Fig. 6. Export of ribosomal subunits in Xenopus oocytes requires CRM1 and Ran·GTP. (A) Outline of the major pathways of rRNA processing in Xenopus (Savino and Gerbi, 1990), including the conversion of 12S rRNA to 6S rRNA, a precursor of 5.8S rRNA, that is matured after export to the cytoplasm (our unpublished results). (B and C) Requirement for the export receptor CRM1. (B) Oocytes were labeled with [³²P]GTP at 0 h, and treatment with LMB (400 ng/ml) was initiated at 6 h. The intracellular distributions of newly made rRNAs in control and LMB-treated oocytes were monitored at the indicated times by analysis of 0.5 oocyte equivalents of total nuclear (N) and cytoplasmic (C) RNAs in a 1.2% agarose gel. (C) PKI NES peptides conjugated to BSA (NES–BSA) were injected into nuclei 1 h prior to labeling with [³²P]GTP for 24 h, and labeled rRNAs of control and treated oocytes were analyzed as in (A). (D) Requirement for Ran·GTP. Oocytes were labeled with [³²P]GTP at 0 h, and RanT24N was injected into the cytoplasm at 24 h; rRNAs of control and treated oocytes were analyzed after 24 and 40 h of labeling, as indicated. The gel mobilities of precursor and mature rRNAs are indicated, and arrowheads show the nuclear accumulation of mature rRNAs upon inhibition of ribosomal subunit export.