Identification and immunoprecipitation of cytoskeletal proteins from murine extra-embryonic endodermal cells

Murine extra-embryonic endodermal cells derived from either teratocarcinomas or cultured mouse blastocysts contain two protein species of Mr = 55,000 and Mr = 50,000 endodermal cytoskeletal proteins A and B, respectively) that are insoluble in nonionic detergent and 1 M NaCl and are not found in abundance in embryonal carcinoma cells, the stem cells of teratocarcinomas. Antiserum raised against the electrophoretically purified endo B protein immunoprecipitated endo B from [35S]methionine-labeled cell lysates of three parietal endodermal cell lines, a presumptive visceral endodermal cell line, and a mouse hepatoma line. Immunoprecipitable endo B was not found in murine embryonal carcinoma cells, fibroblasts, myoblasts, keratinocytes, erythroleukemic or neuroblastoma cells. These results are consistent with the view that endo B is not tubulin, vimentin, desmin, or keratin. Amino acid composition data, partial peptide analysis of immunoprecipitated endo B, and immunoprecipitation analysis with antikeratin serum support the suggestion that endo B is not a keratin. Indirect immunofluorescent staining of parietal endodermal cells with the endo B antiserum resulted in the fluorescence of a fibrillar cytoskeletal network. The synthesis of endo B was increased dramatically when embryonal carcinoma cells were induced to differentiate by treatment with retinoic acid. Endo B appears to be a cytoskeletal protein that is synthesized when malignant embryonal carcinoma cells differentiate to benign extra-embryonic endoderm.