Proteolytic degradation is critical to the maintenance of appropriate levels of short-lived and regulatory proteins as important and diverse as those involved in cellular metabolism, heat shock and stress response, antigen presentation, modulation of cell surface receptors and ion channels, cell cycle regulation, transcription, and signalling factors. The ubiquitin-proteasome pathway deconstructs most proteins in the eukaryotic cell cytosol and nucleus. Others are degraded via the vacuolar pathway which includes endosomes, lysosomes and the endoplasmic reticulum.