Title : Determination of disulfide bond assignments and N-glycosylation
sites of the human gastrointestinal carcinoma antigen
GA733-2 (
CO17-1A, EGP ,
KS1-4 ,
KSA , and
Ep-CAM )
Abstract :
- The GA733-2 antigen is a cell surface glycoprotein highly expressed on most human gastrointestinal carcinoma and at a lower level on most normal epithelia
- It is an unusual cell-cell adhesion protein that does not exhibit any obvious relationship to the four known classes of adhesion molecules
- In this study, the disulfide-bonding pattern of the GA733-2 antigen was determined using matrix-assisted laser desorption/ionization mass spectrometry and N-terminal sequencing of purified tryptic peptides treated with 2-[2'-nitrophenylsulfonyl]-3-methyl-3-bromoindolenine or partially reduced and alkylated
- Numbering GA733-2 cysteines sequentially from the N terminus , the first three disulfide linkages are Cys1-Cys4, Cys2-Cys6, and Cys3-Cys5, which is a novel pattern for a cysteine-rich domain instead of the expected epidermal growth factor-like disulfide structure
- The next three disulfide linkages are Cys7-Cys8, Cys9-Cys10, and Cys11-Cys12, consistent with the recently determined disulfide pattern of the thyroglobulin type 1A domain of insulin-like growth factor-binding proteins 1 1 and 6
- Analysis of glycosylation sites showed that GA733-2 antigen contained N-linked carbohydrate but that no O-linked carbohydrate groups were detected
- Of the three potential N-linked glycosylation sites, Asn175 was not glycosylated, whereas Asn88 was completely glycosylated, and Asn51 was partially glycosylated
- These data show that the extracellular domain of the GA733-2 antigen consists of three distinct domains ; a novel cysteine-rich N-terminal domain (GA733 type 1 motif ), a cysteine-rich thyroglobulin type 1A domain (GA733 type 2 motif ), and a unique nonglycosylated domain without cysteines (GA733 type 3 motif )