CSH1

UniProt ID: P38287
Organism: Saccharomyces cerevisiae S288C
Review Status: COMPLETE
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Gene Description

CSH1 (YBR161W) encodes a GT32-family multi-pass membrane glycosyltransferase that mannosylates inositol phosphorylceramide (IPC) using GDP-mannose to form mannosyl-inositol phosphorylceramide (MIPC), the precursor of M(IP)2C. It is a paralog of Sur1/Csg1 and acts redundantly with it: csg1 cells only reduce mannosylated sphingolipid synthesis, while csg1 csh1 double mutants make none. Csh1 forms a separate complex with the regulatory subunit Csg2, which it absolutely requires for activity, and it differs from Csg1 in ceramide-substrate specificity (revealed in ipt1, sur2 and scs7 backgrounds). Its biosynthetic activity is assigned to the Golgi, although high-throughput GFP screens detect the tagged protein at the vacuolar membrane.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0000030 mannosyltransferase activity
IBA
GO_REF:0000033 		MODIFY
Summary: IBA mannosyltransferase activity from PTN001285482 (C. albicans, A. fumigatus and yeast SUR1/CSH1 donors). Correct but generic; the specific activity is IPC mannosyltransferase.
Reason: Use the specific activity.
Propagation Review
Root cause: TERM SCOPING PROBLEM
Failure modes: GRANULARITY MISMATCH
Sources checked:
PANTHER:PTN001285482 · PTN001285482 SUPPORTS TRANSFER
Node correctly covers the Sur1/Csh1/Imt MIPC synthases; the IBD term is only less specific than GO:0103064.
Supporting Evidence:
PMID:12954640
These results indicated that Csg1p and Csh1p have redundant functions in MIPC synthesis
GO:0000324 fungal-type vacuole
HDA
PMID:14562095
Global analysis of protein localization in budding yeast. 		KEEP AS NON CORE
Summary: Huh et al. 2003 GFP screen: vacuole (membrane). Csh1 is a multi-pass glycosyltransferase of the secretory pathway; MIPC is made in the Golgi (where Csh1 was placed by Uemura et al.), so a steady-state vacuolar GFP signal in a high-throughput screen may reflect delivery of the tagged protein to the vacuole for turnover, or a genuine post-Golgi pool, rather than the site of MIPC synthesis. Two independent screens agree, so it is not dismissed.
Reason: Observed location distinct from the site of MIPC synthesis.
Supporting Evidence:
file:yeast/CSH1/CSH1-uniprot.txt
SUBCELLULAR LOCATION: Vacuole membrane {ECO:0000269|PubMed:14562095};
GO:0000324 fungal-type vacuole
HDA
PMID:26928762
One library to make them all: streamlining the creation of y... 		KEEP AS NON CORE
Summary: Yofe et al. 2016 SWAT-GFP library: vacuole. Csh1 is a multi-pass glycosyltransferase of the secretory pathway; MIPC is made in the Golgi (where Csh1 was placed by Uemura et al.), so a steady-state vacuolar GFP signal in a high-throughput screen may reflect delivery of the tagged protein to the vacuole for turnover, or a genuine post-Golgi pool, rather than the site of MIPC synthesis. Two independent screens agree, so it is not dismissed.
Reason: Observed location distinct from the site of MIPC synthesis.
Supporting Evidence:
PMID:26928762
One library to make them all: streamlining the creation of yeast libraries via a SWAp-Tag strategy.
GO:0005515 protein binding
IPI
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		REMOVE
Summary: Uemura et al. 2003 IPI with Csg2 (P35206). The Csh1-Csg2 interaction defines the Csh1-Csg2 IPC mannosyltransferase complex, which is already captured by the GO:0031501 part_of annotation; 'protein binding' adds nothing.
Reason: Uninformative protein binding term; removal does not mean the interaction is false (captured as complex membership).
Supporting Evidence:
PMID:12954640
We also revealed that Csg2p interacts with both Csg1p and Csh1p
PMID:12954640
These results suggested that two distinct inositol phosphorylceramide mannosyltransferase complexes, Csg1p-Csg2p and Csh1p-Csg2p, exist
GO:0005515 protein binding
IPI
PMID:16093310
Large-scale identification of yeast integral membrane protei... 		REMOVE
Summary: Miller et al. 2005 split-ubiquitin membrane-protein interaction screen: Csh1-Csg2. Confirms the known complex; uninformative term.
Reason: Uninformative protein binding term; removal does not mean the interaction is false.
Supporting Evidence:
PMID:12954640
We also revealed that Csg2p interacts with both Csg1p and Csh1p
GO:0005774 vacuolar membrane
IEA
GO_REF:0000044 		KEEP AS NON CORE
Summary: UniProt SubCell 'Vacuole membrane' mapping, derived from Huh et al. 2003. Csh1 is a multi-pass glycosyltransferase of the secretory pathway; MIPC is made in the Golgi (where Csh1 was placed by Uemura et al.), so a steady-state vacuolar GFP signal in a high-throughput screen may reflect delivery of the tagged protein to the vacuole for turnover, or a genuine post-Golgi pool, rather than the site of MIPC synthesis. Two independent screens agree, so it is not dismissed.
Reason: Observed location distinct from the site of MIPC synthesis.
Supporting Evidence:
file:yeast/CSH1/CSH1-uniprot.txt
SUBCELLULAR LOCATION: Vacuole membrane {ECO:0000269|PubMed:14562095};
GO:0005794 Golgi apparatus
IDA
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		ACCEPT
Summary: Uemura et al. 2003 IDA Golgi apparatus (full text not cached; curator-assigned). Golgi is where IPC is made by Aur1-Kei1 and mannosylated, and is consistent with the Csg1/Imt paralogs; deferring to the curator.
Reason: Site of MIPC synthesis.
Supporting Evidence:
PMID:12954640
These results suggested that two distinct inositol phosphorylceramide mannosyltransferase complexes, Csg1p-Csg2p and Csh1p-Csg2p, exist
GO:0006676 mannosyl diphosphorylinositol ceramide metabolic process
IDA
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		KEEP AS NON CORE
Summary: Uemura et al. 2003 IDA for mannosyl diphosphorylinositol ceramide (M(IP)2C) metabolic process. Csh1 makes MIPC, the substrate that Ipt1 converts to M(IP)2C; csg1 csh1 cells lack all mannosylated sphingolipids including M(IP)2C. Csh1 performs the upstream mannosylation step, not M(IP)2C formation itself.
Reason: Supplies the MIPC precursor of M(IP)2C; the direct process is MIPC biosynthesis.
Supporting Evidence:
PMID:12954640
Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss
GO:0006676 mannosyl diphosphorylinositol ceramide metabolic process
IEA
GO_REF:0000117 		KEEP AS NON CORE
Summary: ARBA rule for M(IP)2C metabolic process. Same reasoning: Csh1 acts one step upstream.
Reason: Precursor supply only.
Supporting Evidence:
PMID:12954640
Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss
GO:0006688 glycosphingolipid biosynthetic process
IEA
GO_REF:0000117 		ACCEPT
Summary: ARBA rule for glycosphingolipid biosynthetic process. MIPC is a mannosylated (glyco)sphingolipid; correct parent.
Reason: Correct.
Supporting Evidence:
PMID:12954640
These results indicated that Csg1p and Csh1p have redundant functions in MIPC synthesis
GO:0006688 glycosphingolipid biosynthetic process
IMP
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		ACCEPT
Summary: Uemura et al. 2003 IMP: csg1 csh1 double mutants make no mannosylated sphingolipids.
Reason: Correct.
Supporting Evidence:
PMID:12954640
Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss
GO:0030148 sphingolipid biosynthetic process
IMP
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		ACCEPT
Summary: Uemura et al. 2003 IMP for sphingolipid biosynthesis. Correct general process.
Reason: Correct.
Supporting Evidence:
PMID:12954640
Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss
GO:0031501 mannosyltransferase complex
IEA
GO_REF:0000117 		ACCEPT
Summary: ARBA rule for mannosyltransferase complex. Csh1 forms a heterodimeric complex with Csg2.
Reason: Correct.
Supporting Evidence:
file:yeast/CSH1/CSH1-uniprot.txt
SUBUNIT: Heterodimer of CSH1 and CSG2. {ECO:0000269|PubMed:12954640}.
GO:0031501 mannosyltransferase complex
IPI
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		ACCEPT
Summary: Uemura et al. 2003 IPI: Csh1-Csg2 forms one of two IPC mannosyltransferase complexes, and Csh1 activity requires Csg2.
Reason: Core complex.
Supporting Evidence:
PMID:12954640
These results suggested that two distinct inositol phosphorylceramide mannosyltransferase complexes, Csg1p-Csg2p and Csh1p-Csg2p, exist
PMID:12954640
Deletion of the CSG2 gene reduced the Csg1p activity and abolished the Csh1p activity
GO:0051999 mannosyl-inositol phosphorylceramide biosynthetic process
IBA
GO_REF:0000033 		ACCEPT
Summary: IBA from PTN001285482 with S. pombe Imt1-3 and A. fumigatus donors. Correct.
Reason: Core process.
Supporting Evidence:
PMID:12954640
These results indicated that Csg1p and Csh1p have redundant functions in MIPC synthesis
GO:0103064 inositol phosphorylceramide mannosyltransferase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Combined IEA (ARBA, RHEA:64596, EC 2.4.1.370). Correct.
Reason: Core MF.
Supporting Evidence:
PMID:12954640
These results indicated that Csg1p and Csh1p have redundant functions in MIPC synthesis
GO:0103064 inositol phosphorylceramide mannosyltransferase activity
IMP
PMID:12954640
Csg1p and newly identified Csh1p function in mannosylinosito... 		ACCEPT
Summary: Uemura et al. 2003 IMP: CSH1 is redundant with CSG1 for IPC mannosylation, and has distinct ceramide-substrate specificity.
Reason: Core MF.
Supporting Evidence:
PMID:12954640
Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss
PMID:12954640
demonstrated that Csh1p has a different substrate specificity from Csg1p

Core Functions

GDP-mannose-dependent IPC mannosyltransferase acting with Csg2 in the Golgi to make MIPC, redundant with the Sur1/Csg1-Csg2 enzyme but with distinct ceramide specificity.

Molecular Function:
inositol phosphorylceramide mannosyltransferase activity
Directly Involved In:
mannosyl-inositol phosphorylceramide biosynthetic process
Cellular Locations:
Golgi apparatus
In Complex:
mannosyltransferase complex
Supporting Evidence:
  • PMID:12954640
    Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss
  • PMID:12954640
    These results indicated that Csg1p and Csh1p have redundant functions in MIPC synthesis
  • PMID:12954640
    Deletion of the CSG2 gene reduced the Csg1p activity and abolished the Csh1p activity
  • PMID:12954640
    These results suggested that two distinct inositol phosphorylceramide mannosyltransferase complexes, Csg1p-Csg2p and Csh1p-Csg2p, exist

References

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Suggested Questions for Experts

Q: Is the vacuolar-membrane signal of Csh1-GFP (two independent screens) a turnover artefact, or does a post-Golgi pool of the Csh1-Csg2 enzyme mannosylate IPC at the vacuole?

Q: Which ceramide features (hydroxylation state, chain length) distinguish Csh1 from Csg1 substrate preference?

Suggested Experiments

Experiment: Endogenously tagged Csh1 colocalisation with medial/late Golgi markers in pep4 versus wild-type cells, with MIPC synthesis measured in Golgi- versus vacuole-enriched fractions.

Hypothesis: Csh1 is active in the Golgi and only transits to the vacuole for degradation

Type: cell biology / lipidomics

Deep Research

SGD

(CSH1-deep-research-sgd.md)
SGD gene description for CSH1 (yeast) SGD

SGD gene description for CSH1 (yeast)

Mannosylinositol phosphorylceramide (MIPC) synthase catalytic subunit; forms a complex with regulatory subunit Csg2p; function in sphingolipid biosynthesis is overlapping with that of Sur1p; CSH1 has a paralog, SUR1, that arose from the whole genome duplication

📚 Additional Documentation

Notes

(CSH1-notes.md)

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