IDENTITY OF THE CORE PROTEINS OF THE LARGE CHONDROITIN SULFATE PROTEOGLYCANS SYNTHESIZED BY SKELETAL-MUSCLE AND PRECHONDROGENIC MESENCHYME

Large, chondroitin sulphate-containing proteoglycans are synthesized b y three prominent tissues in the embryonic chick limb. One of these pr oteoglycans is aggrecan, the phenotype-specific proteoglycan of cartil age. Another, PG-M, is produced by prechondrogenic mesenchymal cells. The third, M-CSPG, is made by developing skeletal muscle cells. While the carbohydrate components of PG-M and M-CSPG share some similarities , both of these proteoglycans clearly have different carbohydrate moie ties from those of aggrecan. To compare these three proteoglycans at a nother level, their core protein structures were analysed in three way s: by the presence or absence of monoclonal antibody epitopes, by one- dimensional peptide display of the cyanogen bromide-cleaved core prote ins and by electron microscopic imaging of the molecules. Monoclonal a ntibodies whose epitopes are present in aggrecan core protein were tes ted with core protein preparations from M-CSPG and PG-M. One of these, 7D1, recognizes both PG-M and M-CSPG, while another, 1C6, shows no re activity for the non-cartilage proteoglycans. The absence of 1C6 react ivity is of interest, as its epitope is in a region of the aggrecan co re protein known to have a functional homologue in the core proteins o f PG-M and M-CSPG. The cyanogen bromide-fragmented peptide pattern of M-CSPG is the same as that of PG-M, and both are different from that o f aggrecan. The aggrecan pattern has one prominent large band (molecul ar mass 130 kDa), some less prominent large bands (molecular mass 70-1 00 kDa) and several smaller bands. In contrast, the PG-M and M-CSPG pa tterns show no bands with molecular masses > 73 kDa, and the smaller b ands (molecular mass < 40 kDa) have a different pattern to that of the smaller bands from aggrecan. The electron microscopic images of aggre can show a core protein with one end having two globular regions separ ated by a short linear segment; adjacent to this is a long linear segm ent, which sometimes contains a third globular region at the end of th e core protein opposite the end with the double-globe structure. M-CSP G and PG-M core proteins never show images with the double-globe struc ture. Instead, one end of the molecule has a single globular domain, a nd a second globular region is variably present at the opposite end of the core protein. Thus, by all three methods, the core proteins of PG -M and M-CSPG appear to be the same and both differ from the core prot ein of aggrecan.