Electron microscopic immunolocalization of a karyoskeletal protein of molecular weight 145 000 in nucleoli and perinucleolar bodies of Xenopus laevis.

Amplified nucleoli of Xenopus laevis oocytes contain a major karyoskeletal protein of Mr 145 000 insoluble in low- and high-salt buffer as well as in non-denaturing detergents. Electron microscopic localization on native and high-salt extracted nucleoli using specific murine antibodies against this polypeptide and gold-coupled antibodies for visualization reveals that the Mr 145 000 protein is located in coils of filaments of ca 4 nm diameter. In addition, this protein occurs in the medusoid filament bodies (MFBs) present in the nucleolar cortex and free in the nucleoplasm. In somatic cells of tissues and in A6 kidney epithelial cells grown in vitro the Mr 145 000 polypeptide or an immunologically related protein is also organized in coiled aggregates of filaments 4-12 nm in diameter present both in the periphery of nucleoli and free in the nucleoplasm. We discuss a possible role of this protein as a karyoskeletal support involved in the storage and transport of preribosomal particles.