An unexpected link between the secretory path and the organization of the nucleus

Yeast sec mutations define the machinery of vesicular traffic. Surprisingly , many of these mutations also inhibit ribosome biogenesis by reducing tran scription of rRNA and genes encoding ribosomal proteins. We observe that th ese mutants reversibly inhibit protein import into the nucleus, with import cargo accumulating at the nucleoplasmic face of nuclear pore complexes, as when Ran-GTP cannot bind importins. They also rapidly and reversibly reloc ate multiple nucleolar and nucleoplasmic proteins to the cytoplasm. The imp ort block and relocation are antagonized by overexpression of yeast Ran, Ho g1p kinase, or Ssa/Hsp70 proteins or by inhibition of protein synthesis. Th ese nucleocytoplasmic signaling events document an extraordinary plasticity of nuclear organization.