polypeptide N-acetylgalactosaminyltransferase 3 (protein-UDP acetylgalactosaminyltransferase 3, UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 3, polypeptide GalNAc transferase 3, GalNAc-T3, pp-GaNTase 3, GALNT3)

Function

• protein glycosylation
• catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a Ser or Thr on the nascent glycoprotein
• glycosylates fibronectin in vivo (probable)
• role in phosphate homeostasis

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, also called GT1 motif
    • Mn+2 coordination
    • substrate binding
  • C-terminal domain (domain B, Gal/GalNAc-T motif)
    • catalytic reaction
    • UDP-Gal binding
• ricin B-type lectin domain binds to GalNAc & contributes to glycopeptide specificity

Compartment

Golgi, trans & medial parts of Golgi stack

Alternative splicing

named isoforms=2

Expression

• expressed in organs that contain secretory epithelial glands
• expressed in pancreas, skin, kidney testis > prostate, ovary, intestine, colon
• expressed in placenta, lung, fetal lung, fetal kidney

Pathology

• activity toward HIV envelope glycoprotein gp120, EA2, Muc2 & Muc5
• defects in GALNT3 are a cause of familial tumoral calcinosis
• overexpressed in many differentiated carcinomas

More general terms

• polypeptide N-acetylgalactosaminyltransferase

Database

• OMIM: https://mirror.omim.org/entry/211900
• OMIM: https://mirror.omim.org/entry/601756
• UniProt: http://www.uniprot.org/uniprot/Q14435.html