We have determined the sequence of cloned cDNAs derived from a 1,66S-nucleotide mRNA which transiently accumulates during Xenopus laevis embryogenesis. Computer analysis of the deduced amino acid sequence revealed that this mRNA encodes a 47-kilodalton type I intermediate filament subunit, i.e., a cytokeratin. As is common to all intermediate filament subunits so far examined, the predicted polypeptide, named XK70, contains N-and C-terminal domains flanking a central cc-helical rod domain. The overall amino acid homology between XK70 and a human 50-kilodalton type I keratin is 47%; homology within the a-helical domain is 57%. The N-terminal domain, which is not completely contained in our cDNAs, is basic, contains 42% serine plus alanine, and includes five copies of a six-amino-acid repeating unit. The C-terminal domain has a high a-helical content and contains a region with sequence homology to the C-terminal domains of other type I and type Ill intermediate filament proteins. We suggest that different keratin flament subtypes may have different functional roles during amphibian oogenesis and embryogenesis.The cytoskeleton of vertebrate cells consists largely of actin microfilaments, intermediate filaments (IFs), and microtubules. The 7-to 15-nm-diameter IFs are divided into five major classes based on their differing subunit composition and cell type specificity: (i) keratin (or cytokeratin) filaments, present in epithelial cells; (ii) desmin filaments, found predominately in myogenic cells; (iii) vimentin filaments, present in mesenchymally derived cells; (iv) neurofilaments, found in neuronal cells; and (v) glial filaments, present in glial cells (26; P. M. Steinert and D. A. D. Parry, Annu. Rev. Cell Biol., in press). Analysis of amino acid sequence, X-ray diffraction, and electron microscopy data have determined that all IF subunits have partial sequence homology and a similar secondary structure. From sequence data comparisons, the IF subunit family has been classified into four types of related polypeptides: type I and II keratins, type III subunits such as desmin and vimentin, and type IV neurofilament subunits (Steinert and Parry, in press). Despite substantial sequence diversity, the basic structure of all IF proteins is similar and consists of a central at-helical coiled-coil rod domain flanked by terminal domains of variable length that are generally not a-helical (8, 15; Steinert and Parry, in press).The 40-to 70-kilodalton (kDa) cytokeratins, the subunits of keratin filaments, constitute a complex family of related polypeptides which are differentially expressed in various epithelial tissues (23,25,33) Xenopus laevis polyadenylated [poly(A)+] RNAs which are not stored in the egg, but are initially transcribed during the midblastula to gastrula stages of embryogenesis (47). As an approach to elucidate the role of the proteins synthesized by these differentially expressed gastrula RNAs, a number of individual cDNA clones have been sequenced. We have previously reported that one of these clones, DG8...