Variation of multifunctional surface binding proteins--a virulence strategy for group A streptococci?

Procollagen and proteoglycan biosynthesis was defined in long-term culture of a human osteogenic sarcoma cell line, SAOS-2. An osteoblast phenotype was maintained by these cells up to 40 days post-confluent in the presence of ascorbic acid. Under these conditions, cells deposited around them an extensive collagenous matrix that was able to mineralize in the presence of an exogenous phosphate donor (beta-glycerophosphate). The collagenous matrix was comprised predominantly of collagen type I with significant amounts of collagen type V, and greater than 80% of the collagen in the matrix was involved in covalent crosslinkages. With increasing time in culture there was a decrease in the collagen synthetic rate, although the collagenous matrix was maintained. The proteoglycans synthesized by nonmineralizing and mineralizing cultures were purified after biosynthetic labeling with 35S]sulfate and 3H]glucosamine. Two major species were apparent: a large chondroitin sulfate proteoglycan (CSPG), and a small chondroitin sulfate proteoglycan, decorin. In nonmineralizing cultures, decorin partitioned equally between the cell layer and culture medium, whereas the large CSPG species partitioned exclusively into the cell layer-associated matrix. In the presence of extensive mineral deposition, greater than 90% of the newly synthesized proteoglycans were secreted into the medium. Northern blot hybridization indicated that SAOS-2 cells express mRNA encoding a range of bone proteins, including decorin, osteonectin, and bone sialoprotein.