The link protein in proteoglycan aggregates from the Swarm rat chondrosarcoma.

Aggregated proteoglycans (70% aggregated), isolated from the Swarm rat chondrosarcoma by extraction with 4 M guanidinium chloride in the presence of protease inhibitors and purified by centrifugation in an associative cesium chloride gradient, were separated into the component parts by centrifugation in a dissociative cesium chloride gradient. The gradient was cut into five equal fractions. The bottom fraction contained 98% of the chondroitin sulfate and 84% of the protein of the aggregate preparation. Sedimentation equilibrium studies on the protein core of this fraction, isolated by column chromatography from chondroitinase ABC digests, suggest that its molecular weight is 2.0 x lo” to 2.2 x 1W. The intermediate fractions of lower buoyant densities contained hyaluronic acid (0.8% of the total weight of the aggregated preparations) and proteoglycan monomers with fewer chondroitin sulfate chains relative to the protein core than in the bottom fraction. The top fraction, in addition to proteoglycans, contained a link protein. The link protein was separated from the proteoglycans by chromatography on Sephadex G-200 in the presence of 4 M guanidinium chloride. Its molecular weight was estimated to be 40,000. It stabilized complexes of proteoglycan monomers and hyaluronic acid so that they could be seen in the analytical ultracentrifuge at pH 5.8. The amino acid composition of the link protein differs significantly from that of the protein core of the proteoglycan monomers and from that of the hyaluronic acid binding region of the protein core of the proteoglycans.