Abstract
Collagens, or more precisely collagen-based extracellular matrices, are often considered as a metazoan hallmark. Among the collagens, fibrillar collagens are present from sponges to humans, and are involved in the formation of the well-known striated fibrils. In this review we discuss the different steps in the evolution of this protein family, from the formation of an ancestral fibrillar collagen gene to the formation of different clades. Genomic data from the choanoflagellate (sister group of Metazoa) Monosiga brevicollis, and from diploblast animals, have suggested that the formation of an ancestral α chain occurred before the metazoan radiation. Phylogenetic studies have suggested an early emergence of the three clades that were first described in mammals. Hence the duplication events leading to the formation of the A, B and C clades occurred before the eumetazoan radiation. Another important event has been the two rounds of “whole genome duplication” leading to the amplification of fibrillar collagen gene numbers, and the importance of this diversification in developmental processes. We will also discuss some other aspects of fibrillar collagen evolution such as the development of the molecular mechanisms involved in the formation of procollagen molecules and of striated fibrils.
Highlights
Collagen, in all its forms, represents the most abundant protein in animals
During the last few years, data arising from sequencing projects have led to a better understand the evolution of the fibrillar collagen genes [30,50], and have highlighted the importance of the chain selection sequence and of the two rounds of genome duplication in the evolution of vertebrate development [53,62,63]
Fibrils are generally heterotypic, their diameters depending on the procollagen types present and their ratio, as well as the N-propeptide maturation of minor procollagens and interactions with other extracellular matrix components [69]
Summary
In all its forms, represents the most abundant protein in animals. The collagens represent a heterogeneous family of extracellular matrix glycoproteins containing at least one triple helical domain and are generally involved in the formation of supramolecular networks [1,2]. Some other metazoan proteins contain a triple helix but are not members of the collagen family. Type IV collagen is one of the major constituents of basement membranes where it forms a threedimensional network It has been characterized in Homoscleromorpha, the only sponge group presenting basement membrane-like structures. In another sponge group devoid of basement membrane structure, the Demospongiae, a short-chain collagen family evolutionarily related to type IV collagens has been described. A prototypal fibrillar procollagen chain consists of an uninterrupted collagenous domain or major triple helix made up of approximately 338 Gly-Xaa-Yaa triplets, this region being flanked by two non-collagenous domains, the N- and the C-propeptides. Chain is related to vertebrate types XXIV and XXVII, but that the N-propeptide of this fibrillar collagen is still unknown [12]
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