UGT8 (CGT) — Review Notes

UniProt: Q16880 (CGT_HUMAN). HGNC:12555. Gene synonyms: CGT, UGT4, UGT8A1.

Identity and core function

UGT8 is 2-hydroxyacylsphingosine 1-beta-galactosyltransferase, also known as UDP-galactose:ceramide
galactosyltransferase (CGT) / ceramide UDP-galactosyltransferase / cerebroside synthase.

• RecName from UniProt: "2-hydroxyacylsphingosine 1-beta-galactosyltransferase" with
  EC=2.4.1.47 [file:human/UGT8/UGT8-uniprot.txt "EC=2.4.1.47"].
• FUNCTION: "Catalyzes the transfer of galactose to ceramide, a key enzymatic step in the
  biosynthesis of galactocerebrosides, which are abundant sphingolipids of the myelin membrane
  of the central nervous system and peripheral nervous system"
  [file:human/UGT8/UGT8-uniprot.txt]. It "Galactosylates both hydroxy- and non-hydroxy fatty
  acid-containing ceramides and diglycerides (By similarity)."
• Catalytic activity (UniProt): "an N-acylsphing-4-enine + UDP-alpha-D-galactose = a beta-D-
  galactosyl-(1<->1')-N-acylsphing-4-enine + UDP + H(+)"; RHEA:13093; EC 2.4.1.47.
• Pathway: "Sphingolipid metabolism; galactosylceramide biosynthesis"
  [file:human/UGT8/UGT8-uniprot.txt].

The transfer reaction produces galactosylceramide (GalCer) + UDP. This is a distinct,
parallel branch from the UGCG glucosylceramide route: UGCG (glucosylceramide synthase) transfers
Glc from UDP-Glc to ceramide on the cytosolic face of the Golgi, whereas UGT8 transfers Gal from
UDP-Gal to ceramide on the luminal side of the ER. The two enzymes share no significant sequence
homology (different evolutionary origin) [PMID:34080016, "the amino acid sequence of UGT8 shows no
significant homology to GCS, which indicates different evolutionary origins of these enzymes"].

Reaction / substrate specificity

The review PMID:34080016 (Reza, Ugorski, Suchański 2021, Glycobiology; full text available)
describes the reaction:
- "GalCer is generated by the transfer of Gal from UDP-galactose to the same hydroxyl group at the
C-1 position of the ceramide backbone by galactosylceramide synthase" PMID:34080016.
- "This enzyme is encoded by the UGT8 gene (Kapitonov and Yu 1997)." PMID:34080016
- GalCer species "are enriched in very-long-chain α-hydroxy fatty acids (C18–C26)" — consistent
with the UniProt note that UGT8 galactosylates hydroxy-fatty-acid ceramides.

Subcellular location

• UniProt SUBCELLULAR LOCATION: "Membrane; Single-pass membrane protein. Endoplasmic reticulum"
  [file:human/UGT8/UGT8-uniprot.txt]. TRANSMEM 472..492; SIGNAL 1..20; type I membrane glycoprotein.
• Active site faces the ER lumen: "GalCer is synthesized on the luminal side of the ER, as UGT8 is
  a type I membrane glycoprotein with its active site directed into the lumen of this organelle"
  PMID:34080016. This supports GO:0098553 (lumenal side of ER membrane) and GO:0005789 (ER
  membrane); the bare GO:0005783 (endoplasmic reticulum) is a valid but less precise location.
• Reactome R-HSA-6785933: "UGT8 localizes to the luminal part of the endoplasmic reticulum (ER)
  membrane."

Biological role — myelin / nervous system

• GalCer (and its 3-O-sulfated derivative sulfatide, made by GAL3ST1) is a major myelin lipid.
  "GalCer is the most abundant single component of the myelin sheath (20–25% of the total lipid
  content) produced by oligodendrocytes in the central nervous system (CNS) and Schwann cells in
  the peripheral nervous system" PMID:34080016.
• PMID:8661025 (Bosio et al. 1996; abstract only) describes CGT as "the key enzyme in the
  biosynthesis of the glycosphingolipids, cerebrosides and sulfatides, essential constituents of
  the myelin membrane of the central nervous system (CNS) and PNS" and states "The CGT gene
  therefore is important in the differentiation program of the oligodendrocyte lineage." This
  supports CNS/PNS development involvement (BP), though the direct enzymatic core is myelin-lipid
  (GalCer) biosynthesis.
• Mouse Ugt8 knockouts (Cgt-/-, Bosio/Coetzee groups) generate GalCer- and sulfatide-deficient
  myelin with tremor, dysmyelination, and disrupted paranodal junctions — the basis for the
  Ensembl-projected paranode/paranodal-junction BP terms. These are downstream physiological
  consequences of loss of the myelin lipid, not the enzyme's molecular action.

Annotation assessment summary

Core molecular function: GO:0003851 N-acylsphingosine galactosyltransferase activity — the OLS
definition ("N-acylsphing-4-enine + UDP-alpha-D-galactose = a beta-D-galactosyl-(1<->1')-N-
acylsphing-4-enine + H+ + UDP") exactly matches the UniProt CGT reaction. This is the correct,
current term. Well supported by ISS-from-rat (Q09426), IBA, and Reactome.

GO:0008489 "UDP-galactose:glucosylceramide beta-1,4-galactosyltransferase activity" is the WRONG
term for UGT8. Its OLS definition is the transfer of Gal onto glucosylceramide to make
lactosylceramide ("a beta-D-glucosyl-(1<->1')-N-acylsphing-4-enine + UDP-alpha-D-galactose = a
beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)-Cer" ) — that is the lactosylceramide-synthase (B4GALT5/6)
reaction, not the CGT (ceramide→GalCer) reaction. UGT8 adds Gal directly to ceramide, not to
glucosylceramide. The two old PINC/TAS (1996) and Ensembl electronic annotations carrying
GO:0008489 conflate the two reactions and should be MODIFY→GO:0003851. (Note the label mismatch:
these appear in GOA under a long, incorrect label.)

Over-annotations / non-core:
- CNS/PNS development (GO:0007417, GO:0007422) and nervous system development (GO:0007399):
physiological consequences of myelin GalCer loss; keep as non-core.
- Paranode/paranodal-junction terms (GO:0002175, GO:0030913, GO:0048812), cytoskeleton
organization (GO:0007010), response to immobilization stress (GO:0035902): Ensembl-projected
from mouse (GO_REF:0000107); downstream phenotypic/loss-of-function consequences, not the
molecular activity. cytoskeleton organization and response to immobilization stress are
over-annotations for a lipid-biosynthetic ER enzyme.
- GO:0008194 UDP-glycosyltransferase activity (InterPro IEA): correct but generic parent of the
specific MF; keep as non-core.
- GO:0006688 glycosphingolipid biosynthetic process (IDA PMID:34080016; TAS Reactome): correct
but a generic parent of the specific GO:0006682 galactosylceramide biosynthetic process. Note
PMID:34080016 is a review; the IDA code is unusual for a review but the biological claim
(UGT8/GalCer is part of glycosphingolipid biosynthesis) is well supported. Keep the more
specific GO:0006682 as core.

Location: ER / ER membrane / luminal side of ER membrane all consistent and supported.