Structure and function of mammalian fibrillin genes
The fibrillins are large extracellular matrix proteins. They are composed primarily of repeating 6-cysteine calcium binding domains, interspersed with 8-cysteine domains homologous to the latent transforming growth factor (TGF) binding proteins. The fibrillins are involved in regulation of active TGF β within the matrix. We have used bioinformatics and molecular studies to examine the fibrillin genes of mammals. The mouse has two fibrillin genes while the human has three. Homologues were found in all mammalian species accessible in the Ensembl data base (http://www.ensembl.org): rat, bovine, chimpanzee, dog and possum. Within a species, the different fibrillin genes show high similarity at the protein and nucleotide level, while between species the genes are conserved at the protein, nucleotide and gene structure level. Expression of the fibrillins is developmentally and spatially regulated, and analysis of microarray results shows that the expression pattern is common to a range of extracellular matrix proteins associated with bone, adipose and vascular smooth muscle cells. Analysis of transcript data (http://fantom3.gsc.riken.jp) shows that the fibrillin genes are controlled by CpG island promoters with diffuse start sites located up to 500 bases 5’ of the initial ATG codon. Both Fbn1 and Fbn2 in mouse have two distinct transcription start site clusters separated by about 200 nucleotides. The human orthologues (FBN1 and FBN2) have a single 5’ promoter region. FBN3 has a low level of expression, and only one promoter region could be detected. A striking level of conservation was observed. For example, transcript start sites were found in FBN1 within a block of 129 bases. 103 of these bases were identical in human, chimpanzee, mouse, rat, bovine and possum, giving nucleotide conservation of 80%. These promoter regions have been cloned into reporter constructs to examine their activity in mammalian cell lines and assess important regulatory molecules.