1-C-6 epitope in cartilage proteoglycan G2 domain is masked by keratan sulphate

The presence of the protein epitope recognized by monoclonal antibody 1-C-6 was investigated on the globular G1 and G2 domains of pig cartilage proteoglycan core protein. After reduction of disulphide bonds and removal of keratan sulphate chains with keratanase, both G1 and G2 domains were shown to contain the epitope. However, without keratanase digestion the epitope on the G2 domain was poorly detected. The results suggest that a keratan sulphate chain substituted close to the epitope sequence in the G2 domain prevents antibody access to the epitope and thus masks its detection. This shows the 1-C-6 epitope to be a conserved protein sequence in the G2 domain of proteoglycans from different species, but its detection may be masked by glycosylation.