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Fibrillin-1 is a protein that is encoded in human bodies by the gene FBN1 situated on chromosome 15. Fibrillin-1 is a single protein chain of 230kb involving 65 exons from the class of glycoproteins with a mass of 350kDa. The protein forms microfibrils located in the extracellular matrix, and thus has a role in the structural support of cells in elastic and nonelastic connective tissues in the human body. ^[1]

Structure

3D model represents these parts of fibrillin-1: , and .

The protein fibrillin-1 contains 59 subunits either called epidermal growth factor-like domain (EGF), or transforming growth factor β binding protein-like domain (8 TGF-bp). EGFs are repeated in tandem along with the whole protein which represents about 75% of the total Fibrillin-1 length, and they are interrupted by the insertion of the TGF-bp units, which contain 8 cysteines each which form . In total, there are 47 motifs of EGF in one Fibrillin-1, but only 43 of them contain calcium-binding sequences. In consequence, these EGF are named cb-EGF for their ability to bind calcium cations. Each EGF or cb-EGF unit contains 6 residues of cysteine which form (CYS1-CYS3, CYS2-CYS4, CYS5-CYS6) stabilizing the secondary structure of the protein. Cb-EGF units contain also a composed especially of amino acids that contain an oxygen atom, or groups with an azote in their lateral chains (aspartic and glutamic acids, serine, asparagine and glutamine). These amino acids stabilize the calcium cation by interactions between positively charged cation and hetero-atoms (oxygen or azote) of the amino acid's lateral chain. Consequently, a pentagonal bipyramidal binding site is created in which one calcium cation is bound in every cb-EGF subunit of the fibrillin-1 protein. ^[2]

Biological Function

Fibrillin-1 is a ubiquitous protein mostly expressed in muscles in its monomeric form. The monomers then polymerize to form the 10 to 12nm of diameter microfibrils. In the microfibrils the fibrillin-1 is associated to various proteins such as MAGP-1, MAGP-2, fibulin 2 and fibulin 5, elastin, versicane and LTBP-1. Those microfibrils constitute the elastic and non-elastic human connective tissues such as the dermis or the organs. This protein plays an important role in the cytokine and growth factor regulation. For example, fibrillin-1 can modulate the bioavailability of TGFβ1, which is a cytokine that regulates cell survival. Changed TGFβ signaling is a significant factor in the development of certain diseases. A fibrillin-1 segment encoded by exons 44-49 triggers the release of TGFβ1 and consequently stimulates TGFβ receptor-mediated Smad2 signaling. Thereby, specific gene activation or repression can be induced. ^{[3] [4]}

Diseases caused by mutation

The Marfan syndrome (MFS) is a genetic disorder due to a mutation of the FBN1 gene. Because Fibrillin-1 is found in connective tissues, having this syndrome can cause severe damages to the ocular, skeletal and cardiovascular systems by affecting the organs’ tissues. Indeed, with fragile connective tissues due to badly synthesized microfibrils, the aorta can be deformed and disrupted which can induce internal bleeding, and lead to death. ^[5]

It exists nearly 1 000 different mutations possible in this gene (), but the most common one is a substitution of guanine by thymine at the 1538 nucleotide of the transcript. This type of mutation leads to a non-synonymous amino acid substitution Cys (cysteine) to Phe (phenylalanine) at the 528 position on the Fibrillin-1 gene. Because this cysteine is present in the calcium-binding domain's polypeptide chain, the epidermal growth factor-like domain's structure of FBN1 is modified by affecting the . The calcium cation cannot bind properly to the and therefore there is no stabilization of cb-EGF interdomain which causes defects in connective tissue. We can thus detect the Marfan syndrome by an increase of TGF-bp in the blood because the factors cannot bind to the protein due to a change in the binding domain's structure. ^[6]

Other diseases can occur by the substitution of other cysteines of the FBN1 transcript such as C1, C2, C3, or C4. But the consequences of these mutations are much more severe. It shows the importance of cysteine localization for the protein structure. Also, a mutation of the TGFBR2 gene coding for the TGF-bp has been found and can cause the "Type 2 Marfan syndrome". However, not much has been discovered on the subject yet. ^{[7] [8]}