KDELC2

Model summary

A soluble glycosyltransferase in human cells that installs O-linked glycans onto serine residues of secretory and extracellular matrix proteins. It uses a β-sandwich recognition platform to capture folded substrates and a catalytic core to transfer sugars from nucleotide-sugar donors, primarily driving glucosyl addition and retaining the capacity for xylosyl transfer in vitro. Operating on the luminal face of the endoplasmic reticulum and continuing through the Golgi apparatus, it modifies collagen and proteoglycan precursors to support their maturation and secretion.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The N-terminal half (residues 26–134) is densely annotated with IPR013783 (Immunoglobulin-like fold, homologous_superfamily, 26–134), IPR014756 (Immunoglobulin E-set, homologous_superfamily, 27–132), and two tandem repeat annotations—IPR017868 (Filamin/ABP280 repeat-like, 24–134) and IPR001298 (Filamin/ABP280 repeat, 28–137). This cluster defines a β-sandwich Ig-like/E-set scaffold that forms rigid, adhesive surfaces well-suited for protein–protein recognition and substrate capture in the secretory pathway. Immediately downstream, the C-terminal half is dominated by IPR006598 (Glycosyl transferase CAP10 domain, 153–486), a catalytic module characteristic of inverting glycosyltransferases that use nucleotide-sugar donors. The entire polypeptide is encompassed by IPR051091 (Protein O-Glucosyltransferase/Glycosyltransferase 90 family, 25–487), which specifies membership in the protein O-glucosyltransferase lineage that modifies secretory and extracellular matrix proteins.

2. 02

   This ordered architecture—an N-terminal Ig-like/filamin-repeat recognition platform feeding into a CAP10 catalytic core—causally establishes glycosyltransferase chemistry.

   The CAP10 domain dictates nucleotide-sugar–dependent transfer to hydroxyl-bearing acceptors, while the Ig-like/filamin-repeat array provides affinity and specificity toward structured extracellular substrates. Together, these features enforce two molecular activities: GO:0035251 UDP-glucosyltransferase activity and GO:0035252 UDP-xylosyltransferase activity. The family context (IPR051091) and catalytic core support glucosyl transfer to serine residues on secretory proteins; the same fold can accommodate alternative donors and acceptors, explaining observed xylosyltransferase capability in vitro.

3. 03

   From these molecular functions, the biological role follows.

   Decorating serine residues with O-linked glycans is a canonical secretory-pathway modification, aligning with GO:0006486 protein glycosylation and specifically GO:0018209 peptidyl-serine modification. The Ig-like/filamin-repeat targeting module rationalizes selective action on extracellular matrix clients such as collagens and proteoglycans, which require O-glycan addition for proper folding, secretion, and matrix assembly.

4. 04

   Cellular localization is dictated by where this chemistry occurs.

   The absence of transmembrane helices and the presence of soluble Ig-like repeats indicate a luminal, membrane-associated enzyme rather than an integral membrane protein. Its activity on nascent secretory cargo positions it within the early secretory system: GO:0005789 endoplasmic reticulum membrane (peripherally associated on the luminal face) and GO:0000139 Golgi membrane (as cargo progresses). This soluble, lumen-facing configuration is consistent with a peripheral membrane enzyme that cycles or concentrates in ER and Golgi compartments.

5. 05

   Mechanistically, the Ig-like/filamin-repeat platform likely binds structured extracellular substrates and accessory chaperones, positioning serine residues for catalysis in the CAP10 active site.

   I hypothesize that the enzyme transiently associates with ER-resident lectins and chaperones to survey client proteins, then uses UDP-glucose (and, under certain conditions, UDP-xylose) to install O-glycans. Likely interaction partners include collagen-modifying factors and ER lectins that guide substrate flow. In particular, association with luminal chaperone networks and collagen-handling machinery would channel modified substrates forward through ER-to-Golgi trafficking toward extracellular matrix assembly.