The chordates combine the vertebrates and the invertebrate phyla of the cephalo- and urochordates (tunicates). Two cytoplasmic intermediate filament (IF) proteins of the urochordate Styela plicata are characterized by cDNA cloning, gene organization, tissue specific expression patterns in the adult animal and the self assembly properties of the recombinant proteins. In line with metazoan phylogeny St-A and St-B have the short length version of the coil 1b domain found in all vertebrate and

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... lochordate IF proteins while protostomic IF proteins have the longer length version with an extra 42 residues. St-A is the first IF protein from a lower chordate which can be unambiguously related to a particular vertebrate IF subfamily. St-A shares 46% sequence identity with desmin, displays the N-terminal motif necessary for filament assembly of type III proteins and forms normal homopolymeric 10 nm filaments in vitro. St-A but not St-B is present in smooth muscle cells of the body wall musculature. St-A and St-B are found as separate networks in some interior epithelia. St-B shares 30 to 35% identity with keratin 8, St-A and desmin and does not form IF under in vitro assembly conditions. Its relation to a particular vertebrate IF type or to the eight currently known IF proteins from the cephalochordate Branchiostoma remains unresolved. The striking relation between St-A and desmin predicts that the common progenitor of the urochordate (tunicate) and the cephalochordate/vertebrate lineages already possessed a type III homologue. Unlike in vertebrates intron patterns cannot be used to classify the tunicate IF genes. Although St-A is a type III homologue its gene shows an intron position which in vertebrates is restricted to keratin type II genes.