Collagen triple helices are stabilized by
4-hydroxyproline residues. No function is known for the much less common
3-hydroxyproline (3Hyp), although genetic defects inhibiting its formation cause recessive
osteogenesis imperfecta. To help understand the pathogenesis, we used mass spectrometry to identify the sites and local sequence motifs of 3Hyp residues in fibril-forming
collagens from normal human and bovine tissues. The results confirm a single, essentially fully occupied 3Hyp site (A1) at Pro(986) in A-clade chains alpha1(I), alpha1(II), and alpha2(V). Two partially modified sites (A2 and A3) were found at Pro(944) in alpha1(II) and alpha2(V) and Pro(707) in alpha2(I) and alpha2(V), which differed from A1 in sequence motif. Significantly, the distance between sites 2 and 3, 237 residues, is close to the
collagen D-period (234 residues). A search for additional D-periodic 3Hyp sites revealed a fourth site (A4) at Pro(470) in alpha2(V), 237 residues N-terminal to site 3. In contrast, human and bovine
type III collagen contained no 3Hyp at any site, despite a candidate
proline residue and recognizable A1 sequence motif. A conserved
histidine in mammalian alpha1(III) at A1 may have prevented 3-hydroxylation because this site in chicken type III was fully hydroxylated, and
tyrosine replaced
histidine. All three B-clade type V/XI
collagen chains revealed the same three sites of 3Hyp but at different loci and sequence contexts from those in A-clade
collagen chains. Two of these B-clade sites were spaced apart by 231 residues. From these and other observations we propose a fundamental role for 3Hyp residues in the ordered self-assembly of
collagen supramolecular structures.