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Using a cDNA clone from the ovary of the frog, Xenopus laevis, we have identified the mRNA and determined the complete amino acid sequence of a major cytoskeletal protein expressed in the oocyte. A comparison with other cytoskeletal proteins of Xenopus and mammals identifies this polypeptide Mr 55,700 as a nonepidermal kind of cytokeratin of the basic (type II) subfamily, which represents the amphibian equivalent to cytokeratin no. 8 of simple epithelia of higher mammals. The sequence data demonstrate the high evolutionary stability of this protein. This cytokeratin and its mRNA are present in oocytes, eggs, embryos, liver, and intestinal mucosa of adult frogs, as well as cultured kidney epithelial cells. We suggest that epithelial cell differentiation in early stages of Xenopus embryogenesis differs from other known pathways of cell differentiation in that major cell-type-specific proteins--i.e., cytokeratins of the simple epithelial type--and their mRNAs are maternally provided and distributed to early epithelial cells by special sorting mechanisms.

Cloning of cDNA and amino acid sequence of a cytokeratin expressed in oocytes of Xenopus laevis.