Alpha 1(XVIII), a collagen chain with frequent interruptions in the collagenous sequence, a distinct tissue distribution, and homology with type XV collagen.

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Abstract
We report on the isolation of mouse cDNA clones which encode a collagenous sequence designated here as the alpha 1 chain of type XVIII collagen. The overlapping clones cover 2.8 kilobases and encode an open reading frame of 928 amino acid residues comprising a putative signal peptide of 25 residues, an amino-terminal noncollagenous domain of 301 residues, and a primarily collagenous stretch of 602 residues. The clones do not cover the carboxyl-terminal end of the polypeptide, since the translation stop codon is absent. Characteristic of the deduced polypeptide is the possession of eight noncollagenous interruptions varying in length from 10 to 24 residues in the collagenous amino acid sequence. Other features include the presence of several putative sites for both N-linked glycosylation and O-linked glycosaminoglycan attachment and homology of the amino-terminal noncollagenous domain with thrombospondin. It is of particular interest that five of the eight collagenous sequences of type XVIII show homology to the previously reported type XV collagen, suggesting that the two form a distinct subgroup among the diverse family of collagens. Northern blot hybridization analysis revealed a striking tissue distribution for type XVIII collagen mRNAs, as the clones hybridized strongly with mRNAs of 4.3 and 5.3 kilobases that were present only in lung and liver of the eight mouse tissues studied.