A protein that is a component of collagen. Collagens are trimeric molecules in which each chain comprises a repeating Gly-X-Y triplet, in which X and Y can be any residue but are usually proline and hydroxyproline, respectively. This results in a left-handed helix that combines with other two to form a triple-helical structure. [ http://www.ncbi.nlm.nih.gov/pubmed/15788652 http://www.ncbi.nlm.nih.gov/pubmed/1639194 http://pfam.xfam.org/family/PF01391 ]