Type XVII collagen (BP180) is a keratinocyte transmembrane protein that exists as the full-length protein in hemidesmosomes and as a 120-kDa shed ectodomain in the extracellular matrix. The largest collagenous domain of type XVII collagen, COL15, has been described previously as a cell adhesion domain (Tasanen, K., Eble, J. A., Aumailley, M., Schumann, H., Baetge, J, Tu, H., Bruckner, P., and Bruckner-Tuderman, L. (2000) J. Biol. Chem. 275, 3093-3099). In the present work, the integrin binding of triple helical, human recombinant COL15 was tested. Solid phase binding assays using recombinant integrin ␣ 1 I, ␣ 2 I, and ␣ 10 I domains and cell spreading assays with ␣ 1  1 -and ␣ 2  1 -expressing Chinese hamster ovary cells showed that, unlike other collagens, COL15 was not recognized by the collagen receptors. Denaturation of the COL15 domain increased the spreading of human HaCaT keratinocytes, which could migrate on the denatured COL15 domain as effectively as on fibronectin. Spreading of HaCaT cells on the COL15 domain was mediated by ␣ 5  1 and ␣ V  1 integrins, and it could be blocked by RGD peptides. The collagen ␣-chains in the COL15 domain do not contain RGD motifs but, instead, contain 12 closely related KGD motifs, four in each of the three ␣-chains. Twenty-two overlapping, synthetic peptides corresponding to the entire COL15 domain were tested; three peptides, all containing the KGD motif, inhibited the spreading of HaCaT cells on denatured COL15 domain. Furthermore, this effect was lost by mutation from D to E (KGE instead of KGD). We suggest that the COL15 domain of type XVII collagen represents a specific collagenous structure, unable to interact with the cellular receptors for other collagens. After being shed from the cell surface, it may support keratinocyte spreading and migration.The collagens are a family of extracellular matrix proteins (1). Among the 19 different collagen subtypes that have been identified, types XIII and XVII are the only transmembrane proteins (2, 3). Type XVII collagen (BP180) is hemidesmosomal transmembrane protein that is mutated in junctional epidermolysis bullosa and is targeted by autoantibodies in blistering skin diseases (4 -6). It occurs in two forms: as a full-length transmembrane protein and as a distinct, soluble ectodomain that is shed from the cell surface by a furin-mediated, proteolytic process (7,8). This collagen possesses intracellular and transmembrane domains that are of 560 and 23 amino acids in length, respectively. In addition, it has an extracellular domain of 914 amino acids that contains multiple noncollagenous interruptions, dividing it into 15 collagenous subdomains (3). The longest of these subdomains, COL15, consists of 242 amino acids (residues 567-808 of collagen XVII). Thus, it is much larger than any of the other collagenous domains, which vary from 14 to 45 residues in length (3).Most collagen subtypes can be recognized by a group of integrin-type cell adhesion receptors. Although all collagenreceptor integrins share the common  1 in...