The epidermal intermediate filament proteins of tunicates are distant keratins; a polymerisation-competent hetero coiled coil of the Styela D protein and Xenopus keratin 8.
Two novel cytoplasmic intermediate filament (IF) proteins (C and D) from the tunicate (urochordate) Styela are characterised as putative keratin orthologs. The coexpression of C and D in all epidermal cells and the obligatory heteropolymeric IF assembly of the recombinant proteins argue for keratin orthologs, but the sequences do not directly reveal which protein behaves as a keratin I or II ortholog. This problem is solved by the finding that keratin 8, a type II keratin from man or Xenopus, forms chimeric IF when mixed with Styela D. Mutant proteins of Styela D and keratin 8 with a single cysteine in equivalent positions show that these chimeric IF are, like vertebrate keratin filaments, based on the hetero coiled coil. We propose that Styela D retains, in spite of its strong sequence drift, important molecular features of type I keratins. By inference Styela C reflects a type II ortholog. We discuss that type I to III IF proteins are expressed along the chordate branch of metazoa.
Wang
J
J
Max Planck Institute for Biophysical Chemistry, Department of Biochemistry, Goettingen, Germany.
Karabinos
A
A
Schünemann
J
J
Riemer
D
D
Weber
K
K
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D003545:Cysteine / Q000737:chemistry; D018076:DNA, Complementary / Q000378:metabolism; D004220:Disulfides; D004591:Electrophoresis, Polyacrylamide Gel; D004817:Epidermis / Q000737:chemistry*; D015723:Gene Library; D006801:Humans; D015151:Immunoblotting; D007381:Intermediate Filament Proteins / Q000737:chemistry*; D007633:Keratins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D009130:Muscle, Smooth / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014018:Tissue Distribution; D014561:Urochordata / Q000737:chemistry* / Q000235:genetics; D014981:Xenopus
C417962:C protein, Styela clava; C417963:D protein, Styela clava; D018076:DNA, Complementary; D004220:Disulfides; D007381:Intermediate Filament Proteins; D011994:Recombinant Proteins; D007633:Keratins; D003545:Cysteine