Two novel matrix proteins isolated from articular cartilage show wide distributions among connective tissues

Two proteins of Mr = 58,000 and 59,000, respectively, were purified from 4 M guanidinium chloride extracts of articular cartilage by dissociative CsCl-density gradient centrifugation followed by gel chromatography on Sephadex G-200 and ion exchange chromatography on DEAE-cellulose. The two proteins differ in ionic properties and only the one with Mr = 59,000 bound to the ion exchanger. Although the two proteins showed dissimilar peptide patterns after proteolysis, their amino acid composition was similar, with very high contents of leucine and aspartic acid/asparagine. The two proteins showed no cross-reactivity in radioimmunoassays. By use of these assays, the proteins were demonstrated in extracts of most connective tissues, with high contents of about 0.1% of tissue wet weight determined in several types of cartilage. Among the non-cartilage connective tissues, tendon and sclera had the highest contents of the proteins, i.e. about 0.1% of the tissue wet weight. Bone extracts, on the other hand, contained insignificant amounts of the proteins. Only the Mr = 59,000 protein was detected in serum, its concentration being about 33 micrograms/l. Both proteins were shown to be localized in the extracellular matrix of cartilage, predominantly in the territorial matrix, by using indirect immunofluorescence.