THE PREDICTED STRUCTURE OF CHICK LENS CP49 AND A VARIANT THEREOF, CP49(INS) THE FIRST VERTEBRATE CYTOPLASMIC INTERMEDIATE FILAMENT PROTEIN WITH A LAMIN-LIKE INSERTION IN HELIX-1B

The full length cDNA sequence for the lens-specific intermediate filam ent protein, CP49, from chicken is presented. The sequence contains fe atures typical of the other intermediate filament proteins, including two major alpha-helical regions, helix I and II and appropriate linker regions. CP49 lacks a C-terminal non-alpha-helical domain and is only the second intermediate filament protein to be described missing this feature. Comparison to the bovine CP49 shows significant homology in all domains except the N-terminal non-alpha-helical domain. Besides bo vine CP49, the other protein most homologous to chicken CP49 in the da tabase was keratin 18, a type I keratin. A variant of CP49 is also des cribed, called CP49(ins). Of the 61 positive clones identified in the library, two encoded CP49(ins), one of these being a full-length clone . The sequence differed to CP49 by the insertion of 49 amino acids in helix IB. This is the first chordate cytoplasmic intermediate filament protein sequence to be identified with an archetypal lamin-like inser tion in this helical subdomain and represents a key discovery in traci ng the evolutionary pathway of intermediate filament protein family.