In 1992 the cDNA sequence of COL16A1 has been discovered in a screening for collagen-like sequences in cDNA banks of a human fibroblast cell line and human placenta tissue. Two laboratories published independently the human COL16A1 cDNA sequence (Pan et al., 1992; Yamaguchi et al., 1992). The coding sequence of authentic collagen type XVI comprises of 1604 amino acids including a 21 amino acid signal peptide, whereas the recombinant version of collagen type XVI contains 1597 amino acids (Kassner et al., 2004). The nucleotide sequences published by Pan et al. and Yamaguchi et al. were completed by Kassner et al. with respect to a missing codon for one amino acid (Kassner et al., 2004). Two predicted imperfections in the collagenous region could not be confirmed (unpublished data).

The cDNA of 5,4 kb comprises a 4809 bp coding sequence, framed by non-translated parts, including a 425 bp 3-non-coding sequence which contains polyadenylating signals. COL16A1 has been localized to chromosome 1, 1p35-p34 (Pan et al., 1992). No splice variants have been described up to now. COL16A1 gene expression and transcription varies in various phases of cell growth in cultured skin fibroblasts. Gene expression was increased in stationary phases (G0/G1) of cell growth when cell proliferation was inhibited by serum deprivation or suspension arrest (Tajima et al., 2000). Transcription activity of the COL16A1 gene appears to be mechanosensitive. It is downregulated in HCS2/8 human chondrosarcoma cells after application of continuous hydrostatic pressure (Sironen et al., 2002). Recently, COL16A1 is described to act as a Smad-signalling specific target gene for BMP-4/BMP-6 in hepatocellular carcinoma (Maegdefrau and Bosserhoff, 2012).

No pseudogenes are described up to now.

Collagen type XVI, by structural analogy a member of the FACIT - (fibril-associated collagens with interrupted triple helices) family of collagens, contains 10 collagenous (COL) domains interspersed with 11 non-collagenous (NC) regions (figure 1A-C). It is a homotrimeric molecule of about 210 kDa for each native α1 chain. 32 cysteine residues, which are almost all located in the non-collagenous domains at the junction to the preceding collagenous regions, contribute to a high thermal stability of the homotrimer in form of disulfide bonds (Pan et al., 1992; Yamaguchi et al., 1992). The prominent non-collagenous NC11 domain consists mainly of a 200-residue motif referred to as proline-arginine-rich protein (PARP) in several other collagen types or as tsp-1 in thrombospondin (figure 1A). It has been recombinantly expressed and used for generation of specific anti-NC11 antibodies (Tillet et al., 1995). 24% of the total number of proline residues and 48% of the total number of lysine residues were hydroxylated in recombinant collagen type XVI. Because only lysine and proline in the Y position of X-Y-Gly amino acid triplets and additionally some lysine residues in the Y position of X-Y-Ser and X-Y-Ala in collagens are subject to hydroxylation, the amino acid sequence of authentic human collagen type XVI would imply that 54% of the available prolines and a maximum of 92% of the available lysines could potentially be hydroxylated in recombinant collagen type XVI. Two of the three potential N-glycosylation sites reside in the N-terminal NC11 region and are glycosylated. The third has been assigned to the NC1 domain whereas here, no evidence has been found for the attachment of a glycosaminoglycan chain (Kassner et al., 2004).
Atomic force images of individual trimeric molecules exhibited a total length of 168 ± 3 nm including the N-terminal NC11 globular domains and the flexible threadlike tail comprising all collagenous regions plus the remaining non-collagenous domains. The height of the NC11 domain constituted 0,94 ± 0.06 nm and the radius at half height (r0.5h) was calculated as 9,48 ± 0,47 nm. The threadlike section of the remaining molecule C-terminal of the NC11 domain appeared to be a thin flat structure with a height between 0,5 and 0,7 nm. Notably, at a distance of 94.8 ± 4.6 nm from the NC11 terminus the molecular measurements increased either in height or diameter. The extension of this section contributed with 73,1 ± 1,6 nm to the total length of the protein (figure 1D). Rotary shadowing images of purified recombinant collagen type XVI exhibited extended rod-like molecules with a globular domain at one end, probably constituting the large N-terminal NC11 domain (figure 1E). The shape of the molecules revealed highly flexible regions, in some molecules even two or more kinks. The length of the molecules varied between 100-240 nm, with the majority being close to 150 nm.
The N-terminal half of human fibrillin-1/-2 binds dose-dependent to collagen type XVI at low salt concentrations of 50-100 mM NaCl, while interaction between collagen type XVI and the C-terminal half of fibrillin-1/-2 under these conditions were considerably lower. These results indicate that monomeric fibrillin-1/-2 can interact with collagen type XVI with low affinity. Both, fibrillin-1 and fibrillin-2 did not interact with the recombinant NC11 domain. Soluble recombinant fibronectin interacted strongly with collagen type XVI at 150 mM NaCl with half maximal binding at ~12 μg/ml (~55 nM fibronectin), indicating that fibronectin can bind to collagen XVI with high affinity (Kassner et al., 2004).
Collagen XVI co-localizes with α2 integrin at the dermal epidermal junction (DEJ) (figure 2 A-C) and with α1 integrin and around fat cells in subdermal layers (figure 2 D-F). Cells bearing the integrins α1β1 and α2β1 attach and spread on recombinant collagen type XVI. Collagen type XVI induces the recruitment of these integrins into focal adhesion plaques, a principal step in integrin signaling. In cell-free binding assays, collagen type XVI is more avidly bound by α1β1 integrin than by α2β1 integrin. Both integrins interact with collagen type XVI via the A-domain of their α-subunits. A tryptic collagen type XVI fragment comprising the collagenous domains 1-3 is recognized by α1β1 integrin. Electron microscopy of complexes of α1β1 integrin with this tryptic collagen XVI fragment or with full-length collagen type XVI revealed a unique α1β1 integrin binding site within collagen type XVI located in the COL 1-3 domains (Eble et al., 2006).

Collagen type XVI is expressed in various cells and tissues. It is synthezised by dermal fibroblasts, smooth muscle cells (Grässel et al., 1996), dermal dendrocytes and dendritic cells in the skin (Akagi et al., 2002), articular and costal chondrocytes (Kassner et al., 2003), endometrial stromal cells (Tierney et al., 2003), basal dermal and oral keratinocytes (Grässel et al., 1999), bone marrow derived mesenchymal stem cells (Grässel et al., 2009), neurons from the dorsal root ganglion (Hubert et al., 2007), glioblastoma/astrocytoma cells (Senner et al., 2008) and intestinal myofibroblasts (Ratzinger et al., 2010). Collagen type XVI is further expressed in the limbal stem/progenitor niche which comprises clusters of cells in the basal epithelium. There it is associated with the corneal-limbal transition zone (Schlötzer-Schrehardt et al., 2007). During early mouse development, collagen type XVI occurs in many tissues and is co-distributed with the major fibrillar collagens. In particular, it is strongly expressed in differentiating chondrocytes and dermal fibroblasts, smooth muscle cells of the heart and dorsal root neural fibers, whereas in adult mice no signal appears in brain tissue. Additional expression is found in the cortical areas of the kidney and ovaries (Lai and Chu, 1996). In adults it is found in skin, cartilage, gastrointestinal tract and glioma tissues.

Extracellular matrix of tissues. For skin and cartilage tissue its suprastructure is known. It is associated to the fibrillin containing microfibrillar apparatus in the dermal-epidermal junction zone in skin and to collagen II containing D-banded cartilage fibrils in costal cartilage (Grässel et al., 1999; Kassner et al., 2003). It is depositred pericellular around fibroblasts and smooth muscle cells (Grässel et al., 1996) and in the territorial region of chondrocytes (Kassner et al., 2003).

Morphogenesis and assembly of distinct suprastructures in different tissues, i.e. microfibrillar apparatus in the dermis and fibrillar networks in various connective tissues (figure 3A, B). Presumably, it is an adaptor protein such as collagen type IX and connects and organizes large fibrillar networks and thus regulates integrity and stability of ECM (Eble et al., 2006; Grässel et al., 1999; Kassner et al., 2003). It is a substrate for adhesion and invasion of tumor cells, i.e. glioblastomas (Senner et al., 2008) and regeneration of connective tissues after neural injury (Hubert et al., 2007). In addition, it induces invasiveness of glioblastoma by altering the cell-matrix interaction (Bauer et al., 2011) and proliferative activity of oral squamous cell carcinoma by altering the cell cycle activity of the tumor cells (Ratzinger et al., 2011). Notably, COL16A1 is a Smad-signalling specific target gene for BMP-4/-6 in hepatocellular carcinoma where these BMPs are involved in the process of carcinogenesis (Maegdefrau and Bosserhoff, 2012).

Based on conserved structural features with other FACIT-collagens, namely: collagens type IX, type XII, type XIV, type XIX, type XX, type XXI and type XXII. These structural features are: 1. The presence of two highly conserved cysteine residues separated by four amino acids at the NC1-junctions and the existence of two G-X-Y triplet imperfections within the COL2 domain. 2. A succession of triple-helical domains connected by short non-collagenous domains and the presence of a large N-terminal domain that always exhibits a TSPN subdomain next to the collagenous domain. 3. Besides from these common criteria, the FACITs display remarkable divergence in the size and composition of their N-terminal domains and in the number of their collagenous domains (Ricard-Blum and Ruggiero, 2005).

There are no reports about mutations in the COL16A1 gene published as yet.

None yet described.

None yet described.