ANALYSIS OF 8 CDNA AND 6 GENES FOR INTERMEDIATE FILAMENT (IF) PROTEINS IN THE CEPHALOCHORDATE BRANCHIOSTOMA REVEALS DIFFERENCES IN THE IF MULTIGENE FAMILIES OF LOWER CHORDATES AND THE VERTEBRATES

We report the sequences of seven new cytoplasmic intermediate filament (IF) proteins of the cephalochordate Branchiostoma. The eight sequenc es currently known describe four subfamilies (A, B, C and D). All eigh t IF proteins show the short-length version of the coil 1b subdomain f ound in vertebrates and lack the additional 42 residues present in all nuclear lamins and the protostomic IF proteins. Although the lancelet is considered to be the closest relative of the vertebrates, it is di fficult to relate its IF subfamilies unambiguously to a particular typ e I-IV subfamily of vertebrates. C1 and C2 have tail domains with two 64 residue repeats of coiled coil-forming ability, a structural featur e unknown for IF proteins from vertebrates or protostomia. The epiderm al protein D1 shows only a slightly better identity score with vertebr ate type II keratins than with type III proteins, but the D1 gene orga nization is that of type III proteins. The same holds for A1, A2, B1, B2 and C2 genes, although the latter has an additional and uniquely po sitioned intron. Antibodies (Ab) raised against recombinant C2 and DI proteins reveal these proteins in epidermis, some internal epithelia a nd parts of the spinal cord. The results on exonic sequences, gene org anization and expression suggest that Branchiostoma IF proteins may re tain a largely archetypal condition, whereas the vertebrates have esta blished the well-known type I-IV IF system. (C) 1998 Elsevier Science B.V. All rights reserved.