Fig. 4. Nuclear export of ribosomal subunits is competed by NES peptides. (A) Scheme illustrating the processing of pre-rRNA along two co-existing pathways in Xenopus oocytes (modified according to Peculis, 1997) (Peculis, 1997). Cleavages occur in an orderly fashion at numerous sites indicated by arrows. This leads to the removal of externally and internally transcribed spacers (ETS, ITS) to yield mature 5.8S, 18S and 28S rRNA. (B) Competition of the CRM1-mediated NES export pathway interferes with nuclear exit of both ribosomal subunits. Xenopus oocytes were injected with [-³²P]GTP. After 3 or 5 hours, PKI-NES wild-type or mutant peptides conjugated to BSA were injected into the nucleus. At the indicated times, RNA was isolated from nuclear (N) or cytoplasmic (C) fractions, separated by agarose gel electrophoresis and analyzed first by phosphoimaging for quantification and thereafter by autoradiography.