Chicken filensin: a lens fiber cell protein that exhibits sequence similarity to intermediate filament proteins.

Filensin, a 100 kDa, membrane-associated, cytoskeletal protein, is uniquely expressed in the lens fiber cell (Merdes, A., Brunkener, M., Horstmann, H., and Georgatos, S. D. (1991) J. Cell Biol. 115, 397-410). I cloned and sequenced a full-length chicken lens cDNA encoding filensin, also known as CP95 (Ireland, M. and Maisel, H. (1989) Lens and Eye Toxicity Research 6, 623-638). The deduced amino acid sequence of 657 residues contained an internal 280 residue heptad repeat domain with sequence similarities to the rod domain of intermediate filament proteins. The putative filensin rod domain could be divided into three alpha-helical segments (1A, 1B and 2) separated by short, non-helical linkers. The sequence of the amino-terminal end of the filensin rod domain contained the highly conserved intermediate filament segment 1A motif (Conway, J. F. and Parry, D. A. D. (1988) Int. J. Biol. Macromol. 10, 79-98). Allowing conservative amino acid substitutions, the sequence of the carboxy-terminal end of the filensin rod domain was similar to that of the highly conserved intermediate filament rod carboxy terminus. The alpha-helical segments of the shorter filensin rod domain aligned with the corresponding segments of intermediate filament proteins by allowing a gap of four heptad repeats in the amino-terminal half of filensin segment 2. Filensin rod segment 2 contained the characteristic stutter in heptad repeat phasing, nine heptads from the end of the intermediate filament rod. The overall sequence identity between the rod domains of filensin and individual intermediate filament proteins was 20 to 25%, approximately the level of sequence identity observed between intermediate filament proteins of different types. The open reading frame of chicken filensin predicted a 657 amino acid protein with molecular mass of 76 kDa. Embryonic chicken filensin migrated in SDS-PAGE as a triplet of 102, 105 and 109 kDa, while rooster filensin migrated as a 105 and 109 kDa doublet. Antibodies to filensin labeled lens fiber cells but not lens epithelial cells. By immunofluorescence methods filensin was localized to the fiber cell plasma membranes, including the ends of elongated fiber cells.