Assembly and nuclear export of ribosomal subunits in Saccharomyces cerevisiae

Eukaryotic ribosome biogenesis requires movement of macromolecules into and out of the nucleus. Ribosomal subunits are assembled in the nucleus while they function in the cytoplasm. I developed and used an assay to monitor the assembly and nuclear export of the small ribosomal subunit in yeast.; The nuclear transport importers Kap60, Kap95, Kap104, Kap121/Pse1, Mtr10, and Nmd5 are required for the assembly of the small ribosomal subunit. Mammalian orthologs of four of these receptors have been shown to import ribosomal proteins, and the ribosome assembly defect in these yeast mutants may be attributed to defective ribosomal protein import. The nuclear pore complex is important for ribosome biogenesis in that a subset of nucleoporin mutants is defective in the assembly of the small ribosomal subunit, and an overlapping subset of nucleoporins is required for small ribosomal subunit export. This ribosome assembly defect can be attributed to defective import of ribosomal proteins or defective export of mRNAs encoding ribosomal proteins.; The RanGTPase cycle is required for the nuclear export of the small ribosomal subunit. Furthermore, the Ran-dependent transport receptor Crm1/Xpo1 and its accessory factor Yrb2 are involved in this process. Inhibition of the Ran-GTPase cycle or Crm1 results in a block of ribosome export while loss of Yrb2 results in a delay. In sum, my results indicate that the nuclear export of the small ribosomal subunit occurs through a Crm1-dependent pathway.; In a separate project, I determined that Cgr1 is involved in the assembly of the large ribosomal subunit in yeast. I also characterized the involvement of the RSC complex (R&barbelow;emodels the S&barbelow;tructure of C&barbelow;hromatin) in the regulation of ribosome biogenesis. RSC appears to be involved in this process because it regulates gene expression that is controlled by the TOR signaling pathway. I determined that mutants in RSC components are defective in rRNA production but are not altered in the rates of pre-rRNA processing. Lastly, I characterized SWR1 (s&barbelow;ick w&barbelow;ith R&barbelow;SC) which encodes a protein similar to Swi2/Snf2 and the RSC component Sth1.