Proteolytic Processing Causes Extensive Heterogeneity of Tissue Matrilin Forms

The matrilins are a family of multidomain extracellular matrix proteins with adapter functions. The oligomeric proteins have a bouquet-like structure and bind to a variety of different ligands whereby the avidity of their interactions is dependent on the number of subunits and domains present. Here we show the contribution of post-translational proteolytic processing to the heterogeneity of matrilins seen in tissue extracts and cell culture supernatants. A cleavage site after two glutamate residues in the hinge region close to the C-terminal coiled-coil oligomerization domain is conserved among the matrilins. Cleavage at this site yields molecules that lack almost complete subunits. The processing is least pronounced in matrilin-1 and particularly complex in matrilin-2, which contains additional cleavage sites. Replacement of the hinge region in matrilin-4 by the matrilin-1 hinge region had no marked effect on the processing. A detailed study revealed that matrilin-4 is processed already in the secretory pathway and that the activation of the responsible enzymes is dependent on proprotein convertase activity. Matrilin-3 and -4, but not matrilin-1 subunits present in matrilin-1/-3 heterooligomers, were identified as substrates for ADAMTS4 and ADAMTS5, whereas ADAMTS1 did not cleave any matrilin. A neo-epitope antibody raised against the N terminus of the C-terminal cleavage product of matrilin-4 detected processed matrilin-4 in cultures of primary chondrocytes as well as on cartilage sections showing that the conserved cleavage site is used in vivo.