Intermediate-size filaments in a germ cell: Expression of cytokeratins in oocytes and eggs of the frog Xenopus.

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Abstract
Vitellogenic oocytes and eggs of the frog X. laevis contain intermediate-size filaments that are resistant to extractions in high-salt buffers and Triton X-100 and are specifically stained with antibodies to cytokeratins. Gel electrophoresis of cytoskeletal proteins from Xenopus oocytes shows a specific enrichment of 3 polypeptides designated components 1 (Mr [relative MW], 56,000; IEP [pI obtained by 2-dimensional gel eletrophoresis in the presence of 9.5 M urea], .apprx. 5.9), 2(Mr, 46,000; IEP, 5.38) and 3(Mr, 42,000; IEP, .apprx. 5.3). The same 3 cytoskeletal polypeptides are found in eggs and early embryos, in intestinal mucosa of adult frogs and in cultured kidney epithelial cells. They are different from amphibian vimentin and desmin and from the keratins present in the epidermis of adult frogs. Peptide mapping and immunoblotting experiments indicate that Xenopus cytokeratin component 1 is related to cytokeratin A of higher vertebrates but is different from the 2 smaller cytoskeletal polypeptides 2 and 3. Incorporation of [35S]methionine shows that all 3 polypeptides are synthesized in both oocytes and embryos. Maternal storage is not only restricted to proteins serving basic cellular functions but also can extend to proteins related to a specific form of cell differentiation (i.e., epithelial formation) in the early embryo. Apparently, mechanisms of epithelial differentiation in Xenopus embryogenesis are different from those of early mammalian embryos in which no such intermediate-size-filament storage pool has been detected.