Epidermal keratin gene expressed in embryos of Xenopus laevis

DG81 is a cDNA clone derived from a subtracted library containing those RNA molecules that are present in gastrulae but absent from eggs of the frog Xenopus laevis. DG RNAs (where DG indicates differentially expressed in gastrula) represent the products of new transcription activated in the embryo at the midblastula transition or shortly thereafter. DG81 RNA is first detected in middle to late gastrulae, peaks in abundance in early tadpoles, and declines to background levels by the end of metamorphosis. Sequence analysis of an almost full-length cDNA clone homologous to DG81 allows deduction of a protein sequence that shows extensive homology to known intermediate filament proteins, most notably to epidermal type I cytokeratins. Consequently, the protein encoded by DG81 has been named XK81, for Xenopus keratin 81. In concert with keratins analyzed previously, XK81 has a central coiled-coil alpha-helical domain of 312 amino acids, which accounts for most of the homology to other keratins. This rod-like region is flanked by more divergent domains of 73 amino acids at the NH2 terminus and 44 amino acids at the COOH terminus. XK81 provides an example of a cytokeratin whose expression is limited to pre-adult developmental stages. We suggest that XK81 functions specifically in the differentiation of the tadpole epidermis.