HUMAN P80-COILIN IS TARGETED TO SPHERE ORGANELLES IN THE AMPHIBIAN GERMINAL VESICLE

Cultured vertebrate cells often display one or more coiled bodies in t heir nuclei. These are spherical structures similar to-0.5-1.0 mu m in diameter that contain high concentrations of small nuclear ribonucleo proteins (snRNPs); they are distinct from nuclear speckles and nucleol i, the other major sites of snRNP concentration. Coiled bodies in huma n cells contain a unique protein, p80-coilin, that has an M(r) = 80 kD a. Cloned p80-coilin cDNA encodes 576 amino acids with a calculated mo lecular weight of 62.6 kDa. To determine which of several snRNP-contai ning structures in the amphibian germinal vesicle (GV) might be the ho mologue of coiled bodies, we injected myc-tagged transcripts of full-l ength human p80-coilin into the cytoplasm of Xenopus oocytes and follo wed the fate of the translated proteins with an antibody specific for the tag. Western blots of GV proteins showed rapid appearance of both full-length and truncated p80-coilin in the nucleus. Immunofluorescent staining of spread GV contents demonstrated specific uptake of p80-co ilin by the sphere organelle within 1 h after injection. Similar exper iments were performed with a series of deletion constructs that lacked progressively longer segments from the carboxy terminus. A construct that contained only the first 102 amino acids (18% of the molecule) wa s specifically targeted to the sphere organelle. Conversely, a constru ct lacking the first 92 amino acids failed to localize, although it wa s imported into the GV. Thus, a relatively short region at the amino t erminus of human p80-coilin is both necessary and sufficient for local ization in the sphere organelle. Sphere organelles in;the GV; and,coil ed bodies in somatic nuclei are clearly related in composition. We sug gest that they are:homologous organelles with similar functions in pre assembly and sorting of RNA processing components. Differences in thei r composition suggest functional specialization in the two cell types.