A Role for Prolyl 3-Hydroxylase 2 in Post-translational Modification of Fibril-forming Collagens

The fibrillar collagen types I, II, and V/XI have recently been shown to have partially 3-hydroxylated proline (3Hyp) residues at sites other than the established primary Pro-986 site in the collagen triple helical domain. These sites showed tissue specificity in degree of hydroxylation and a pattern of D-periodic spacing. This suggested a contributory role in fibril supramolecular assembly. The sites in clade A fibrillar alpha 1(II), alpha 2(V), and alpha 1(I) collagen chains share common features with known prolyl 3-hydroxylase 2 (P3H2) substrate sites in alpha 1(IV) chains implying a role for this enzyme. We pursued this possibility using the Swarm rat chondrosarcoma cell line (RCS-LTC) found to express high levels of P3H2 mRNA. Mass spectrometry determined that all the additional candidate 3Hyp substrate sites in the pN type II collagen made by these cells were highly hydroxylated. In RNA interference experiments, P3H2 protein synthesis was suppressed coordinately with prolyl 3-hydroxylation at Pro-944, Pro-707, and the C-terminal GPP repeat of the pN alpha 1(II) chain, but Pro-986 remained fully hydroxylated. Furthermore, when P3H2 expression was turned off, as seen naturally in cultured SAOS-2 osteosarcoma cells, full 3Hyp occupancy at Pro-986 in alpha 1(I) chains was unaffected, whereas 3-hydroxylation of residue Pro-944 in the alpha 2(V) chain was largely lost, and 3-hydroxylation of Pro-707 in alpha 2(V) and alpha 2(I) chains were sharply reduced. The results imply that P3H2 has preferred substrate sequences among the classes of 3Hyp sites in clade A collagen chains.