Evidence for a glycosaminoglycan chain on a portion of articular cartilage fibronectins

Fibronectin heterogeneity is, in part, the result of post-translational modifications. In these experiments, cartilage fibronectins were purified by anion exchange chromatography, followed by gelatin affinity chromatography or immunoprecipitation, and, finally, sodium dodecyl sulfate--polyacrylamide gel electrophoresis (NaDodSO4 PAGE). A substantial, although variable, portion of the fibronectins from canine and equine cartilages of all ages required salt concentrations from 0.2 to 1.0 M for elution from DEAE-cellulose. This was in contrast to plasma fibronectin which eluted with 0.1 M NaCl, but these results were consistent with observations made on human cartilage by Brown and Jones (1990 J. Rheumatol. 17, 65-72). When cartilage explants were incubated with Na2 35SO4=, the cartilage fibronectins were sulfated and the fibronectins which eluted with high salt contained from 5- to 50-fold more radiosulfate than the fibronectins which eluted with 0.1 M NaCl. A fraction of the 35SO4= which copurified with the cartilage fibronectin and comigrated with it in NaDodSO4-PAGE could be removed by digestion with chondroitinase ABC. This suggested that a percentage of cartilage fibronectins are covalently linked to a chondroitin sulfate or dermatan sulfate chain and thus might also appropriately be called proteoglycans. Alternatively, there is a proteoglycan which binds so tightly to fibronectin that separation is not achieved even in the presence of urea, sodium dodecyl sulfate, and mercaptoethanol.