Cells in the stratified squamous epithelium of bovine esophagus contain abundant tonofilaments measuring 6-10 nm in diameter. Two polypeptides, extracted from esophageal epithelium with 0.05 M Tris, pH 7.4, containing 8 M urea and 25 mM beta-mercaptoethanol, comprise 35% of the total extractable protein. These polypeptides have apparent molecular weights of 46,000 and 56,000 daltons and are rich in glutamic acid-glutamine, glycine, and serine. Each polypeptide can be partially purified by DEAE-cellulose chromatography. Mixtures of the purified polypeptides from filaments in vitro that measured 6-10 nm in diameter. Neither polypeptide formed filaments by itself. Filaments formed in vitro give an alpha-keratin type x-ray diffraction pattern.. These data indicate that the tonofilaments in esophageal epithelium are formed primarily from these two polypeptides.

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