Schematic representation of collagen XVII, its shed ectodomain, the recombinant fragments, and the specificities of the antibodies used. Collagen XVII is a type II transmembrane protein (upper scheme) with an amino-terminal intracellular domain, a transmembrane domain, and a carboxy-terminal extracellular domain about 1000 amino acids in length. The soluble ectodomain (lower scheme) consists of 15 collagenous and 16 noncollagenous domains. Shedding occurs by proteolytic cleavage carboxy-terminally from the transmembrane domain, presumably within the NC16a region. White bars: Collagenous sequences; black bars: noncollagenous sequences; the white oval indicates the transmembrane domain (TM). The domains covered by the eucaryotic recombinant fragments, ERF-ColXVII-Col15 and ERF-ColXVII-Ecto2 (designated Col15 and Ecto2, respectively), are shown as light gray bars. The dark gray bars indicate the bacterial fusion proteins and the epitopes of the corresponding antibodies Ab-ColXVII-NC16a recognizing the NC16a domain, Ab-ColXVII-Col15-2 for the amino terminus of the Col15 domain (Col15-2), and Ab-ColXVII-Ecto3 (Ecto3) for the Col2 and NC3 domains. Ab-ColXVII-Ecto1 (Ecto1) against the distal carboxy terminus of the molecule has been described previously. 1