Collagens at a glance.

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. Structure and composition Collagens contain three polypeptide ( ) chains, displaying an extended polyproline-II conformation, a righthanded supercoil and a one-residue stagger between adjacent chains (Brodsky and Persikov, 2005). Each polypeptide chain has a repeating Gly-XY 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 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 1955 Cell Science at a Glance