Analysis of the cDNA and gene encoding a cytoplasmic intermediate filament (IF) protein from the cephalochordate Branchiostoma lanceolatum; implications for the evolution of the IF protein family.

We report the molecular cloning of a full-length cDNA encoding a non-neuronal cytoplasmic intermediate filament (IF) protein of the cephalochordate Branchiostoma lanceolatum. Sequence and structural characteristics of IF-1 reveal a close relation to vertebrate IF proteins: they all lack the extended coil 1b version and the lamin tail homology found in protostomic IF proteins. This implies that divergence of type I to IV IF genes from a common ancestor either coincided with the origin of chordates or occurred at an earlier stage in the evolution of deuterostomes. The structural organization of the cephalochordate gene shows a closer relation to vertebrate type III genes than to type I or II genes. The single gene (approximately 19 kb) is composed of 7 exons and 6 introns which are all located within the sequence encoding the rod domain. The positions and phases of the introns show perfect homology to vertebrate type III genes. In line with the absence of protein sequence similarity of the tail domain, the Branchiostoma gene does not possess the introns interrupting this region in type III genes of vertebrates.