An alpha-N-acetylgalactosaminylation at the threonine residue of a defined peptide sequence creates the oncofetal peptide epitope in human fibronectin.
The monoclonal antibody FDC-6 defines a structure specific to oncofetal fibronectins (onf-FN) isolated from fetal and malignant cells and tissues. The absence of this structure is characteristic of normal fibronectin (nor-FN) isolated from plasma and adult normal tissue (Matsuura, H., and Hakomori, S. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 6517-6521). The minimum structure required for FDC-6 reactivity was determined to be Val-Thr-His-Pro-Gly-Tyr (VTHPGY) with alpha-N-acetylgalactosamine (alpha-GalNAc) at Thr, although alpha-GalNAc per se is not involved in the FDC-6 epitope (Matsuura, H., Takio, K., Titani, K., Greene, T., Levery, S. B., Salyan, M. E. K., and Hakomori, S. (1988) J. Biol. Chem. 263, 3314-3322). Thus, a single glycosylation on the normally occurring peptide of FN may induce conformational changes in the peptide to form the specific oncofetal epitope recognized by FDC-6 antibody. The FDC-6-nonreactive synthetic peptide containing the VTHPGY sequence was converted into FDC-6-reactive form on incubation with alpha-N-acetylgalactosaminyltransferase and UDP-[3H]GalNAc in the homogenate of hepatoma cell HUH-7, human fetal fibroblast cell line WI-38, or human epidermoid carcinoma cell line A431. Such a conversion did not take place when the same enzyme fraction of normal adult tissue was incubated with the VTHPGY peptide under the same conditions. Thus, the occurrence of alpha-GalNAc transferase recognizing the VTHPGY peptide sequence (UDP-GalNAc:VTHPGY alpha-GalNAc transferase) is specific for fetal and cancer tissues, and absent in normal adult tissues. However, a similar alpha-GalNAc transferase activity capable of transferring the GalNAc residue to other Ser or Thr hydroxyl groups of nor-FN, and presumably located at the type III connecting segment region, was detectable in homogenate of various normal tissues. Such enzyme activity was determined with the use of enzymatically de-O-glycosylated nor-FN. Thus, the enzymatic basis of FDC-6 epitope formation is a subtle change in the substrate specificity of alpha-GalNAc transferase. The normal enzyme is incapable of transferring alpha-GalNAc from UDP-GalNAc to the Thr residue of the VTHPGY sequence, but is capable of transferring alpha-GalNAc to other Ser or Thr residues of FN. In contrast, alpha-GalNAc transferase of fetal and cancer tissues may have broader specificity and the capability to transfer GalNAc to Thr or Ser residues, including those of the VTHPGY sequence.
Matsuura
H
H
Biomembrane Institute, Seattle, Washington.
Greene
T
T
Hakomori
S
S
D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal; D000951:Antigens, Neoplasm / Q000276:immunology; D002413:Cations, Divalent; D002460:Cell Line; D000939:Epitopes / Q000032:analysis*; D005353:Fibronectins / Q000276:immunology*; D005700:Galactosyltransferases / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D017350:N-Acetylgalactosaminyltransferases; D009928:Organ Specificity; D013347:Subcellular Fractions / Q000201:enzymology; D013379:Substrate Specificity; D014407:Tumor Cells, Cultured / Q000201:enzymology*
D000911:Antibodies, Monoclonal; D000951:Antigens, Neoplasm; D002413:Cations, Divalent; D000939:Epitopes; D005353:Fibronectins; C031192:oncofetal antigens; D005700:Galactosyltransferases; D017350:N-Acetylgalactosaminyltransferases; C060025:fibronectin hexapeptide N-acetylgalactosaminyltransferase