The yeast FIT2 homologs are necessary to maintain cellular proteostasis and membrane lipid homeostasis

Abstract

Lipid droplets (LDs) are implicated in conditions of lipid and protein dysregulation. The fat storage inducing transmembrane (FIT) family induces LD formation. Here, we establish a model system to study the role of S. cerevisiae FIT homologues (ScFIT), SCS3 and YFT2, in proteostasis and stress response pathways. While LD biogenesis and basal endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) remain unaltered in ScFIT mutants, SCS3 was found essential for proper stress-induced UPR activation and for viability in the absence of the sole yeast UPR transducer IRE1. Devoid of a functional UPR, muted SCS3 exhibited accumulation of triacylglycerol within the ER along with aberrant LD morphology, suggesting a UPR-dependent compensatory mechanism. Additionally, SCS3 was necessary to maintain phospholipid homeostasis. Strikingly, global protein ubiquitination and the turnover of both ER and cytoplasmic misfolded proteins is impaired in ScFITΔ cells, while a screen for interacting partners of Scs3 identifies components of the proteostatic machinery as putative targets. Together, our data support a model where ScFITs play an important role in lipid metabolism and proteostasis beyond their defined roles in LD biogenesis.

Destabilization of β-cell FIT2 by saturated fatty acids contribute to ER stress and diabetes

“…Further inhibition of ceramide synthesis by Fumonisin B1 ameliorated palmitatemediated ER stress in MIN6 cells. When coupled with increased C16:0 ceramides levels in islets that lack FIT2, the data suggests that increased ceramides link β-cell FIT2 loss with enhanced ER stress, adding to current knowledge of FIT2 loss leading to ER lipid imbalance and ER dysregulation (23,24). We present a pathway for how palmitate drives ER stress, with FIT2 stability and LD accumulation taking the center stage in β-cells.…”

“…FIT2, a critical protein for LD formation, has been shown to promote ER lipid coalescence and LD formation, a function that is likely linked to its catalytic lipid phosphatase activity (15,18,22,23). Most recently, FIT2 was implicated in ER protein dysregulation, ER structure maintenance and the maladaptive ER response in both yeast and mammalian cells (23,24). Despite this, the role and importance of FIT2 in β-cell LD formation, function and survival, especially during lipotoxicity, remain unknown.…”

“…CC-BY 4.0 International license perpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in mammalian cells (23,24). Despite this, the role and importance of FIT2 in β-cell LD formation, function and survival, especially during lipotoxicity, remain unknown.…”

The small GTPase Arf1 regulates ATP synthesis and mitochondria homeostasis by modulating fatty acid metabolism

“…Therefore, we first asked whether increase in LD biogenesis was due to ER stress. We deleted in wildtype yeast, the two mammalian FIT2 homologues, SCS3 and YFT2, known to connect ER stress response and LD biogenesis [47], and to maintain cellular proteostasis and membrane lipid homeostasis at the ER (Figure S6A, B) [48]. In these strains, LD accumulation and mitochondrial morphology remained unaffected and did not phenocopy yarf1-11.…”

FIT2 organizes lipid droplet biogenesis with ER tubule-forming proteins and septins

“…Conversely, LD formation can be induced when FITs are overexpressed ( Kadereit et al, 2008 ). FIT2 is capable of binding to DAG and triacylglycerol ( Gross et al, 2011 ), regulates lipid homeostasis in the ER ( Becuwe et al, 2020 ; Yap et al, 2020 ), and has been implicated in the regulation of nascent LD budding ( Choudhary et al, 2015 ). A second key protein is the ER-localized integral membrane protein seipin.…”