The recombinant transmembrane protein type XIII collagen is shown to reside on the plasma membrane of insect cells in a 'type II' orientation. Expressions of deletion constructs showed that sequences important for the association of three alpha1(XIII) chains reside in their N- rather than C-terminal portion. In particular, a deletion of residues 63-83 immediately adjacent to the transmembrane domain abolished the formation of disulfide-bonded trimers. The results imply that nucleation of the type XIII collagen triple helix occurs at the N-terminal region and that triple helix formation proceeds from the N- to the C-terminus, in opposite orientation to that of the fibrillar collagens. Interestingly, a sequence homologous to the deleted residues was found at the same plasma membrane-adjacent location in other collagenous transmembrane proteins, suggesting that it may be a conserved association domain. The type XIII collagen was secreted into insect cell medium in low amounts, but this secretion was markedly enhanced when the cytosolic portion was lacking. The cleavage occurred in the non-collagenous NC1 domain after four arginines and was inhibited by a furin protease inhibitor.

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...

The angiogenesis inhibitor histidine-rich glycoprotein (HRG) constitutes one of several examples of molecules regulating both angiogenesis and hemostasis. The antiangiogenic properties of HRG are mediated via its proteolytically released histidine-and proline-rich (His/Pro-rich) domain.

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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. ...

Expression of TLR9 is increased in prostate cancer specimens, especially in the most poorly differentiated forms.

Transmembrane type XIII collagen resides in adhesive structures of cells and tissues, and has therefore been implicated in cell adhesion and in adhesion-dependent cell functions. This collagen also exists as a soluble protein in the pericellular matrix, as the ectodomain is released from the plasma membrane by proteolytic cleavage. Analysis with various protease inhibitors led to confirmation of the furin family of proprotein convertases as the protease group responsible for the shedding of the ectodomain, cleaving at a site conforming to the consensus sequence for the proprotein convertases at the stem of the ectodomain. Both the trans -Golgi network and the plasma membrane were used as cleavage locations. Mammalian cells employed various intracellular mechanisms to modulate shedding of the ectodomain, all resulting in a similar cleavage event. Cell detachment from the underlying substratum was also found to augment the excision. The released ectodomain rendered the pericellular surroundings less supportive of cell adhesion, migration and proliferation, as seen specifically on a vitronectin substratum. Type XIII collagen ectodomain shedding thus resulted in the formation of a soluble, biologically active molecule, which eventually modulated cell behaviour in a reciprocal and substratum-specific manner. The dual existence of membrane-bound and soluble variants widens our biological understanding of type XIII collagen.