Type XIII collagen:structural and functional characterization of the ectodomain and identification of the binding ligands

Abstract Type XIII collagen is a transmembrane protein consisting of a short intracellular portion, a transmembrane anchor, and a long extracellular domain with a mainly collagenous sequence. Histochemical and cell biological studies have revealed that type XIII collagen has a wide distribution in various tissues and that it is mostly localized to cell-cell and cell-matrix contacts. In order to study type XIII collagen at the molecular level, the protein was expressed in insect cells as a homotrimer. The recombinant protein was found to reside in the plasma membrane of insect cells with its N-terminus intracellular and C-terminal part extracellular, i. e. in a type II orientation. The trimerization of type XIII collagen chains was initiated by 21 amino acid residues adjacent to the transmembrane domain on the extracellular side, and this sequence was found to be conserved in several other collagenous transmembrane proteins. In addition to the transmembrane form, the ectodomain of type XIII collagen was secreted into the cell culture medium, a result of proteolytic cleavage by furin-like proteases at the non-collagenous NC1 domain. The ectodomain was purified from the insect cell culture medium with a typical collagenous composition and conformation, and it showed as a 150 nm-long rod in rotary shadowing electron microscopy. Furthermore, the recombinant ectodomain showed high affinity binding to several extracellular matrix proteins, e. g. fibronectin, nidogen-2, and perlecan, as well as to heparin. The type XIII collagen ectodomain also showed selective recognition to collagen receptor integrins. Integrin α1 and α11 I domains bind to type XIII collagen with a high affinity, and both integrins α1β1 and α11β1 mediate cell attachment to type XIII collagen. The present results suggest that type XIII collagen shares common aspects with other collagenous transmembrane proteins in terms of chain association and ectodomain shedding. However, it is notably distinct in its structure and binding specificity compared to other types of collagen and cell-surface proteins. The data imply that type XIII collagen might participate in multiple cell-cell and cell-matrix interactions