ERG9 (YHR190W) encodes squalene synthase (farnesyl-diphosphate farnesyltransferase, EC 2.5.1.21) of Saccharomyces cerevisiae, which catalyses the first committed step of sterol biosynthesis: head-to-head condensation of two molecules of farnesyl diphosphate (FPP) to presqualene diphosphate, followed by NAD(P)H-dependent reductive rearrangement to squalene. Erg9 thus sits at the branch point that partitions FPP between sterols and non-sterol isoprenoids (dolichol, ubiquinone, prenylated proteins). It is a member of the phytoene/squalene synthase family, requires Mg2+, and is anchored to the ER membrane by a hydrophobic C-terminal segment; the truncated soluble catalytic domain retains full two-step activity. The microsomal enzyme requires phospholipid for activity. Erg9 is enriched in plasma-membrane-associated ER and erg9 mutants are sterol auxotrophs.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:16300994 A systematic study of yeast sterol biosynthetic protein-prot... | REMOVE | Summary: Protein binding (IPI) with Erg26 (P53199) from the Mo & Bard 2005 split-ubiquitin screen of sterol enzymes. The interaction suggests co-organisation of ergosterol enzymes ('ergosome') in the ER membrane, but no functional consequence for Erg9 is known and 'protein binding' conveys no function. Removed per the protein-binding policy; the interaction itself is not disputed. Supporting Evidence: PMID:16300994 We employed a split-ubiquitin membrane protein yeast two-hybrid system to characterize interactions between sterol biosynthetic proteins. PMID:16300994 ergosterol biosynthetic enzymes display specific protein-protein interactions forming a functional complex we designate, the ergosome. |
| GO:0005515 protein binding | IPI PMID:37968396 The social and structural architecture of the yeast protein ... | REMOVE | Summary: Protein binding (IPI) with Erg26 (P53199) from high-throughput affinity-enrichment MS (Michaelis et al. 2023). Uninformative term; the co-purification likely reflects shared ER membrane residence of sterol enzymes, with no shown functional role for Erg9. Removed per the protein-binding policy; the interaction itself is not disputed. Supporting Evidence: PMID:37968396 Here we develop a sensitive high-throughput method using highly reproducible affinity enrichment coupled to mass spectrometry |
| GO:0005739 mitochondrion | HDA PMID:24769239 Quantitative variations of the mitochondrial proteome and ph... | MARK AS OVER ANNOTATED | Summary: Renvoise et al. 2014 mitochondrial proteome detected Erg9. Erg9 is a C-terminally anchored ER membrane enzyme; detection in mitochondrial preparations is most plausibly ER co-purification (ER-mitochondria contacts), not a mitochondrial function. Supporting Evidence: PMID:24769239 we performed an overall quantitative proteomic and phosphoproteomic study of isolated mitochondria extracted from yeast grown on fermentative (glucose or galactose) and respiratory (lactate) media. PMID:1807826 The N-terminus of the deduced protein strongly resembles the signal sequence of yeast invertase suggesting a specific mechanism of integration into the membranes of the endoplasmic reticulum. file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005741 mitochondrial outer membrane | HDA PMID:16407407 Proteomic analysis of the yeast mitochondrial outer membrane... | MARK AS OVER ANNOTATED | Summary: Zahedi et al. 2006 mitochondrial outer membrane proteome detected Erg9; the authors note unexpected proteins in this fraction. Erg9 is an ER membrane enzyme; the outer-membrane assignment is an over-annotation. Supporting Evidence: PMID:16407407 Surprisingly, a subclass of proteins known to reside in internal mitochondrial compartments were found in the outer membrane proteome. file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005783 endoplasmic reticulum | HDA PMID:14562095 Global analysis of protein localization in budding yeast. | ACCEPT | Summary: Huh et al. 2003 global GFP localisation placed Erg9 in the ER; consistent with all other data. Supporting Evidence: PMID:14562095 We classify these proteins, representing 75% of the yeast proteome, into 22 distinct subcellular localization categories file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005783 endoplasmic reticulum | HDA PMID:26928762 One library to make them all: streamlining the creation of y... | ACCEPT | Summary: SWAT N-terminal GFP library localisation of Erg9 to the ER. Supporting Evidence: PMID:26928762 With this N′-tag library, which we call SWAT-GFP, we were able to uncover the sub-cellular localization of hundreds of proteins never visualized before file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005783 endoplasmic reticulum | IDA PMID:1807826 Isolation and primary structure of the ERG9 gene of Saccharo... | ACCEPT | Summary: Fegueur et al. 1991 cloned ERG9 and inferred ER membrane integration from its sequence; the SGD curator recorded ER as the site of activity. Correct and supported by later localisation data. Supporting Evidence: PMID:1807826 The N-terminus of the deduced protein strongly resembles the signal sequence of yeast invertase suggesting a specific mechanism of integration into the membranes of the endoplasmic reticulum. PMID:1807826 The amino acid sequence reveals one to four potential transmembrane domains with a hydrophobic segment in the C-terminal region. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:14562095 Global analysis of protein localization in budding yeast. | ACCEPT | Summary: Huh et al. 2003 GFP localisation cited by UniProt for ER membrane; Erg9 is a C-terminally anchored single-pass ER membrane protein. Most informative location. Supporting Evidence: PMID:14562095 We classify these proteins, representing 75% of the yeast proteome, into 22 distinct subcellular localization categories file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:1807826 Isolation and primary structure of the ERG9 gene of Saccharo... | ACCEPT | Summary: Fegueur et al. 1991: sequence analysis with a hydrophobic C-terminal segment and ER integration; UniProt cites this paper for ER membrane. Accepted (supported by microsomal localisation of the activity). Supporting Evidence: PMID:1807826 The N-terminus of the deduced protein strongly resembles the signal sequence of yeast invertase suggesting a specific mechanism of integration into the membranes of the endoplasmic reticulum. PMID:1807826 The amino acid sequence reveals one to four potential transmembrane domains with a hydrophobic segment in the C-terminal region. PMID:350878 Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. |
| GO:0005789 endoplasmic reticulum membrane | IBA GO_REF:0000033 | ACCEPT | Summary: PANTHER IBA (PTN000179294) for ER membrane across squalene synthases (Arabidopsis SQS1, mouse/rat Fdft1, S. pombe erg9, S. cerevisiae Erg9); correct. Supporting Evidence: file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane PMID:350878 Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt SubCell mapping (SL-0097); correct. Supporting Evidence: file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005829 cytosol | RCA GO_REF:0000123 | MODIFY | Summary: YeastPathways RCA (ERGOSTEROL-SYN-PWY-1) default cytosol compartment. Erg9's catalytic domain is soluble when the C-terminal anchor is removed, but the native enzyme is a microsomal, ER-membrane-anchored protein; the location of the activity is the ER membrane. Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: PMID:350878 Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. PMID:8323279 Soluble truncated squalene synthase is monomeric and catalyzes the two-step conversion of farnesyl diphosphate (FPP) to squalene via presqualene diphosphate in the presence of Mg2+ and NADPH. file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005829 cytosol | RCA GO_REF:0000123 | MODIFY | Summary: YeastPathways RCA (PWY-5670-1, epoxysqualene biosynthesis) default cytosol compartment; native Erg9 is ER-membrane anchored and active in microsomes. Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: PMID:350878 Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005829 cytosol | RCA GO_REF:0000123 | MODIFY | Summary: YeastPathways RCA (PWY3O-31704 superpathway) default cytosol compartment; native Erg9 is ER-membrane anchored and active in microsomes. Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: PMID:350878 Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. file:yeast/ERG9/ERG9-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0006696 ergosterol biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: PANTHER IBA (PTN000907895) for ergosterol biosynthesis in the fungal squalene synthase clade; correct. Supporting Evidence: PMID:1807826 The ERG9 gene of Saccharomyces cerevisiae has been cloned by complementation of the erg9-1 mutation which affects squalene synthetase. |
| GO:0006696 ergosterol biosynthetic process | IDA PMID:8323279 Yeast squalene synthase: expression, purification, and chara... | ACCEPT | Summary: Zhang et al. 1993 expressed and purified recombinant Erg9 and showed it converts FPP to squalene, the first pathway-specific sterol step. Correct. Supporting Evidence: PMID:8323279 Soluble truncated squalene synthase is monomeric and catalyzes the two-step conversion of farnesyl diphosphate (FPP) to squalene via presqualene diphosphate in the presence of Mg2+ and NADPH. PMID:8323279 Squalene synthase is the first pathway-specific enzyme in the cholesterol biosynthetic pathway. |
| GO:0006696 ergosterol biosynthetic process | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA IEA for ergosterol biosynthetic process; correct. Supporting Evidence: PMID:8323279 Squalene synthase is the first pathway-specific enzyme in the cholesterol biosynthetic pathway. |
| GO:0006696 ergosterol biosynthetic process | IMP PMID:1807826 Isolation and primary structure of the ERG9 gene of Saccharo... | ACCEPT | Summary: Fegueur et al. 1991 cloned ERG9 by complementation of the sterol-requiring erg9-1 mutant (IMP). Supporting Evidence: PMID:1807826 The ERG9 gene of Saccharomyces cerevisiae has been cloned by complementation of the erg9-1 mutation which affects squalene synthetase. |
| GO:0006696 ergosterol biosynthetic process | IMP PMID:200835 Ertosterol biosynthesis in Saccharomyces cerevisiae: mutants... | ACCEPT | Summary: Karst & Lacroute 1977 isolated ergosterol-auxotrophic erg9 mutants lacking squalene synthetase (IMP). Supporting Evidence: PMID:200835 These groupes lack the following enzymatic activities: squalene epoxidase (erg 1), 2,3-oxidosqualene-lanosterol cyclase (erg 7), phosphomevalonic kinase (erg 8), mevalonic kinase (erg12) and squalene synthetase (erg 9, erg 10, erg 11). |
| GO:0006696 ergosterol biosynthetic process | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA (ERGOSTEROL-SYN-PWY-1); correct process. Supporting Evidence: PMID:8323279 Squalene synthase is the first pathway-specific enzyme in the cholesterol biosynthetic pathway. |
| GO:0006696 ergosterol biosynthetic process | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA (PWY3O-31704 superpathway); correct, Erg9 catalyses the first committed sterol step. Supporting Evidence: PMID:8323279 Squalene synthase is the first pathway-specific enzyme in the cholesterol biosynthetic pathway. |
| GO:0008610 lipid biosynthetic process | IEA GO_REF:0000002 | MODIFY | Summary: InterPro IPR006449 (squalene synthase) mapping to lipid biosynthetic process. True but very general; Erg9 acts in ergosterol biosynthesis. Proposed replacements: ergosterol biosynthetic process Supporting Evidence: PMID:8323279 Squalene synthase is the first pathway-specific enzyme in the cholesterol biosynthetic pathway. PMID:1807826 The ERG9 gene of Saccharomyces cerevisiae has been cloned by complementation of the erg9-1 mutation which affects squalene synthetase. |
| GO:0016020 membrane | IDA PMID:350878 Squalene synthetase. Solubilization from yeast microsomes of... | MODIFY | Summary: Agnew & Popjak 1978 solubilised squalene synthetase from yeast microsomal membranes and showed a phospholipid requirement (IDA for membrane). Sound, but the specific membrane is the ER membrane. Proposed replacements: endoplasmic reticulum membrane Supporting Evidence: PMID:350878 Squalene synthetase was solubilized from yeast microsomal membranes with deoxycholate. PMID:350878 There was an apparently absolute requirement for phospholipid by the enzyme. |
| GO:0016104 triterpenoid biosynthetic process | RCA GO_REF:0000123 | KEEP AS NON CORE | Summary: YeastPathways RCA (PWY-5670-1, epoxysqualene biosynthesis) to triterpenoid biosynthetic process. Squalene is a C30 triterpene, so the assignment is correct; GO lacks a squalene-specific biosynthesis term. Non-core relative to ergosterol biosynthesis. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. |
| GO:0016765 transferase activity, transferring alkyl or aryl (other than methyl) groups | IEA GO_REF:0000002 | MODIFY | Summary: InterPro IPR019845 (squalene/phytoene synthase conserved site) mapping to the generic transferase-of-alkyl/aryl-groups class. Correct class (prenyltransferase) but the specific activity is squalene synthase. Proposed replacements: squalene synthase [NAD(P)H] activity Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. file:yeast/ERG9/ERG9-uniprot.txt Reaction=2 (2E,6E)-farnesyl diphosphate + NADPH + H(+) = squalene + 2 |
| GO:0045338 farnesyl diphosphate metabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PANTHER IBA (PTN001700951) for farnesyl diphosphate metabolic process. Erg9 consumes FPP and controls the flux of FPP into sterols versus non-sterol isoprenoids, so involvement in FPP metabolism is accurate, but it describes substrate consumption rather than the pathway Erg9 serves. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. PMID:8323279 Soluble truncated squalene synthase is monomeric and catalyzes the two-step conversion of farnesyl diphosphate (FPP) to squalene via presqualene diphosphate in the presence of Mg2+ and NADPH. |
| GO:0045338 farnesyl diphosphate metabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro IPR044844 mapping to farnesyl diphosphate metabolic process; as for the IBA row, true (FPP is Erg9's substrate) but non-core. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. |
| GO:0051996 squalene synthase [NAD(P)H] activity | EXP PMID:1807826 Isolation and primary structure of the ERG9 gene of Saccharo... | ACCEPT | Summary: Fegueur et al. 1991 showed squalene synthetase activity in E. coli expressing the cloned ERG9 fragment. Core function. Supporting Evidence: PMID:1807826 The presence of squalene synthetase activity in E. coli bearing the yeast DNA fragment isolated, indicates that the structural gene encoding squalene synthetase has been cloned. file:yeast/ERG9/ERG9-uniprot.txt Reaction=2 (2E,6E)-farnesyl diphosphate + NADPH + H(+) = squalene + 2 |
| GO:0051996 squalene synthase [NAD(P)H] activity | IBA GO_REF:0000033 | ACCEPT | Summary: PANTHER IBA (PTN001700951) for squalene synthase activity across plant, animal and fungal SQS orthologs; Erg9 is a canonical member. Supporting Evidence: PMID:8323279 Soluble truncated squalene synthase is monomeric and catalyzes the two-step conversion of farnesyl diphosphate (FPP) to squalene via presqualene diphosphate in the presence of Mg2+ and NADPH. file:yeast/ERG9/ERG9-uniprot.txt SIMILARITY: Belongs to the phytoene/squalene synthase family. |
| GO:0051996 squalene synthase [NAD(P)H] activity | IDA PMID:8157649 Yeast squalene synthase. A mechanism for addition of substra... | ACCEPT | Summary: Mookhtiar et al. 1994 kinetic analysis of recombinant yeast squalene synthase forming PSPP and squalene from FPP with NADPH activation. Direct evidence. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. |
| GO:0051996 squalene synthase [NAD(P)H] activity | IDA PMID:8323279 Yeast squalene synthase: expression, purification, and chara... | ACCEPT | Summary: Zhang et al. 1993: purified recombinant (C-terminally truncated) Erg9 catalyses the two-step FPP -> PSPP -> squalene conversion with Mg2+ and NADPH. Supporting Evidence: PMID:8323279 Soluble truncated squalene synthase is monomeric and catalyzes the two-step conversion of farnesyl diphosphate (FPP) to squalene via presqualene diphosphate in the presence of Mg2+ and NADPH. |
| GO:0051996 squalene synthase [NAD(P)H] activity | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA (ARBA, InterPro, RHEA:32295/32299, EC 2.5.1.21); correct. Supporting Evidence: file:yeast/ERG9/ERG9-uniprot.txt Reaction=2 (2E,6E)-farnesyl diphosphate + NADPH + H(+) = squalene + 2 |
| GO:0051996 squalene synthase [NAD(P)H] activity | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA (ERGOSTEROL-SYN-PWY-1). The pathway splits the reaction into FPP -> presqualene diphosphate (EC 2.5.1.103) and PSPP -> squalene; GO:0051996 covers the overall two-step reaction carried out by Erg9. Correct. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. |
| GO:0051996 squalene synthase [NAD(P)H] activity | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA (PWY-5670-1, epoxysqualene biosynthesis); both half-reactions are catalysed by Erg9; correct. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. |
| GO:0051996 squalene synthase [NAD(P)H] activity | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA (PWY3O-31704 superpathway); correct. Supporting Evidence: PMID:8157649 Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. |
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Download this section (compressed HTML)Q: How is Erg9 activity/abundance regulated post-translationally (e.g. sterol-dependent degradation as for mammalian SQS) to balance FPP use between sterol and non-sterol isoprenoid branches in yeast?
Experiment: Measure Erg9 protein turnover (cycloheximide chase) under ergosterol depletion and repletion, and in ERAD mutants (hrd1, doa10), to test for sterol-regulated degradation.
Farnesyl-diphosphate farnesyl transferase (squalene synthase); joins two farnesyl pyrophosphate moieties to form squalene in the sterol biosynthesis pathway
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