Collagen XIII alpha 1 is an 85 kDa - 95 kDa protein in the type 2 transmembrane collagen family (1). Mature mouse Collagen XIII alpha 1 consists of a 40 amino acid (aa) cytoplasmic domain, a 19 aa transmembrane segment, and a 692 aa extracellular domain (ECD). The ECD contains three collagenous regions separated by shorter non‑collagenous regions (2, 3). Within comparable regions of the ECD, mouse Collagen XIII alpha 1 shares 85% and 88% aa sequence identity with human and rat Collagen XIII alpha 1, respectively. Mouse Collagen XIII alpha 1 is extensively spliced, with some isoforms showing a tissue specific distribution (2, 4). Collagen XIII alpha 1 is widely expressed during development and in the adult (4, 5). It localizes to intercellular adherens junctions and cell-matrix focal adhesions (6, 7). Collagen XIII alpha 1 assembles into disulfide-linked trimers, a process that is enhanced by proline hydroxylation (2, 8). Trimerization involves triple helix formation within the collagenous domains, although portions of the non‑collagenous regions can also form coiled coils (8 ‑ 10). The ECD of trimeric Collagen XIII alpha 1 is an extended rod-like structure with two flexible hinges that correspond to non‑collagenous regions (11). Collagen XIII alpha 1 clusters in cholesterol-rich domains on the plasma membrane (2, 12), and it can be cleaved from the cell surface or intracellularly by a furin-like protease (12). Collagen XIII alpha 1 binds the extracellular matrix molecules fibronectin, heparin, integrin alpha 1, nidogen‑2, and perlecan (11, 13). The shed ECD retains its ability to bind fibronectin and can interfere with matrix formation (14).
