Structural and functional investigations of Matrilin-1 A-domains reveal insights into their role in cartilage ECM assembly.

Matrilin-1 is expressed predominantly in cartilage and co-localizes with matrilin-3 with which it can form hetero-oligomers. We recently described novel structural and functional features of the matrilin-3 A-domain (M3A) and demonstrated that it bound with high affinity to type II and IX collagens. Interactions preferentially occurred in the presence of Zn(2+) suggesting that matrilin-3 has acquired a requirement for specific metal ions for activation and/or molecular associations. To understand the interdependence of matrilin-1/-3 hetero-oligomers in extracellular matrix (ECM) interactions, we have extended these studies to include the two matrilin-1 A-domains (i.e. M1A1 and M1A2 respectively). In this study we have identified new characteristics of the matrilin-1 A-domains by describing their glycosylation state and the effect of N-glycan chains on their structure, thermal stability, and protein-protein interactions. Initial characterization revealed that N-glycosylation did not affect secretion of these two proteins, nor did it alter their folding characteristics. However, removal of the glycosylation decreased their thermal stability. We then compared the effect of different cations on binding between both M1A domains and type II and IX collagens and showed that Zn(2+) also supports their interactions. Finally, we have demonstrated that both M1A1 domains and biglycan are essential for the association of the type II·VI collagen complex. We predict that a potential role of the matrilin-1/-3 hetero-oligomer might be to increase multivalency, and therefore the ability to connect various ECM components. Differing affinities could act to regulate the integrated network, thus coordinating the organization of the macromolecular structures in the cartilage ECM.