The 26S proteasome of the fission yeast Schizosaccharomyces pombe

Abstract

The 26S proteasome is the multiprotein complex that degrades proteins that have been marked for destruction by the ubiquitin pathway. It is made up of two multisubunit complexes, the 20S catalytic core and the 19S regulatory complex. We describe the isolation and characterisation of conditional mutants in the regulatory complex and their use to investigate interactions between different subunits. In addition we have investigated the localisation of the 26S proteasome in fission yeast, by immunofluoresence in fixed cells and live cells using a GFP tagged subunit. Surprisingly we find that in mitotic cells the 26S proteasome occupies a discrete intracellular compartment, the nuclear periphery. EM analysis demonstrates that the complex resides inside the nuclear envelope. During meiosis the localisation showed a more dynamic distribution. In meiosis I the proteasome remained around the nuclear periphery. However, during meiosis II there was a dramatic relocalisation wherebye initially the signal occupied the area between the dividing nuclei. At the end of mitosis the signal dispersed returning to the nuclear periphery upon ascospore formation. This observation implies that the nuclear periphery is a major site of proteolysis in yeast during mitotic growth and raises important questions about the function of the 26S proteasome in protein degradation.