THE CHICKEN CP49 GENE CONTAINS AN EXTRA EXON COMPARED TO THE HUMAN CP49 GENE WHICH IDENTIFIES AN IMPORTANT STEP IN THE EVOLUTION OF THE EYELENS INTERMEDIATE FILAMENT PROTEINS

The gene structure for chicken CP49 gene is presented. It differs from the human CP49 gene with the presence of an extra exon in helix IB an d the apparent loss of an intron, intron H. The CP49 gene localises to chromosome 2 in the chicken genome where it is flanked by homologues that map to human chromosome 10p13 (VIM) 6p24-p23 (BMP6). Two transcri pts, CP49 and CP49ins, are produced from the single chicken CP49 gene. The difference is a 49-amino-acid insertion in helix IB of CP49 that is encoded by a novel exon found in the chicken CP49 gene. An extended helix IB is believed to be a characteristic of the ancestral intermed iate filament protein as it is found in many invertebrate intermediate filament proteins but has been lost from all vertebrate intermediate filament proteins except the nuclear lamins. Although the intron posit ion and length of the helix IB insert sequences in CP49ins differ to t hose found both in the invertebrate intermediate filament proteins and the vertebrate lamins, the CP49 gene is the first vertebrate cytoplas mic intermediate filament protein to be described with an extended hel ix IB. The chicken CP49 gene is also the first where differential spli cing can remove such a feature. Human and bovine CP49 appear to have l ost the helix IB insert sequences, and so the avian CP49 gene provides an interesting evolutionary link between the eye lens proteins and th e ancestral intermediate filament protein. (C) 1998 Elsevier Science B .V. All rights reserved.