The major function of the link protein (LP) of proteoglycan aggregates (PG) appears to be to stabilize the aggregation of PG monomers with hyaluronic acid (HA). Such aggregates form ordered structures with collagen and together these form the majority of the macromolecular species in cartilage. The mechanism of assembly is not known, but is thought to involve the formation of LP-PG binary complexes which subsequently bind to HA extracellularly. The mechanisms for the modulation of this process, either intrinsic or via cellular processes, are currently unknown. We have previously described the primary structures of LP and the PG region which binds to HA (HABR) and found them to be very similar. In addition to HABR, another domain in PG is similar to LP. This is the recently described G2 domain. This is very similar to both the COOH-terminal half of LP and HABR but as yet has not been assigned a function. We propose to extend the descriptions of the primary structures of these regions by outlining the detailed functions of these domains. We intend to define the exact regions of LP and HABR which bind to HA and which regions bind to HABR and LP respectively. Peptides derived from the intact proteins or synthesized de novo will be analyzed for their ability to (1) inhibit aggregation and (2) bind to the macromolecules in question. Some synthetic peptides have already been synthesized and display interesting properties. In addition, maps of exposed residues in HABR and LP will be developed by differential derivatiztion of the monomers, the binary complexes and the ternary complex of PG-HA and LP. This will enable us to define the residues which are directly involved in aggregation. Throughout these studies, we will utilize the similarity between LP and HABR as an internal control: results obtained with one molecule should be reflected in the other. Eventually, we will develop a model for aggregation which can be used in defining cartilage repair processes.
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