N-linked keratan sulfate in the aggrecan interglobular domain potentiates aggrecanase activity

Keratan sulfate is thought to influence the cleavage of aggrecan by metalloenzymes. We have therefore produced a recombinant substrate, substituted with keratan sulfate, suitable for the study of aggrecanolysis in vitro. Recombinant human G1-G2 was produced in primary bovine keratocytes using a vaccinia virus expression system. Following purification and digestion with specific hydrolases, fluorophore-assisted carbohydrate electrophoresis was used to confirm the presence of the monosulfated Gal-GlcNAc6S and GlcNAc6s-Gal disaccharides and the disulfated Gal6S-GlcNAc6S disaccharides of keratan sulfate. Negligible amounts of fucose or sialic acid were detected, and the level of unsulfated disaccharides was minimal. Treatment with keratanases reduced the size of the recombinant G1-G2 by approximately 5 kDa on SDS-PAGE. Treatment with N-glycosidase F also reduced the size of G1-G2 by approximately 5 kDa and substantially reduced G1-G2 immunoreactivity with monoclonal antibody 5-D-4, indicating that keratan sulfate on the recombinant protein is N-linked. Cleavage of G1-G2 by aggrecanase was markedly reduced when keratan sulfate chains were removed by treatment with keratanase, keratanase II, endo-beta-galactosidase, or N-glycosidase F. These results indicate that modification of oligosaccharides in the aggrecan interglobular domain with keratan sulfate, most likely at asparagine residue 368, potentiates aggrecanase activity in this part of the core protein.