Evolutionary aspects of IF proteins

Abstract

Abstract Extensive work on cytoplasmic intermediate filament (IF) proteins of vertebrates has established a very complex multigene family with close to 50 members in a mammal. These proteins include IF types I to IV as well as additional species. Variability arises primarily from the N- and C-terminal head and tail domains but also from the central α-helical rod domain, which, except for its ends, preserves sequence principles rather than actual sequences. In spite of such differences the helix 1B subdomain keeps a constant length in all cytoplasmic IF proteins so far characterized from vertebrates. In contrast, in nuclear lamins, a special subtype (type V) of IF proteins, from both vertebrates and invertebrates, the length of helix 1B is increased by six heptads or 42 residues. Recent evidence shows that the long helix 1B form is also characteristic of the cytoplasmic IF proteins of various protostomic animals.H Thus in metazoan evolution the long helix 1B form was changed by a deletion to the short helix 1B version prior to the diversification into types I to IV. Current results suggest that this deletion occurred either with the deuterostomic branch or prior to the chordates. Sequence comparisons further suggest that cytoplasmic IF proteins arose possibly from a mutated lamin early in evolution.