Intermediate Filaments: Problems and Perspectives

Abstract

Even though intermediate filaments (IF) appear morphologically similar in the cytoplasm of cells from different tissues, they show a considerable diversity with regard to their composition (Zackroff et al. 1981; Steinert et al., 1984; Steinert & Roop, 1988). The IF proteins are classified into several types: acidic keratins (type I); neutral-basic keratins (type II); desmin, vimentin, glial fibrillary acidic protein, and a newly described 57kD neuron-specific protein (Parysek & Goldman, 1987; type III); and neurofilament triplet proteins (type IV). Recently, the nuclear lamins were also recognized as members of the IF protein family (type V; Fisher et al., 1986; McKeon et al., 1986; Steinert & Roop, 1988). The most prominent structural feature, common to all IF protein molecules, is a conserved central ‘rod’ domain, greatly enriched in alpha helix (Steinert et al., 1984; Steinert & Roop, 1988). In contrast, the amino and carboxy terminal domains of these proteins are variable with respect to both sequence and size. In Types I–IV IF proteins, the central domain contains three short non-alpha helical stretches termed L1, L1–2, and L2 (Steinert et al., 1984; Steinert & Roop, 1988), while in the case of the nuclear lamins the alpha helical region is longer and relatively uninterrupted (Steinert et al., 1984; Steinert & Roop, 1988).