BOVINE FILENSIN POSSESSES PRIMARY AND SECONDARY STRUCTURE SIMILARITY TO INTERMEDIATE FILAMENT PROTEINS

The cDNA coding for calf filensin, a membrane-associated protein of th e lens fiber cells, has been cloned and sequenced. The predicted 755-a mino acid-long open reading frame shows primary and secondary structur e similarity to intermediate filament (IF) proteins. Filensin can be d ivided into an NH2-terminal domain (head) of 38 amino acids, a middle domain (rod) of 279 amino acids, and a COOH-terminal domain (tail) of 438 amino acids. The head domain contains a di-arginine/aromatic amino acid motif which is also found in the head domains of various interme diate filament proteins and includes a potential protein kinase A phos phorylation site. By multiple alignment to all known IF protein sequen ces, the filensin rod, which is the shortest among IF proteins, can be subdivided into three subdomains (coils 1a, 1b, and 2). A 29 amino ac id truncation in the coil 2 region accounts for the smaller size of th is domain. The filensin tail contains 6 1/2 tandem repeats which match analogous motifs of mammalian neurofilament M and H proteins. We sugg est that filensin is a novel EF protein which does not conform to any of the previously described classes. Purified filensin fails to form r egular filaments in vitro (Merdes, A., M. Brunkener, H. Horstmann, and S. D. Georgatos. 1991. J. Cell Biol. 115:397410), probably due to the missing segment in the coil 2 region. Participation of filensin in a filamentous network in vivo may be facilitated by an assembly partner.