Recombinant Saccharomyces cerevisiae FIT family protein YFT2 (YFT2)

What is YFT2 and how is it related to other FIT family proteins?

YFT2 is one of two Saccharomyces cerevisiae homologs of the mammalian Fat storage-Inducing Transmembrane (FIT) protein family, with SCS3 being the other yeast homolog. These proteins are conserved ER-resident transmembrane proteins that facilitate fat storage by partitioning energy-rich triglycerides into lipid droplets (LDs) . The FIT protein family was initially identified in mammals, with FIT2 being the ancient ortholog that has homologs across various species . In S. cerevisiae and other fungi of the Saccharomycotina lineage, FIT2 has evolved into two distinct homologs: SCS3 and YFT2 . These proteins maintain significant sequence conservation, particularly within their predicted transmembrane domains, with human FIT2 showing E values of 3.7E-5 and 5.5E-7 for SCS3 and YFT2 respectively in BLAST searches .

What is the evolutionary relationship between YFT2, SCS3, and mammalian FIT2?

YFT2 and SCS3 have coevolved for more than 170 million years in fungi while diverging from higher eukaryotes . Despite this evolutionary distance, these proteins retain remarkable functional conservation with mammalian FIT proteins. This conservation is demonstrated by cross-species complementation: expression of human FIT2 in yeast rescues the inositol auxotrophy and other phenotypes of strains lacking SCS3, while expression of either yeast YFT2 or SCS3 in human embryonic kidney cells promotes lipid droplet formation . The ability of these proteins to function across evolutionary boundaries suggests they maintain core structural and functional properties that have been preserved since their divergence . This evolutionary conservation makes YFT2 an excellent model for studying fundamental aspects of lipid metabolism that apply across eukaryotic species.

What is the membrane topology and structure of YFT2?

YFT2, like other FIT family proteins, is predicted to have an even number of transmembrane (TM) helices with both N and C termini facing the cytosol . Experimentally-constrained topology studies of the yeast proteome predict that Yft2 has cytosolic N and C termini, consistent with findings for murine FIT proteins that demonstrated a six-transmembrane domain organization . The most highly conserved amino acid residues across species are located within the fourth transmembrane domain . This domain appears to be functionally significant, as a gain-of-function mutation in the conserved TM4 domain of mouse FIT2 (FLL(157-9)AAA) increases triglyceride binding and lipid droplet size while altering the conformation of a cytoplasmic loop connecting TM domains 2 and 3 . Recent studies have identified the catalytic site of a lipid phosphatase in ScFIT protein sequences, suggesting enzymatic activity that may be essential for their function .

What are the molecular mechanisms by which YFT2 influences lipid metabolism?

YFT2 appears to influence lipid metabolism through multiple interconnected molecular mechanisms. Genetic interaction studies indicate that YFT2, together with SCS3, functions in a network connecting lipid metabolism, vesicular trafficking, transcription of phospholipid biosynthetic genes, and protein synthesis . The lipid phosphatase activity identified in FIT family proteins suggests that YFT2 may directly modify lipid species, potentially converting phosphatidic acid to diacylglycerol, which could influence the balance of membrane phospholipids and neutral storage lipids . YFT2 and SCS3 are required for normal ER membrane biosynthesis, particularly in response to perturbations in lipid metabolism and ER stress . The genetic data indicate that optimal strain fitness requires a balance between phospholipid synthesis and protein synthesis, and deletion of YFT2 and SCS3 impacts a regulatory mechanism that coordinates these processes . This suggests that YFT2 may be part of a feedback system that helps maintain homeostasis between different metabolic pathways involved in membrane biogenesis and protein production.

How do YFT2 and SCS3 interact with the unfolded protein response (UPR) pathway?

YFT2 and SCS3 have distinct interactions with the unfolded protein response (UPR) pathway, which is activated during ER stress. While deletion of YFT2 or SCS3 does not significantly alter basal ER stress-induced UPR, SCS3 is essential for proper stress-induced UPR activation . SCS3 is also essential for viability in the absence of IRE1, the sole yeast UPR transducer . This is evidenced by the creation of a temperature-sensitive allele of SCS3 that is functional at a permissive temperature of 25°C but not at the restrictive temperature of 37°C in strains lacking IRE1 . In contrast, IRE1 is not essential in the absence of YFT2, indicating a specific functional relationship between SCS3 and the UPR pathway that is not shared by YFT2 . Cells with mutated SCS3 exhibited an accumulation of triacylglycerol within the ER along with aberrant LD morphology, suggesting that there is a UPR-dependent compensatory mechanism that acts to mitigate lack of SCS3 . These findings suggest that the mammalian FIT genes may play an important role in ER stress pathways, which are linked to obesity and type 2 diabetes .

What genetic interactions have been identified for YFT2, and what do they reveal about its function?

Genetic interaction studies have placed YFT2 and SCS3 within a complex network that connects multiple cellular processes. Using synthetic genetic array (SGA) analysis, researchers have identified numerous synthetic genetic interactions for YFT2 and SCS3 that link them to lipid metabolism, vesicular trafficking, transcription of phospholipid biosynthetic genes, and protein synthesis . Particularly notable is the synthetic lethality between SCS3 and IRE1, indicating that each is required for viability in the absence of the other . A split-ubiquitin-based membrane yeast two-hybrid (MYTH) screen has also identified potential protein-protein interactions for YFT2 and SCS3, with 664 colonies positive for bait-prey interactions out of 1344 colonies screened . These genetic and protein interaction data collectively indicate that YFT2 and SCS3 have both shared and unique functions that are integral to cellular homeostasis, particularly under conditions of metabolic or ER stress . The extensive network of genetic interactions suggests that YFT2 functions at the intersection of multiple cellular processes rather than being restricted to a single pathway.

What experimental approaches can be used to study YFT2 function across different model systems?

Several experimental approaches have proven valuable for studying YFT2 function across different model systems. Cross-species complementation assays have been particularly informative, demonstrating functional conservation between yeast and mammalian FIT proteins . Expression of YFT2 in human cell lines can be used to assess its capacity to induce lipid droplet formation, while expression of human FIT2 in yeast can test for complementation of yft2Δ or scs3Δ phenotypes . Chemical genetic approaches, testing growth of deletion strains under various chemical stresses, can reveal condition-specific requirements for YFT2 . Synthetic genetic array (SGA) analysis can identify genetic interactions that place YFT2 in broader cellular networks . Protein-protein interaction studies, such as split-ubiquitin-based membrane yeast two-hybrid (MYTH) screening, can identify physical interaction partners . Advanced microscopy techniques using lipid-specific dyes like BODIPY 493/503 allow visualization of lipid droplets and assessment of YFT2's impact on their formation and morphology . Biochemical assays to test for lipid phosphatase activity with various substrates can help determine YFT2's enzymatic functions .

How does YFT2 contribute to cellular stress responses beyond lipid metabolism?

YFT2, together with SCS3, contributes to cellular stress responses beyond direct lipid metabolism through multiple mechanisms. These proteins are required for normal ER membrane biosynthesis in response to perturbations in lipid metabolism and ER stress . While YFT2's role appears less critical than SCS3 in UPR activation, the double deletion strain shows more pronounced defects in stress responses . YFT2 may influence cellular proteostasis, as genetic data indicate that optimal strain fitness requires a balance between phospholipid synthesis and protein synthesis . The mechanism involves communication of changes in the ER (such as low inositol levels) to transcriptional regulators of phospholipid biosynthetic genes, particularly Opi1-regulated genes . Additionally, genetic interaction studies suggest connections between YFT2 and vesicular trafficking pathways, which are essential for cellular adaptation to various stresses . The evolutionary conservation of YFT2 function across species suggests it plays a fundamental role in cellular stress adaptation that has been maintained throughout eukaryotic evolution .

Comparative Properties of YFT2, SCS3, and Mammalian FIT2