The immunoglobulin J chain (joining chain) participates in the effective di-/polymerization of either IgA or IgM and is essential for the secretion of these immunoglobulins into the mucosa
In literature the J chain was reported to be N-glycosylated at Asn49 (60, 63, 64) ; however, O-glycosylation has hitherto not been described for the molecule
Interestingly, two O-glycopeptides detected in HILIC fractions #13 and #14 might correspond to the J chain and suggest O-glycosylation at Thr97 (95DPEV99m/z 608.722+, m/z 608.712+) (supplemental Figs
S3 and S5)
This potentially new O-glycosylation site is in close vicinity to a cysteine ( Cys91 ) that can form a disulfide-bridge to IgM molecules
Hence, one might speculate that an occupied O-glycosylation site in this region might function in the establishment/preservation of this inter-molecular bond
However, the number of present fragment ions in the corresponding CID-MS3 spectra did not allow an unambiguous identification of the peptide, as evidenced by several potential peptide hits being equally scored by the search engine
Manual fragment spectra annotation, though, suggest the identification of immunoglobulin J chain —nevertheless, this identification deserves further validation
Both identified O-glycopeptides were found to be decorated with monosialylated T-antigens