Nuclear export of the small ribosomal subunit requires the Ran-GTPase cycle and certain nucleoporins

After their assembly in the nucleolus, ribosomal subunits are exported from the nucleus to the cytoplasm. After export, the 20S rRNA in the small ribo somal subunit is cleaved to yield 18S rRNA and the small 5' ITS1 fragment. The 5' ITS1 RNA is normally degraded by the cytoplasmic Xrn1 exonuclease, b ut in strains lacking XRN1, the 5' ITS1 fragment accumulates in the cytopla sm. Using the cytoplasmic localization of the 5' ITS1 fragment as an indica tor for the export of the small ribosomal subunit, we have identified genes that are required for ribosome export. Ribosome export is dependent on the Ran-GTPase as mutations in Ran or its regulators caused 5' ITS1 to accumul ate in the nucleoplasm. Mutations in the genes encoding the nucleoporin Nup 82 and in the NES exporter Xpo1/Crm1 also caused the nucleoplasmic accumula tion of 5' ITS1. Mutants in a subset of nucleoporins and in the nuclear tra nsport factors Srp1, Kap95, Pse1, Cse1, and Mtr10 accumulate the 5' ITS1 in the nucleolus and affect ribosome assembly. In contrast, we did not detect nuclear accumulation of 5' ITS1 in 28 yeast strains that have mutations in other genes affecting nuclear trafficking.