The
cartilage oligomeric matrix protein (COMP) and
matrilins are abundant non-collagenous
proteins in the cartilage extracellular matrix. In the presence of
calcium, COMP and
matrilin-1 elute together in the gel filtration of cartilage extracts and can be co-immunoprecipitated. In a screen for
ligands of
matrilin-1, -3, and -4 using an ELISA-style binding assay, COMP was identified as a prominent binding partner for all three, indicating a conservation of the COMP interaction among
matrilins. The interaction of COMP and
matrilin-4 is saturable, and an apparent K(D) of 1 nm was determined. However, only the full-length COMP and the full-length
matrilin-4 proteins showed a strong interaction, indicating that the oligomeric structures markedly increase the affinity. Mutations in COMP or
matrilin-3 cause related forms of human chondrodysplasia, and the COMP mutation D469Delta, which is found in patients with
pseudoachondroplasia, has been shown to cause a reduced
calcium binding. Despite this, the mutation causes only a slight decrease in
matrilin-4 binding. This indicates that impaired binding of COMP to
matrilins does not cause the
pseudoachondroplasia phenotype but rather that
matrilins may be coretained in the rough endoplasmatic reticulum where COMP accumulates in the chondrocytes of patients.