Collagens at a glance

Abstract

Collagens are a large family of triple helical proteins that are widespread throughout the body and are important for a broad range of functions, including tissue scaffolding, cell adhesion, cell migration, cancer, angiogenesis, tissue morphogenesis and tissue repair. Collagen is best known as the principal tensile element of vertebrate tissues such as tendon, cartilage, bone and skin, where it occurs in the extracellular matrix as elongated fibrils. Collagen is also well known for its location in basement membranes – for example, in the kidney glomerulus, where it functions in molecular filtration. However, the identification of transmembrane collagens on the surfaces of a wide variety of cells and collagens that are precursors of bioactive peptides that have paracrine functions has resulted in a revival of interest in collagen. Moreover, new developments in 3D reconstruction electron microscopy have led to new opportunities for studying intracellular trafficking of collagen. Newcomers to the field face the daunting task of sifting through 100,000 research papers that span 40 years. Here, we provide `the collagen basics'. Several excellent reviews are cited that are sources of more detailed descriptions and discussions.Collagens contain three polypeptide (α) chains, displaying an extended polyproline-II conformation, a right-handed supercoil and a one-residue stagger between adjacent chains (Brodsky and Persikov, 2005). Each polypeptide chain has a repeating Gly-X-Y triplet in which glycyl residues occupy every third position and the X and Y positions are frequently occupied by proline and 4-hydroxyproline, respectively. The three α chains are held together by interchain hydrogen bonds. Highly ordered hydration networks surround the triple helices. The significance of these interactions to collagen stability remains a matter of debate. Some collagens have interruptions (containing numerous residues) and imperfections (one to three residues) in the triple helix. The conformational changes derived from some simple imperfections have been visualised in crystal structures of model peptides (Bella et al., 2006). Integrin adhesion sequences [e.g. Arg-Gly-Asp (RGD) and Gly-Phe-O-Gly-Arg, where O is hydroxyproline] occur in the triple helical domain of several collagens and contribute to integrin-ligand-binding specificity (Bella and Humphries, 2005).There is no agreed definition for a collagen; there are triple helical proteins that are called collagens and there are proteins that have triple helical domains that are not regarded as collagens. In general, collagens are regarded as triple helical proteins that have functions in tissue assembly or maintenance. Inevitably, the line between `collagens' and `collagen-like' proteins is blurred. Vertebrate collagens are given a Roman numeral.Collagen I is the archetypal collagen in that it is trimeric, it is triple helical, its triple helix has no imperfections, it assembles into fibrils, and it has a predominately structural role in the tissue. However, most collagens differ from collagen I in one or more respects. For example, other collagens can have interruptions in the triple helix and do not necessarily assemble (in their own right) into fibrils. Furthermore, transmembrane collagens have numerous interruptions in the triple helix, do not self-assemble into fibrils, and have roles in cell adhesion and signaling.At least 28 different collagens occur in vertebrates (numbered I-XXVIII; some with common names), together with a large group of collagen-like proteins (e.g. acetyl cholinesterase, adiponectin, C1q, ficolin, macrophage receptor and surfactant protein) (for a review, see Myllyharju and Kivirikko, 2001). In general, invertebrates have far fewer collagen genes but most have examples of fibrillar and basement membrane collagen (Huxley-Jones et al., 2007). Most collagens have evolved from orthologues present in invertebrates but little is known about the molecular composition of these molecules and no systematic nomenclature exists. Notably, ∼200 cuticle-forming collagens are found in Caenorhabditis elegans (Page and Winter, 2003).Collagens can be heterotrimeric – for example, type I collagen, which contains two identical α chains and a third chain that differs, [α1(I)]2α2(I). However, the majority of collagens are homotrimers – for example, collagen II, which contains three identical α chains [α1(II)]3. Note that the α1 chain of one type of collagen (e.g. collagen I) has a primary structure different from that of the α1 chain of another type of collagen (e.g. collagen II). Collagens have non-triple helical domains at their N- and C-termini. These domains are called `non collagenous' (NC) domains and are numbered from the C-terminus (NC1, NC2, etc.).Fibril-forming collagens occur as 67-nm D-periodic fibrils that are the principal source of tensile strength in animal tissues (Kadler et al., 1996). The fibrils are indeterminate in length and range in diameter from