The proteoglycans aggrecan and Versican form networks with fibulin-2 through their lectin domain binding

Abstract

Aggrecan, versican, neurocan, and brevican are important components of the extracellular matrix in various tissues. Their amino-terminal globular domains bind to hyaluronan, but the function of their carboxyl-terminal globular domains has long remained elusive. A picture is now emerging where the C-type lectin motif of this domain mediates binding to other extracellular matrix proteins. We here demonstrate that aggrecan, versican, and brevican lectin domains bind fibulin-2, whereas neurocan does not. As expected for a C-type lectin, the interactions are calcium-dependent, with K(D) values in the nanomolar range as measured by surface plasmon resonance. Solid phase competition assays with previously identified ligands demonstrated that fibulin-2 and tenascin-R bind the same site on the proteoglycan lectin domains. Fibulin-1 has affinity for the common site on versican but may bind to a different site on the aggrecan lectin domain. By using deletion mutants, the interaction sites for aggrecan and versican lectin domains were mapped to epidermal growth factor-like repeats in domain II of fibulin-2. Affinity chromatography and solid phase assays confirmed that also native full-length aggrecan and versican bind the lectin domain ligands. Electron microscopy confirmed the mapping and demonstrated that hyaluronan-aggrecan complexes can be cross-linked by the fibulins.