Focus on Molecules: Collagens V and XI

Abstract

The collagens are a family of 28 extracellular matrix glycoproteins. Collagens are composed of 3 polypeptides, called alpha chains. Each α chain contains at least one stiff, rod-like domain of varying length with a glycine at every third residue that can form a triple helix, i.e., collagenous domain. Collagens V and XI are members of the fibril-forming class of collagens, along with collagens I, II, III, XXIV and XXVII. Collagens V and XI comprise a subclass of regulatory fibrillar collagens that co-assemble with collagens I, II and III. All collagens are trimers composed of 3 α chains with at least one triple helical or collagen (COL) domain. Alpha chains of collagens V (P20908 and P25940) and XI (P12107 and P13942) are encoded by genes with different phylogenetic origins, belonging to different clades or `ancestral groups' (clade 2) from collagens I, II and III (clade 1) or XXIV and XXVII (clade 3). Clade 1 also contains the α2(V) gene (P05997; i.e., the α2 chain of collagen V) and the α1(II) gene, termed α3(XI) when part of a collagen XI trimer (P02458). These genes code for multi-domain polypeptide chains with features common to all fibrillar collagens: a signal peptide; an N-terminal non-collagenous region (NC3); a short N-terminal triple helical domain of interrupted Gly-X–Y repeats (COL2), where X and Y can be any amino acid, but are often proline and hydroxyproline, respectively; a short non-collagenous domain (NC2 or `hinge'); the main triple helical or collagenous domain of continuous (GlyX–Y)n repeats (COL1); and a non-collagenous C-propeptide (NC1) (Fig.1).