TSC3 (YBR058C-A) encodes an 80-residue ER-membrane protein that is a small activating subunit of serine palmitoyltransferase (SPT), the Lcb1-Lcb2 enzyme catalysing the first committed step of sphingolipid synthesis (serine + acyl-CoA to 3-ketosphinganine). Tsc3 co-immunoprecipitates with Lcb1/Lcb2 and is a stoichiometric component of the SPOTS complex (SPT, Orm1/2, Tsc3, Sac1). tsc3 cells have severely reduced SPT activity and die at high temperature unless supplied with long-chain bases; Tsc3 also promotes C20 long-chain base formation, implying a role in acyl-CoA chain-length selection. It is fungus-specific and not homologous to the animal SPTSSA/B small subunits, although it fills an analogous activating role.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005783 endoplasmic reticulum | HDA PMID:26928762 One library to make them all: streamlining the creation of y... | ACCEPT | Summary: Yofe et al. 2016 SWAT-GFP library: endoplasmic reticulum. Consistent with Tsc3 as a subunit of the ER-membrane SPT complex. Reason: Correct. Supporting Evidence: PMID:26928762 One library to make them all: streamlining the creation of yeast libraries via a SWAp-Tag strategy. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt SubCell 'Endoplasmic reticulum membrane' (Gable et al. 2000 and Huh et al. 2003); Tsc3 has a hydrophobic C-terminal membrane segment. Reason: Site of SPT activity. Supporting Evidence: file:yeast/TSC3/TSC3-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane PMID:10713067 a hydrophobic carboxyl terminus that is membrane-associated |
| GO:0008047 enzyme activator activity | IDA PMID:10713067 Tsc3p is an 80-amino acid protein associated with serine pal... | ACCEPT | Summary: Gable et al. 2000 IDA: Tsc3 copurifies with Lcb1/Lcb2 and tsc3 membranes have severely reduced SPT activity. Tsc3 is a small activating subunit of SPT, not catalytic itself. Reason: Core function: activates the Lcb1-Lcb2 SPT enzyme. Supporting Evidence: PMID:10713067 The tsc3 mutant cells have severely reduced serine palmitoyltransferase activity PMID:10713067 Tsc3p coimmunoprecipitates with Lcb1p and/or Lcb2p but does not bind as tightly as Lcb1p and Lcb2p bind to each other |
| GO:0008047 enzyme activator activity | IMP PMID:10713067 Tsc3p is an 80-amino acid protein associated with serine pal... | ACCEPT | Summary: Gable et al. 2000 IMP: tsc3 cells have reduced SPT activity and a temperature-sensitive lethality rescued by long-chain bases. Reason: Core function. Supporting Evidence: PMID:10713067 The tsc3 mutant cells have severely reduced serine palmitoyltransferase activity PMID:10713067 S. cerevisiae cells lacking the TSC3 gene have a temperature-sensitive lethal phenotype that is reversed by supplying 3-ketosphinganine, dihydrosphingosine, or phytosphingosine in the growth medium |
| GO:0017059 serine palmitoyltransferase complex | IDA PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | ACCEPT | Summary: Breslow et al. 2010 IDA: mass spectrometry of the Orm-associated complex identified Lcb1, Lcb2 and Tsc3 together with Sac1 (SPOTS complex). Reason: Tsc3 is a stoichiometric SPT complex subunit. Supporting Evidence: PMID:20182505 Mass spectrometry showed that this complex comprises the serine palmitoyltransferase proteins Lcb1, Lcb2 and Tsc3, in addition to the phosphoinositide phosphatase Sac132 |
| GO:0017059 serine palmitoyltransferase complex | IPI PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | ACCEPT | Summary: Breslow et al. 2010 IPI: co-purification of Tsc3 with the SPT/Orm complex. Reason: Correct. Supporting Evidence: PMID:20182505 which we term the SPOTS complex (Serine Palmitoyltransferase, Orm1/2, Tsc3, and Sac1) |
| GO:0090154 positive regulation of sphingolipid biosynthetic process | IMP PMID:10713067 Tsc3p is an 80-amino acid protein associated with serine pal... | ACCEPT | Summary: Gable et al. 2000 IMP: loss of Tsc3 reduces SPT activity and long-chain base synthesis. As an activating subunit, Tsc3 positively regulates the first step of sphingolipid synthesis. Reason: Correct description of the activator role at process level. Supporting Evidence: PMID:10713067 The tsc3 mutant cells have severely reduced serine palmitoyltransferase activity PMID:10713067 S. cerevisiae cells lacking the TSC3 gene have a temperature-sensitive lethal phenotype that is reversed by supplying 3-ketosphinganine, dihydrosphingosine, or phytosphingosine in the growth medium |
| GO:0090154 positive regulation of sphingolipid biosynthetic process | IMP PMID:23286903 Iron, glucose and intrinsic factors alter sphingolipid compo... | ACCEPT | Summary: Lester et al. 2013 IMP: tsc3 deletion lowers C20 long-chain bases by 85% after the diauxic shift and shifts the PHS:DHS ratio. Reason: Correct; also shows a role in C20-LCB (acyl-CoA chain length) selection. Supporting Evidence: PMID:23286903 We conclude that Tsc3 promotes synthesis of C20-LCBs during and after the diauxic shift and adjusts the mix to favor formation of PHS over DHS. |
| GO:0090156 intracellular sphingolipid homeostasis | NAS PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | KEEP AS NON CORE | Summary: SGD NAS from Breslow et al. 2010, a paper on Orm1/2-mediated sphingolipid homeostasis in which Tsc3 is identified as a SPOTS complex subunit. The homeostatic feedback is executed by Orm1/2 phosphorylation; Tsc3's own contribution is SPT activation. Reason: Tsc3 is part of the complex that is homeostatically regulated, but the regulation is Orm-mediated. Supporting Evidence: PMID:20182505 which we term the SPOTS complex (Serine Palmitoyltransferase, Orm1/2, Tsc3, and Sac1) |
Loading supporting content…
Download this section (compressed HTML)Q: How does Tsc3 increase SPT activity mechanistically: by stabilising the Lcb1-Lcb2 active site, improving acyl-CoA access, or altering acyl-CoA chain-length preference (C18-CoA for C20 LCBs)?
Experiment: Measure SPT kinetics with C14-C18 acyl-CoAs using purified Lcb1-Lcb2 with and without Tsc3 (or with Tsc3 point mutants at the Lcb2 interface).
Hypothesis: Tsc3 shifts SPT acyl-CoA specificity toward longer chains
Type: enzymology
Protein that stimulates the activity of serine palmitoyltransferase; involved in sphingolipid biosynthesis; Lcb1p and Lcb2p are the two components of serine palmitoyltransferase
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)