polypeptide N-acetylgalactosaminyltransferase 4 (protein-UDP acetylgalactosaminyltransferase 4, UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 4, polypeptide GalNAc transferase 4, GalNAc-T4, pp-GaNTase 4, GALNT4)

Function

• catalyzes initial reaction in O-linked oligosaccharide biosynthesis, transfer of an N-acetyl-D-galactosamine to Ser or Thr on the nascent glycoprotein
• highest activity toward Muc7, EA2 & Muc2, lower activity than GALNT2
• glycosylates Thr-57 of SELPLG

 ,UDP-N-acetyl-D-galactosamine + polypeptide <--> ,            UDP + N-acetyl-D-galactosaminyl-polypeptide

Cofactor: Mn+2, Ca+2

Structure

• 2 conserved domains in the glycosyltransferase region
  • N-terminal domain (domain A, GT1 motif) with role in Mn+2 coordination & substrate binding
  • C-terminal domain (domain B, Gal/GalNAc-T motif) with role in catalytic reaction & UDP-Gal binding
• ricin B-type lectin domain binds to GalNAc with role in glycopeptide specificity
• belongs to the glycosyltransferase 2 family, GalNAc-T subfamily

Compartment

Golgi membrane

Alternative splicing

named isoforms=2 Expresison:

• ubiquitous
• highly expressed in mucous cells

More general terms

• polypeptide N-acetylgalactosaminyltransferase

Database

• OMIM: https://mirror.omim.org/entry/603565
• UniProt: http://www.uniprot.org/uniprot/Q8N4A0.html