ALG5 encodes dolichyl-phosphate beta-glucosyltransferase (Dol-P-Glc synthase), a glycosyltransferase family 2 enzyme anchored in the endoplasmic reticulum membrane by an N-terminal signal-anchor helix. It transfers glucose from UDP-glucose to dolichyl phosphate, and the resulting dolichyl-phosphate glucose is the donor that the ER-lumenal glucosyltransferases ALG6, ALG8 and ALG10 use to add the three terminal glucose residues of the Glc3Man9GlcNAc2-PP-dolichol precursor of protein N-glycosylation. Horse ALG5 is the ortholog of human ALG5 within PANTHER subfamily PTHR10859:SF91. The UniProt entry A0A5F5PM72 is a 294-residue alternatively spliced product equivalent to human ALG5 isoform 2. It keeps the membrane anchor and the DXD motif but lacks 30 residues (human 95-124), and this segment contains a strand and a DDGS motif that are conserved in ALG5 from yeast to mammals. Whether this isoform is catalytically active has not been tested in any species.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004581 dolichyl-phosphate beta-glucosyltransferase activity | IEA GO_REF:0000003 | UNDECIDED | Summary: The ALG5 gene encodes this activity: horse ALG5 sits in subfamily PTHR10859:SF91, and its human ortholog is experimentally supported. This UniProt product, however, is the isoform-2 equivalent and lacks a subfamily-invariant segment of the catalytic domain. Reason: Gene-level function is not in doubt. A0A5F5PM72 is in PTHR10859:SF91 with human, mouse, fly and yeast ALG5, and SF91 is separate from the DPM-type mannosyltransferases. PAINT places GO:0004581 as an IBD at PTN001653583, with experimental donors that include human Q9Y673 and yeast ALG5. Human ALG5 complements yeast alg5 deficiency. The annotation here, though, is an EC-mapping IEA on a specific gene model. That model pairs gaplessly with human isoform 2 (VSP_041019, residues 95-124 missing), and the missing segment includes the hydrophobic strand and V-V-D-D-G-S-x-D motif that are conserved in every SF91 ortholog compared, inside the Pfam GT2 domain. No catalytic data exist for isoform 2 in any species, so neither ACCEPT nor REMOVE can be justified for this product. The DXD motif (ADADG) and the C-terminal region are retained. UniProt has no full-length horse ALG5 entry to carry the annotation instead. Supporting Evidence: PMID:10359825 Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. file:human/ALG5/ALG5-uniprot.txt /note="Missing (in isoform 2)" file:HORSE/ALG5/ALG5-bioinformatics/RESULTS-rereview.md | P40350 ALG5_YEAST | `-KEKYGSRWEIVIVDDGSTDNTTQYCLKIC` | file:HORSE/ALG5/ALG5-bioinformatics/RESULTS-rereview.md | A0A5F5PM72 | 294 | `------------------------------` | |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: The N-terminal signal-anchor helix is preserved, which supports ER membrane residence. Reason: The horse model keeps the hydrophobic segment that corresponds to the human signal-anchor transmembrane helix (residues 8-28), and the 30-residue deletion is cytoplasmic and downstream of it. ER membrane localization of human ALG5 is experimentally recorded, and PAINT places GO:0005789 as an IBD across eukaryotic ALG5 (PTN000092356). This localization therefore holds regardless of whether the isoform is catalytically competent. Supporting Evidence: file:human/ALG5/ALG5-uniprot.txt FT TRANSMEM 8..28 file:HORSE/ALG5/ALG5-bioinformatics/RESULTS.md | TRANSMEM | 8–28 | 8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28 | 16/21 | |
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Download this section (compressed HTML)Q: Does ALG5 isoform 2 (lacking human residues 95-124, which include the conserved DDGS motif), represented in horse by A0A5F5PM72, retain dolichyl-phosphate glucosyltransferase activity, or is it a non-catalytic or regulatory product?
Q: Should UniProt or Ensembl provide a full-length (isoform-1-equivalent) horse ALG5 product, so that gene-level Dol-P-Glc synthase annotations do not rest on a deletion isoform?
Experiment: Express human or horse ALG5 isoform 1 and isoform 2 in an S. cerevisiae alg5 deletion strain. Compare complementation of CPY hypoglycosylation, and measure Dol-P-Glc formation from UDP-[3H]glucose and dolichyl phosphate in microsomes.
Type: complementation and enzyme assay
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