We for the tissues. The nonfibrillar group comprises collagen types IV, VI-X, and XII-XVII (1-3). These molecules display great heterogeneity in structure, tissue location, macromolecular organization, and function. One common feature is that they all have one or more interruptions in the collagenous sequence. Their collagenous sequences vary in length from about 330 to 1400 aa, the shortest being found in type VI collagen molecules and the longest in type VII. Their carboxyl-terminal and amino-terminal noncollagenous domains also are highly variable in both sequence and length, the latter varying in both domains from <20 aa to several hundred amino acids. One subgroup among the nonfibrillar collagens is formed by the fibril-associated collagens with interrupted triple helices (FACMI): types IX, XII, and XIV (2). These collagens share sequence homology and do not appear to form polymers alone but are associated with fibrils composed of fibrillar collagens. Another subgroup is formed by the structurally homologous types VIII and X, which are thought to form sheets in the extracellular matrix (2). The recently described types XV (4), XVI (5), and XVII (6) differ from the other nonfibrillar collagens in being characterized by numerous interruptions in their triple-helical regions. Type XVI collagen shares some structural features with the FACIT collagens, as also does another recently characterized form called Y-collagen (7,8). Type XVII (6), a hemidesmosomal protein also known as the 180-kDa bullous pemphigoid antigen, is unique among the collagens in that it is thought to be a transmembrane protein.Collagen types XIII and XV and the a5 chain of type IV collagen were identified in our laboratory by screening of cDNA libraries under low stringency with probes encoding collagenous sequences (4, 9, 10). Recently we screened a mouse cDNA library to obtain clones coding for the mouse counterpart to the previously characterized human type XIII collagen (9). One of the positive clones was found to encode a collagenous protein not described before.t We present here a partial characterization of this polypeptide, which is characterized by multiple interruptions in the triple helix, and suggest that it should be designated the al chain of type XVIII collagen. Our findings indicate that type XVIII collagen has an unusual tissue location. Furthermore, type XVIII was found to be homologous with type XV, and the two thus form a subgroup among the collagens.
MATERIALS AND METHODS[solation of cDNA Clones and DNA Sequencing. A 500-bp clone, G2, encoding the al chain ofmurine type XIII collagen (unpublished results) was used as a probe to screen a mouse embryo Agtll cDNA library (ML 1027a, Clontech) under stringent conditions (11). The final wash for the filters was at 50°C in 0.5x standard saline citrate (SSC)/0.1% NaDodSO4.The recombinant phage ME-1 was isolated and the insert DNA was subcloned to the EcoRI site of pBluescript SK (Stratagene). The nucleotide sequence was determined for Abbreviation: FACIT, fibril-associ...