The origin of human cartilage proteoglycan link-protein heterogeneity and fragmentation during aging

Human articular-cartilage link proteins are resolved into three components by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, indicative of three different structures. The action of the proteinase clostripain yields a single link-protein component with electrophoretic properties analogous to those of the smallest (most mobile) native link protein, suggesting that this link protein may be derived naturally from one or both of the larger molecules by proteolytic cleavage in situ. Upon chemical deglycosylation of native link protein two components are resolved, suggesting that two of the link proteins differ only in their degree and/or type of oligosaccharide substitution. This pattern is compatible with a proteolytic origin for the smallest link protein. During aging further proteolytic fragmentation occurs, though it is only apparent on reduction of disulphide bonds. This fragmentation occurs at identical sites in all three native link proteins, indicating the existence of a large region common to all the link proteins, which appears to consist predominantly of the C-terminal half of the molecules. These observations are compatible with the variation in oligosaccharide and proteolytic heterogeneity occurring at the N-terminus of the link proteins.