The 22 S cylinder particles of Xenopus laevis. I. Biochemical and electron microscopic characterization

Supernatant fractions obtained after high speed centrifugation (1 h at 100 000 X g) of homogenates from whole ovaries, oocytes as well as from separated nuclei and ooplasms of Xenopus laevis contain distinct 22 S particles which have been purified and characterized by sucrose gradient centrifugation, ion exchange chromatography on DEAE-Sephacel and fast protein liquid chromatography (FPLC). The purity of the particle fraction has been assessed by electron microscopy as well as one- and two-dimensional gel electrophoresis. The particles appear as hollow cylinders of 10 nm outer diameter and 16 nm length, showing a composition of four stacked annuli which often reveal 6 symmetrically distributed granular subunits of approximately 3 nm diameter. Biochemically the particles are characterized by a group of 12 polypeptides with Mr values from 22 000 to 30 000 which in urea-denatured state markedly differ in their isoelectric values, ranging from pH 5.4 to ca. 8.2. Tryptic peptide mapping has demonstrated that all 12 major polypeptides are different. No evidence for association with nucleic acids has been found. The particles are very stable and resist treatments with low and high salt buffers, chelating agents, various non-denaturing detergents, and 3 M urea. They occur in relatively high concentrations both in the nucleus and in the cytoplasm. Structurally and compositionally identical cylinder particles have also been found in cultures of kidney epithelial cells of Xenopus and in human carcinoma (HeLa) cells, indicating that this is a rather widespread component of diverse cell types and species. The significance of this particle and its relationship to morphologically similar particles described in the literature is discussed.