The triple helix of collagens - an ancient protein structure that enabled animal multicellularity and tissue evolution

Abstract

The cellular microenvironment, characterized by an extracellular matrix (ECM), played an essential role in the transition from unicellularity to multicellularity in animals (metazoans), and in the subsequent evolution of diverse animal tissues and organs. A major ECM component are members of the collagen superfamily -comprising 28 types in vertebrates - that exist in diverse supramolecular assemblies ranging from networks to fibrils. Each assembly is characterized by a hallmark feature, a protein structure called a triple helix. A current gap in knowledge is understanding the mechanisms of how the triple helix encodes and utilizes information in building scaffolds on the outside of cells. Type IV collagen, recently revealed as the evolutionarily most ancient member of the collagen superfamily, serves as an archetype for a fresh view of fundamental structural features of a triple helix that underlie the diversity of biological activities of collagens. In this Opinion, we argue that the triple helix is a protein structure of fundamental importance in building the extracellular matrix, which enabled animal multicellularity and tissue evolution.

Keywords: Cell biology; Collagen; Evolution; Extracellular matrix; Multicellularity; Triple helix.

Fig. 1.

Collagen IV is the evolutionarily most ancient of the vertebrate collagen superfamily. The collagen superfamily has 28 members in vertebrates, each comprising three of 46 α-chains – the basis of the diverse suprastructures distributed across different tissues – invertebrates have less members. A hallmark feature of all collagens is the triple helix, which is characterized by three intertwined polypeptide chains. A collagen IV-like gene probably first appeared in the last common ancestor (LCA) to filastereans, choanoflagellates and animals. The phylogenetic distribution suggests that collagen IV played a critical role in the transition of unicellular organisms to multicellular animals. FACITs, fibril-associated collagens with interrupted triple helices; MACITs, membrane-associated collagens with interrupted triple helices; Misc., miscellaneous.

Fig. 2.

Building a collagen triple helix and encoding information. (A) The building blocks (polypeptide chains) of the triple helix are left-handed polyproline type-II helices, i.e. non-extensible structures that have structural and mechanical roles in the ECM of plants and some animals. Replacement of every third proline (Pro) residue with a glycine Gly) residue results in increased flexibility. (B) Three left-handed superhelices wind together and pack tightly owing to these Gly residues, thereby forming a right-handed triple helix. Once associated, the non-extensibility of the structure is restored, and the combination of three chains results in three binding modes (one, two or three chains) with three levels of variation to these modes, i.e. level 1: 20 variable aa residues, level 2: post-translational modifications (PTMs) and, level 3: chain stagger. Together, these specify the numerous possible binding motifs that are positioned along the length of the triple helix of any of the 28 collagen types to bind various macromolecules.

Fig. 3.

The triple helix of collagen IV scaffolds encodes information for the tethering of macromolecules. (A) Assembly begins with collagen IV monomers (α-chains) transcribed and assembled into protomers inside of many cell types. Protomers are composed of three intertwining α-chains that form a triple helix. On the outside of cells, the NC1- and 7S-domains direct the assembly of protomers into network structures of higher order crosslinked by sulfilimine and lysyl oxidase-like protein 2 (LOXL2). The higher Cl⁻ concentration of the ECM induces NC1-domain-directed oligomerization of protomers that form ‘smart’ scaffolds. (B) Once assembled, collagen IV networks function as smart scaffolds, bestowing BMs that underlie and surround cells of several capabilities. Vast amounts of structural information is encoded in motifs located at specific sites along the surface of the triple helix to tether macromolecules. The mode of variation is based on 20 variable aa residues on a single chain, or the combination of one, two or three chains, post-translational modifications (PTMs) and chain stagger (see Fig. 2). The tethering at specific sites spatially organizes molecules along the triple helix, resulting in a populated scaffold within the BM that provides tensile strength to tissues, and influences cell behavior, adhesion and migration during tissue development and regeneration. Partly modified, with permission (Protein Science) from Brown et al., 2017.

Fig. 4.

Mutations in the triple helix can result in genetic diseases. (A) A single missense mutation of a glycine to another residue results in disruption of the triple helix's function. (B) Glycine mutations are responsible for a number of genetic diseases involving the kidney, teeth, muscle, joints, cartilage, brain, skin, vasculature, bone, inner ear and eye in humans (see Table 1). (B) Diseases and disorders due to replacement of glycine residue(s) in the triple helix of collagens. The type of collagen is indicated in bold Roman numerals. AVN, avascular necrosis of femoral head; CCDD, congenital cranial dysinnervation disorder; CSVD, cerebral small-vessel disease; HANAC, hereditary angiopathy with nephropathy, aneurysms and muscle cramps; UCMD, Ullrich congenital muscular dystrophy; LCPD, Legg–Calvé–Perthes disease; OSMED, otospondylomegaepiphyseal dysplasia; syn, syndrome.

Fig. 5.

The fundamental importance of a triple helix in enabling animal multicellularity and tissue evolution. (A) The unique structure and vast encoding properties of the collagen triple helix outside the cell (left) evokes an analogy to the DNA double helix inside the cell (right). (B) The triple helix protein structure was present in unicellular organisms and was co-opted in the form of collagen IV, enabling the transition to multicellular animals. The triple helix was also adapted as a key feature of all members of the diverse collagen superfamily in the ECM, enabling tissue evolution. LCA, last common ancestor.