Filarin, a novel invertebrate intermediate filament protein present in axons and perikarya of developing and mature leech neurons.

The lan 3-8 monoclonal antibody recognizes a 63 kD antigen that is associated with the cytoskeleton in leech neurons. We have used this antibody to clone a novel invertebrate neuronal intermediate filament protein, filarin, by screening an expression vector library. A full-length clone of 2.2 kb identified by the antibody was isolated and sequenced. The protein contains a coiled-coil rod domain typical of the superfamily of intermediate filament proteins flanked by unique N- and C-terminal domains. The highest homology of filarin is to the alternatively spliced squid brain intermediate filament protein (Szaro et al., 1991, J. Biol. Chem. 266:15035-15041), the only other invertebrate neuronal intermediate filament in the data bank. However, apart from extensive homology in the two end regions of the rod domain, the similarity of the two proteins is limited to the general coiled-coil structure of intermediate filaments. Thus, filarin may represent a novel type of invertebrate neuronal intermediate filament protein. Filarin contains the extra 6 heptads characteristic of lamins and of all cytoplasmic invertebrate intermediate filaments analyzed so far. By Northern analysis, it appears that filarin is not alternatively spliced, since only a single transcript of 2.2 kb is recognized by the clone. Using the lan 3-8 antibody to follow its developmental expression, we found that filarin is present in all known neurons in the central and peripheral nervous system.