Enhanced Activity of Transforming Growth Factor β1 (TGF-β1) Bound to Cartilage Oligomeric Matrix Protein

Cartilage oligomeric matrix protein (COMP) is an important non-collagenous cartilage protein that is essential for the structural integrity of the cartilage extracellular matrix. The repeated modular structure of COMP allows it to “bridge” and assemble multiple cartilage extracellular matrix components such as collagens, matrilins, and proteoglycans. With its modular structure, COMP also has the potential to act as a scaffold for growth factors, thereby affecting how and when the growth factors are presented to cell-surface receptors. However, it is not known whether COMP binds growth factors. We studied the binding interaction between COMP and TGF-β1 in vitro and determined the effect of COMP on TGF-β1-induced signal transduction in reporter cell lines and primary cells. Our results demonstrate that mature COMP protein binds to multiple TGF-β1 molecules and that the peak binding occurs at slightly acidic pH. These interactions were confirmed by dual polarization interferometry and visualized by rotary shadow electron microscopy. There is cation-independent binding of TGF-β1 to the C-terminal domain of COMP. In the presence of manganese, an additional TGF-β-binding site is present in the TSP3 repeats of COMP. Finally, we show that COMP-bound TGF-β1 causes increased TGF-β1-dependent transcription. We conclude that TGF-β1 binds to COMP and that TGF-β1 bound to COMP has enhanced bioactivity. Background: Cartilage oligomeric matrix protein (COMP) is a cartilage protein with a repeated modular structure that binds to and assembles extracellular matrix proteins but is not known to bind growth factors. Results: TGF-β family growth factors bind to multiple sites on COMP. Binding enhances TGF-β-dependent gene expression. Conclusion: COMP enhances the cellular response to TGF-β. Significance: A new function of COMP is to modulate TGF-β activity.