Proteoglycan Lectin Domain Interactions in Extracellular Matrix Assembly

Abstract: This thesis focuses on the C-terminal G3 region of the large aggregating proteoglycans that are found in complex with hyaluronan in the extracellular matrix. The G3 region contains a C-type lectin domain that binds different extracellular matrix proteins playing an integral role in tissue organization during development and remodeling. Here we affinity purify the proteoglycan aggrecan via one of its G3 ligands. It is then investigated for other ligand affinities together with a recombinant version of the lectin domain. Fibulin-2 is a G3 lectin ligand that here is shown to mediate the formation of large hyaluronan-proteoglycan network structures via its dimerization and aggrecan cross-linking. Fibulin-2 binds with an affinity in the nanomolar range to the same site on the aggrecan lectin domain as the previously identified ligands tenascin-R and fibulin-1. In the G3 region additional modules flank the C-type lectin. We investigate the effects of these on lectin ligand binding. The findings suggest that alternative splicing of the G3 region could be a mechanism for modulating the lectin interactions. Tenascin-C is another G3 ligand identified in this work. We also identify the leucine-rich repeat protein PRELP as a novel G3 ligand. This interaction is apparently mediated by PRELP N-linked oligosaccharide structures. It is however not dependent of calcium, which would have been expected by a C-type lectin interaction. PRELP may function as a multifunctional linker between the hyaluronan-aggrecan network and other of its ligands, such as matrix collagens and heparan sulphate proteoglycans on the chondrocyte surface. Furthermore, we show that PRELP form ternary complexes with fibulin-2 and G3 lectin suggesting an intricate network formation mechanism. The fourth ligand identified is collagen IX. It binds the G3 lectin dependent on an N-linked oligosaccharide and calcium. This interaction may provide a link between the aggregates of hyaluronan-proteoglycan and the heterofibrillar structures of collagen II, IX and XI. We crystallize and start to determine the structure of the G3 lectin as it is binding a protein fragment of its ligand tenascin-R.