) proteins are conserved between yeast and humans and that SCS3 and YFT2, the yeast homologs of mammalian FIT2, are part of a large genetic interaction network connecting lipid metabolism, vesicle trafficking, transcription, and protein synthesis
Given the known localization of Scs3 and mammalian FIT proteins in the ER membrane [10,77] and the negative genetic interactions with IRE1 and HAC1, we examined whether deletion of SCS3 and YFT2 creates an ER stress that constitutively activates the unfolded protein response (UPR)

Summary

Introduction

Eukaryotic cells store neutral lipids (triglycerides, TGs and steryl esters, SEs) in cytoplasmic lipid droplets (LDs) surrounded by a monolayer of phospholipids and associated proteins [1]. These sub-cellular structures are mobile, dynamic organelles that grow and shrink depending on metabolic conditions [2,3,4,5,6]. The FIT proteins exhibit different tissue distributions with FIT1 highly expressed skeletal muscle, lower levels in heart, and with FIT2 broadly distributed and most abundant in adipose tissue Studies in both cultured cells and mice have shown that overexpression of the FIT genes promotes the accumulation of LDs [10,11]. The data suggest that FIT protein function is critical for LD formation and can drive this process without affecting TG biosynthesis or turnover [10]

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