The complete primary structure of the mouse type XIII collagen chain was 
determined by cDNA cloning. Comparison of the mouse amino acid sequences with 
the previously determined human sequences revealed a high identity of 90%. 
Surprisingly, the mouse cDNAs extended further in the 5' direction than the 
previously identified human clones. The 5' sequences contained a new in-frame 
ATG codon for translation initiation which resulted in elongation of the 
N-terminal noncollagenous domain by 81 residues. These N-terminal sequences lack 
a typical signal sequence but include a highly hydrophobic segment that clearly 
fulfills the criteria for a transmembrane domain. The sequence data thus 
unexpectedly suggested that type XIII collagen may be located on the plasma 
membrane, with a short cytosolic N-terminal portion and a long collagenous 
extracellular portion. These sequence data prompted us to generate antipeptide 
antibodies against type XIII collagen in order to study the protein and its 
subcellular location. Western blotting of human tumor HT-1080 cell extract 
revealed bands of over 180 kDa. These appeared to represent disulfide-bonded 
multimeric polypeptide forms that resolved upon reduction into 85-95-kDa bands 
that are likely to represent a mixture of splice forms of monomeric type XIII 
collagen chains. These chains were shown to contain the predicted N-terminal 
extension and thus also the putative transmembrane segment. Immunoprecipitation 
of biotinylated type XIII collagen from surface-labeled HT-1080 cells, 
subcellular fractionation, and immunofluorescence staining were used to 
demonstrate that type XIII collagen molecules are indeed located in the plasma 
membranes of these cells.