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.The collagen family of proteins presently includes 19 types of collagen, and several additional proteins have collagen-like domains (1, 2). The collagens can be divided into two subgroups in terms of their structural and functional characteristics, the fibril-forming and the nonfibril-forming collagens. Members of the former group, i.e. types I-III, V, and XI, aggregate into prominent fibrillar structures in many collagen-containing tissues. These molecules are structurally homologous and characterized by a long, uninterrupted collagen triple helix. The other collagens are unable to form fibrils, and they show considerable diversity in structure, macromolecular organization, tissue distribution, and function. One common feature is that they all have one or more interruptions in the collagenous sequence. Several subfamilies can be distinguished among the nonfibril-forming collagens as follows: the network-forming collagens (types IV, VIII and X), fibril-associated collagens with interrupted triple helices (which include types IX, XII, XIV, XVI and XIX), a beaded filament-forming collagen (type VI), the family of types XV and XVIII collagens, and a collagen with a transmembrane domain (type XVII). The last mentioned collagen is distinct from the other family members, because it is not secreted into the extracellular matrix.Type XIII collagen is...

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Type XIII collagen is a type II transmembrane protein predicted to consist of a short cytosolic domain, a single transmembrane domain, and three collagenous domains flanked by noncollagenous sequences. Previous studies on mRNAs indicate that the structures of the collagenous domain closest to the cell membrane, COL1, the adjacent noncollagenous domain, NC2, and the C-terminal domains COL3 and NC4 are subject to alternative splicing. In order to extend studies of type XIII collagen from cDNAs to the protein level we have produced it in insect cells by means of baculoviruses. Type XIII collagen ␣ chains were found to associate into disulfidebonded trimers, and hydroxylation of proline residues dramatically enhanced this association. This protein contains altogether eight cysteine residues, and interchain disulfide bonds could be located in the NC1 domain and possibly at the junction of COL1 and NC2, while the two cysteine residues in NC4 are likely to form intrachain bonds. Pepsin and trypsin/chymotrypsin digestions indicated that the type XIII collagen ␣ chains form homotrimers whose three collagenous domains are in triple helical conformation. The thermal stabilities (T m ) of the COL1, COL2, and COL3 domains were 38, 49 and 40°C, respectively. The T m of the central collagenous domain is unusually high, which in the light of this domain being invariant in terms of alternative splicing suggests that the central portion of the molecule may have an important role in the stability of the molecule. All in all, most of the type XIII collagen ectodomain appears to be present in triple helical conformation, which is in clear contrast to the short or highly interrupted triple helical domains of the other known collagenous transmembrane proteins.The collagens are classically divided into two major groups, fibrillar and nonfibrillar, depending on their structural and functional characteristics, and the heterogeneous group of nonfibrillar collagens can be further divided into several subgroups (1, 2). Type XIII collagen belongs to the group of membraneassociated collagens, together with the hemidesmosomal component type XVII collagen. These two nonfibrillar collagen types are not structurally homologous except that both have been predicted to have a transmembrane domain near their N terminus. Immunoprecipitation of biotinylated type XIII collagen from surface-labeled HT-1080 cells, subcellular fractionation, and immunofluorescence staining have been used to demonstrate that type XIII collagen molecules are indeed located in the plasma membranes of these cells (3). Type XIII collagen molecules are presumed to reside in the plasma membrane in a "type II" orientation with a cDNA-derived predicted structure consisting of a short N-terminal cytosolic domain, a single transmembrane domain, and a large, mainly collagenous ectodomain. The N-terminal noncollagenous domain, NC1, 1 of the human ␣1(XIII) chain encompasses a 38-residue cytosolic domain, a 23-residue transmembrane domain, and the first 60 residues of the noncollagenous ...

Type XIII collagen is a type II transmembrane protein found at many sites of cell adhesion in tissues. Homologous recombination was used to generate a transgenic mouse line (Col13a1 N/N ) that expresses N-terminally altered type XIII collagen molecules lacking the short cytosolic and transmembrane domains but retaining the large collagenous ectodomain. The mutant molecules were correctly transported to focal adhesions in cultured fibroblasts derived from the Col13a1 N/N mice, but the cells showed decreased adhesion when plated on type IV collagen. These mice were viable and fertile, and in immunofluorescence stainings the mutant protein was located in adhesive tissue structures in the same manner as normal ␣1(XIII) chains. In immunoelectron microscopy of wild-type mice type XIII collagen was detected at the plasma membrane of skeletal muscle cells whereas in the mutant mice the protein was located in the adjacent extracellular matrix. Affected skeletal muscles showed abnormal myofibers with a fuzzy plasma membrane-basement membrane interphase along the muscle fiber and at the myotendinous junctions, disorganized myofilaments, and streaming of z-disks. The findings were progressive and the phenotype was aggravated by exercise. Thus type XIII collagen seems to participate in the linkage between muscle fiber and basement membrane, a function impaired by lack of the cytosolic and transmembrane domains. The collagen superfamily of proteins consists of more than 19 types of collagen and several other proteins with collagen-like domains.1 Type XIII collagen and the hemidesmosomal component type XVII collagen form a subfamily of transmembrane collagens. 2 The genes of human and mouse type XIII collagen are 135 to 138 kb in size, consisting of 42 exons, and they are localized to chromosome 10 in both species.3-5 The encoded type XIII collagen consists of three collagenous domains (COL1 to COL3) separated and flanked by four noncollagenous domains (NC1 to NC4). 6,7 The precursor RNAs that encode type XIII collagen undergo complex alternative splicing, which is predicted to affect the structures of the COL1, NC2, and COL3 domains of the human and mouse chains. 5,6,8 -10 Type XIII collagen produced in insect cells forms ␣1(XIII) homotrimers, and the three collagenous domains fold into a stable triple-helical conformation. 11 The type XIII collagen molecules have been shown to reside on the plasma membranes of cells in a type II orientation with a short N-terminal cytosolic portion, a transmembrane domain, and an extensive collagenous ectodomain.12 Sequences that are important for association of the three ␣1(XIII) chains reside in the Nterminal region, and hence triple helix formation is thought to proceed from the N terminus to the C terminus, in the opposite orientation to that known to occur in the fibrillar collagens.12 The extracellular ligands of type XIII collagen have not been identified, but recent studies with recombinant protein demonstrate that its ectodomain interacts strongly with the I domain of ␣1 integrin. ...

Type XIII collagen is a short chain collagen which has recently been shown to be a transmembrane protein. The purpose of this study was to elucidate the presence and localization of type XIII collagen in normal human skin and cultured keratinocytes. Expression of type XIII collagen was demonstrated in normal human skin and epidermis at the RNA level using reverse transcription followed by polymerase chain reaction and at the protein level using western blotting and indirect immunofluorescence labeling. Immunolabeling of epidermis revealed type XIII collagen both in the cell-cell contact sites and in the dermal-epidermal junction. In cultured keratinocytes type XIII collagen epitopes were detected in focal contacts and in intercellular contacts. The results of this study show very little colocalization of type XIII collagen and desmosomal components at the light microscopic level. Thus, these results suggest that type XIII collagen is unlikely to be a component of desmosomes. Instead, the punctate labeling pattern of type XIII collagen at the cell-cell contact sites and high degree of colocalization with E-cadherin suggests that type XIII collagen is very likely to be closely associated with adherens type junctions, and may, in fact, be a component of these junctions.