Association of the aggrecan keratan sulfate-rich region with collagen in bovine articular cartilage

Aggrecan, the predominant large proteoglycan of cartilage, is a multidomain macromolecule with each domain contributing specific functional properties . One of the domains contains the majority of the keratan sulfate (KS) chai n substituents and a protein segment with a proline-rich hexapeptide repeat sequence. The function of this domain is unknown but the primary structure suggests a potential for binding to collagen fibrils. We have examined bin ding of aggrecan fragments encompassing the KS-rich region in a solid-phase assay, A moderate affinity (apparent K-d = 1.1 mu M) for isolated collagen II, as well as collagen I, was demonstrated. Enzymatic digestion of the RS chains did not alter the capacity of the peptide to bind to collagen, wher eas cleavage of the protein core abolished the interaction. The distributio n of the aggrecan KS-rich region in bovine tarsometatarsal joint cartilage was investigated using immunoelectron microscopy. Immunoreactivity was rela tively low in the superficial zone and higher in the intermediate and deep zones of the uncalcified cartilage. Within the pericellular and territorial matrix compartments the epitopes representing the aggrecan KS-rich region were detected preferentially near or at collagen fibrils. Along the fibrils , epitope reactivity was non-randomly distributed, showing preference for t he gap region within the D-period. Our data suggest that collagen fibrils i nteract with the KS-rich regions of several aggrecan monomers aligned withi n a proteoglycan aggregate. The fibril could therefore serve as a backbone in at least some of the aggrecan complexes.