Comparison of mouse and human keratin 18: a component of intermediate filaments expressed prior to implantation.

Keratin 18 is a type-I keratin that is found in a variety of simple epithelial tissues. In mice, the corresponding protein, called Endo B, is expressed at the 4- to 8-cell stage of mouse development and may be one of the first intermediate-filament proteins synthesized after fertilization. A cDNA clone for keratin 18, designated pK18, was isolated from a human placental cDNA library by hybridization with the mouse Endo-B probe. It was characterized by hybridization selection of RNA, translation, immunoprecipitation, Northern blotting, and sequence analysis. Synthetic T7 polymerase transcripts of the cDNA were indistinguishable in size from keratin-18 mRNA, suggesting that pK18 represents a full-length copy of the RNA. The cDNA insert is 1,428 nucleotides long and contains a single open reading frame of 1,342 nucleotides coding for 429 amino acids. The deduced amino acid sequence is 89.7% identical with that of Endo B. The only extensive difference between the two sequences is due to 9 additional amino acids being present in the last half of the N-terminal domain of keratin 18. The 38-nucleotide-long 3' noncoding region of the cDNA is 75% identical with the corresponding portion of Endo B. The 5' noncoding regions are 59% identical. The expression of keratin-18 mRNA was found to vary more than tenfold when HeLa cells and BeWo trophoblastic cells were compared.