Hul5 ubiquitin ligase: good riddance to bad proteins

Failure to eliminate abnormal proteins in the cell is associated with numerous aggregation diseases. Misfolded proteins are normally detected by protein quality control and either refolded or eliminated. The ubiquitin-proteasome system is a major pathway that degrades these unwanted proteins. Ubiquitin ligases are central to these degradation pathways as they recognize aberrant proteins and covalently attach a polyubiquitin chain to target them to the proteasome. We discovered that the Hul5 ubiquitin ligase is a major player in a novel protein quality control pathway that targets cytosolic misfolded proteins. Hul5 is required for the maintenance of cell fitness and the increased ubiquitination of low solubility proteins after heat-shock in yeast cells. We identified several low-solubility substrates of Hul5, including the prion-like protein Pin3. It is now apparent that in the cytoplasm, misfolded proteins can be targeted by multiple degradation pathways. In this review, we discuss how the Hul5 protein quality control pathway may specifically target low solubility cytosolic proteins in the cell.