Fibrillins constitute a family of large extracellular proteins that form the core of highly ordered extended and ubiquitously distributed aggregates, termed microfibrils 
. Evolutionarily, fibrillins and microfibrils are widely distributed from cnidarians to mammals 
. Microfibrils confer structural integrity to individual organ systems and regulate the bioavailability of growth factors of the TGF-β superfamily, including TGF-β and bone morphogenetic proteins 
. In elastin-expressing vertebrate organisms, microfibrils are believed to provide a scaffold for elastogenesis in blood vessels, lung, skin and other elastic tissues 
. A number of hereditary connective tissue disorders in humans, including Marfan syndrome, dominant Weill-Marchesani syndrome, geleophysic and acromicric dysplasia, stiff skin syndrome and others are associated with mutations in fibrillin-1, whereas mutations in fibrillin-2 lead to congenital contractural arachnodactyly 
Most of the important structural and functional properties in fibrillins were identified and described with fibrillin nucleotide and protein sequences from mammalian organisms, in particular humans. Thus, the following introductory description of fibrillin properties refers primarily to information available on human fibrillins. The fibrillin family consists of three homologous isoforms, fibrillin-1, -2 and -3. Fibrillins are composed of a typical sequence of individual domains containing between 40–80 amino acid residues (). This characteristic domain signature is 100% conserved between all three fibrillins of mammalian organisms with the exception of an alternatively spliced domain at the N-terminus of fibrillin-3 
. Almost all of the fibrillin domains are characterized by a typical number of cysteine residues ranging between 6–9 cysteine residues per domain. Generally, fibrillins are the extracellular proteins with the highest content of cysteine residues (12–13%) 
Overview of the domain structure of the human fibrillin family.
The most prominent domain in fibrillins is the epidermal growth factor-like (EGF) domain which is present 46–47 times in fibrillins. This domain contains six characteristic cysteine residues which form three stabilizing disulfide bonds in a 1–3, 2–4, 5–6 arrangement 
. The majority of the EGF-like domains contain a consensus sequence ((D/N)X(D/N)(E/Q)Xm
(Y/F)) for calcium-binding (cb) at their N-terminus where m
are variable numbers of amino acid residues, and the asterisk indicates a potential hydroxylation site 
. Homologous EGF and cbEGF domains are found in numerous extracellular matrix, cell surface and blood proteins throughout all metazoan organisms 
Tandem arrays of EGF and cbEGF domains in fibrillins separate two other types of domains, the transforming growth factor (TGF)-β binding protein domains (TB) and the hybrid domains. These two domains are unique to fibrillins and to the latent TGF-β binding proteins (LTBPs) 
. The TB domains occur seven times in fibrillins and are characterized by eight cysteine residues that form four stabilizing disulfide bonds organized in a 1–3, 2–6, 4–7, 5–8 pattern 
. Three of these cysteine residues (Cys3, Cys4, Cys5) are present in a characteristic triple Cys-Cys-Cys motif. Sequence and structural analysis demonstrated that the two hybrid domains present in fibrillins have phylogenetically evolved by fusion of the N-terminus of a TB domain with the C-terminus of a cbEGF domain 
. Hybrid domains are characterized by a typical Cys-Cys repeat and a 1–3, 2–5, 4–6, 7–8 disulfide-bond pattern 
. The first hybrid domain in mammalian fibrillins contains an extra cysteine inserted between Cys2 and Cys3. This unpaired cysteine residue were shown to be surface-exposed, suggesting a role in intermolecular disulfide-bond formation 
Fibrillins can be structurally distinguished by a characteristic domain without cysteine residues immediately following the first TB domain. This unique domain is rich in proline residues in fibrillin-1, rich in glycine residues in fibrillin-2 and rich in both proline and glycine residues in fibrillin-3 
. For fibrillin-1, the proline-rich region has been suggested as a hinge region facilitating a folding mechanism of the fibrillin molecule in microfibrils 
. The proline-rich region in fibrillin-1 and possibly the glycine-rich region in fibrillin-2 are involved in the interaction with tropoelastin 
. The fibrillin N-terminal domain contains four cysteine residues and the C-terminal domain contains 2 cysteine residues. Both terminal domains also contain a tribasic consensus sequence (Arg-Xaa-(Lys/Arg)-Arg) that are recognition signals for proprotein convertases of the Furin/PACE type 
. Both the N- and the C-terminal domains are proteolytically processed at these sites 
. It has been shown for fibrillin-1 that only the processed form becomes incorporated into the extracellular matrix, suggesting that profibrillin-1 processing within its terminal domains plays a regulatory role for its assembly into microfibrils 
. All three fibrillins in mammalian organisms contain one (fibrillin-1) or two (fibrillin-2 and -3) Arg-Gly-Asp (RGD) sites. Common to all three mammalian fibrillins is a RGD site present in the TB4 domain at the tip of an extensible loop 
. This site mediates the interactions with αvβ3, α5β1 and αvβ6 integrins 
. Human fibrillin-2 and fibrillin-3 contain a second RGD site in TB3 and cbEGF18, respectively. It is not known whether these RGD sites represent functional integrin-binding epitopes. Another differential feature between the three fibrillins is the number and position of predicted N-linked glycosylation sites (see ).
Recently, a global evolutionary analysis of TB domain-containing proteins highlighted that the characteristic fibrillin domain organization emerged over 600 million years ago prior to the divergence of cnidarians and bilaterians and before LTBPs emerged 
. This study further described the unpaired cysteine residue in the first hybrid domain, as well as the CXXC motif in the C-terminus as absolutely conserved between species, while the RGD site in TB4 is not conserved outside vertebrates.
In the present study we have retrieved and extensively curated 78 fibrillin gene and protein sequences from 39 organisms ranging from cnidarians to mammals. We have developed evolutionary trees suggesting that a single ancestral fibrillin gene still present in invertebrates and jawless fish (agnathans) underwent duplication. One of the resulting genes evolved to become fibrillin-1, while the other underwent evolutionary changes, including a second duplication, to produce fibrillin-2 and fibrillin-3. We have further analyzed the evolution of critical functional motifs and domains in fibrillins.