2E). chaperone-dependent delivery to nuclear San1. The broad conservation of Ubr ligases and the relevant chaperones indicates that these mechanisms will be important in understanding both basic and biomedical aspects of cellular proteostasis. Keywords:chaperone, proteostasis, misfolding Protein quality control (QC) functions to ensure that damaged and misfolded proteins are maintained at acceptable levels to limit their stress-causing, or proteotoxic, effects. One strategy of protein QC is the selective degradation of misfolded proteins. For degradative QC pathways to be effective, they must be specific for aberrant proteins; sufficiently general to recognize selectively common structural hallmarks shared by numerous unrelated proteins; and physiologically important, better allowing the cell to survive proteotoxic stress. Because protein QC underlies many pressing maladies, such as parkinsonism, cystic fibrosis, and aging, discovery of the rules of substrate selectivity and destruction is a key step in understanding these conditions and designing appropriate therapeutical interventions to combat them. In eukaryotes, the ubiquitin proteasome system is employed in the selective degradation of many proteins (1). A substrate protein is marked for degradation by assembly of a polyubiquitin chain, initiated by covalent addition of the small (7.6 kDa) protein ubiquitin to a lysine in an isopeptide bond, followed by iterative addition of the next ubiquitin to the previously added one to create a polyubiquitin chain that is uniquely recognized Chenodeoxycholic acid by the 26S proteasome. Protein ubiquitination is catalyzed by a three-enzyme cascade. The single E1 ubiquitin-activating enzyme hydrolyzes ATP to acquire ubiquitin in labile thioester linkage, which is then transferred in thioester linkage to one of a small group of E2s or ubiquitin-conjugating enzymes (UBCs). E2-bound ubiquitin Rabbit Polyclonal to STAT1 (phospho-Ser727) is finally transferred to an isopeptide linkage on the target protein or the growing polyubiquitin chain by the action of the E3 ubiquitin ligase. It is the E3 ubiquitin ligase that determines the specificity of a given ubiquitination process; identifying and understanding the E3s involved in a degradative pathway are thus key parts of understanding the mechanisms of substrate selection and modification. E3s for several QC pathways have been discovered and include the endoplasmic reticulum-associated ligases Hrd1 and Doa10 involved in endoplasmic reticulum-associated degradation (ERAD) and the San1 ubiquitin ligase that mediates destruction of misfolded nuclear proteins (24). The mechanism used by the QC ligases to detect misfolded substrates can vary, with some employing chaperones (5) and others not. The details of substrate recognition are key to understanding the envelope of structures subject to destruction by a given pathway. So far, no widely conserved ubiquitination pathway has been described for cytoplasmic QC. Metazoans express the CHIP ubiquitin ligase that mediates cytoplasmic QC, using Hsp70 chaperones to detect misfolded proteins (6,7). However, CHIP is not conserved in all eukaryotes. For example, no CHIP is encoded in yeast. Nevertheless, chaperone-dependent ubiquitination of misfolded proteins has been observed in yeast (8), indicating that previously undescribed, and probably highly conserved, cytoplasmic QC pathways remain to be discovered. To that end, we have investigated the E3 ligases involved in ubiquitination of misfolded cytoplasmic proteins in yeast. We have discovered that two E3 ligases collaborate in ubiquitination of a diverse set of misfolded proteins, including full-length substrates with point mutations and truncated proteins. The two E3s are the nuclear E3 San1 (9,10) and Ubr1, best known in the N-end rule pathway (11,12). A variety of misfolded substrates undergo selective chaperone-dependent ubiquitination by either ligase. In this function, Ubr1 and San1 appear to function independently. In vitro experiments indicate that the Ubr1 ligase directly employs chaperones in substrate ubiquitination, whereas the San1 E3 may require chaperones for delivery to the nucleus. Our phenotypic studies show that Ubr1 had the principal role in mediating cytoplasmic proteotoxic stress imposed by model substrates or chemical stressors. This QC function of Ubr1 was independent of its function in the well-described N-end rule, and so represents a previously undescribed physiologically important Chenodeoxycholic acid role for this molecule. Our demonstration of parallel pathways indicates the importance and complexity of Chenodeoxycholic acid cytoplasmic proteostasis. Understanding them will provide.