**3. Conclusion**

146 Rheumatoid Arthritis – Etiology, Consequences and Co-Morbidities

the first sequence (1285-1300) was found distributed throughout the gel; while the second sequence (1373-1385) was only detected in area 3 and 5, areas lower than the lubricin area (Fig. 4B and C). Additionally, several new peptides derived from both *N*-terminus and *C*-termini were found in this study. These fragments could be solely by-products of degenerative joint. Alternatively, these fragments might play a regulatory role. For example, fragments from fibronectin and aggrecan have been reported to correlate with joint diseases (Homandberg et al., 1997a; Homandberg et al., 1997b; Struglics et al., 2009). Interestingly, these two proteins were also found in this study (Table I). It is not clear whether they form a protein complex

Fig. 8. MS2 spectrum of the peptide derived after desialylated and *O*-glycanase treated lubricin sample searched against UniProt and NCBI human protein database using GPM software. The *m/z* 420.73 is the [M+H]2+ precursor ion. The assigned ID number for the peptide in the GPM database is 3740. This peptide sequence is in the mucin domain of

lubricin and is repeated 18 times in lubricin.

together with lubricin or just happened to be co-purified with lubricin.

In summary, using glycoproteomics we fully characterized the major glycoprotein in SF as lubricin. With knowledge of *O*-glycosylation and proteomic properties of lubricin, it allowed us to identify RA or OA-specific glyco-epitopes and fragments, enabling us to better understand how the glycosylation of lubricin is influenced by inflammation of the joint.
