Proline hydroxylation in collagen supports integrin binding by two distinct mechanisms

Collagens are the most abundant extracellular matrix proteins in vertebrates and have a characteristic triple-helix structure. Hydroxylation of proline residues is critical for helix stability, and diminished prolyl hydroxylase activity causes wide-spread defects in connective tissues. Still, the role of proline hydroxylation in the binding of collagen receptors such as integrins is unclear. Here, we isolated skin collagen from genetically modified mice having reduced prolyl 4-hydroxylase activity. At room temperature, the reduced proline hydroxylation did not affect interactions with the recombinant integrin 2I domain, but at 37 degrees C, collagen hydroxylation correlated with the avidity of 2I domain binding. Of note, LC-MS/MS analysis of isolated skin collagens revealed no major changes in the hydroxyproline content of the main integrin-binding sites. Thus, the disrupted 2I domain binding at physiological temperatures was most likely due to structural destabilization of the collagenous helix. Integrin 2I binding to the triple-helical GFPGER motif was slightly weaker than to GFOGER (O = hydroxyproline). This phenomenon was more prominent when 1 integrin was tested. Integrin 11 expressed on CHO cells and recombinant 1I domain showed remarkably slower binding velocity and weaker avidity to GFPGER when compared with GFOGER. Structural modeling revealed the critical interaction between Arg-218 in 1I and the hydroxyproline residue in the integrin-binding motif. The role of Arg-218 was further validated by testing a variant R218D 1I domain in solid-phase binding assays. Thus, our results show that the lack of proline hydroxylation in collagen can affect integrin binding by a direct mechanism and via structural destabilization of the triple helix.