Electron microscopic studies of cartilage proteoglycans. Direct evidence for the variable length of the chondroitin sulfate-rich region of proteoglycan subunit core protein.

Electron microscopic studies of bovine nasal cartilage proteoglycans have been carried out to examine the hypothesis that the size of a proteoglycan subunit is determined mainly by the length of the chondroitin sulfate-rich region of proteoglycan subunit core protein. A polydisperse population of proteoglycan subunits f rom ma tu re bovine nasal cartilage was separated into a series of relatively monodisperse fractions with sedimentation coefficients (s$o,solvent) of 8.3 S, 10.3 S., 14.0 S, 19.9 S, and 21.1 S. There was no difference in the lengths of the chondroitin sulfate chains or the spacing between chains in the different subunit fractions, indicating that neither increased chondroitin sulfate chain length nor decreased spacing between chains contributed to increases in subunit molecular weight. However, the lengths of the proteoglycan subunit core proteins increased, and the numbers of chondroitin sulfate chains per subunit increased, as the sedimentation coefficients of the subunits increased, demonstrating that the length of the chondroitin sulfate-rich region increases as the molecular weight of the proteoglycan subunit increases. Proteoglycan aggregates were prepared by equilibrium density gradient centrifugation under associative conditions. O n nitrocellulose films, proteoglycan subunits bound to hyaluronic acid in the proteoglycan aggregates consisted of two distinct segments: 1) a peripheral thick segment corresponding predominant ly to the chondroitin sulfate-rich region; and 2) a cent ra l thin segment which could be traced directly to the hyaluronic acid central fi lament and which contains the hyaluronic acid-binding region. To show that the length of the proteoglycan subunit is determined mainly by the length of the chondroitin sulfate-rich region, total subunit lengths, th ick segment lengths, and thin segment lengths were measured. The percentage of the total subunit length contributed by each segment was calculated and plotted against total subunit length. As the total subunit length increased, the percentage of the core protein consisting of the thick segment increased, and the percentage of the core protein consisting of the thin segment decreased. Hyperbolic equations describing these relationships, calculated f rom the experimental measurements, were similar to a hyperbolic equation derived f rom a theoretical model of a proteoglycan subunit composed of a