Dieter H. Wolf

ER degradation of a misfolded luminal protein by the cytosolic ubiquitin-proteasome pathway

Secretion of proteins is initiated by their uptake into the endoplasmic reticulum (ER), which possesses a proteolytic system able to degrade misfolded and nonassembled proteins. The ER degradation system was studied with yeast mutants defective in the breakdown of a mutated soluble vacuolar protein, carboxypeptidase yscY (CPY*). The ubiquitin-conjugating enzyme Ubc7p participated in the degradation process, which was mediated by the cytosolic 26S proteasome. It is likely that CPY* entered the ER, was glycosylated, and was then transported back out of the ER lumen to the cytoplasmic side of the organelle, where it was conjugated with ubiquitin and degraded.

Mutant analysis links the translocon and BiP to retrograde protein transport for ER degradation

Proteins enter the secretory pathway through the endoplasmic reticulum, which delivers properly folded proteins to their site of action and contains a quality-control system to monitor and prevent abnormal proteins from being delivered. Many of these proteins are degraded by the cytoplasmic proteasome, which requires their retrograde transport to the cytoplasm,. Based on a co-immunoprecipitation of major histocompatibility complex (MHC) class I heavy-chain breakdown intermediates with the translocon subunit Sec61p (refs 9, 10), it was speculated that Sec61p may be involved in retrograde transport. Here we present functional evidence from genetic studies that Sec61p mediates retrograde transport of a mutated lumenal yeast carboxypeptidase ycsY (CPY*) in vivo. The endoplasmic reticulum lumenal chaperone BiP (Kar2p) and Sec63p, which are also subunits of the import machinery,, are involved in export of CPY* to the cytosol. Thus our results demonstrate that retrograde transport of proteins is mediated by a functional translocon. We consider the export of endoplasmic reticulum-localized proteins to the cytosol by the translocon for proteasome degradation to be a general process in eukaryotic cell biology.

Protein dislocation from the ER requires polyubiquitination and the AAA-ATPase Cdc48.

Endoplasmic reticulum (ER)-associated protein degradation by the ubiquitin–proteasome system requires the dislocation of substrates from the ER into the cytosol. It has been speculated that a functional ubiquitin proteasome pathway is not only essential for proteolysis, but also for the preceding export step. Here, we show that short ubiquitin chains synthesized on proteolytic substrates are not sufficient to complete dislocation; the size of the chain seems to be a critical determinant. Moreover, our results suggest that the AAA proteins of the 26S proteasome are not directly involved in substrate export. Instead, a related AAA complex Cdc48, is required for ER-associated protein degradation upstream of the proteasome.

Isolation of autophagocytosis mutants of Saccharomyces cerevisiae

Protein degradation in the vacuole (lysosome) is an important event in cellular regulation. In yeast, as in mammalian cells, a major route of protein uptake for degradation into the vacuole (lysosome) has been found to be autophagocytosis. The discovery of this process in yeast enables the elucidation of its mechanisms via genetic and molecular biological investigations. Here we report the isolation of yeast mutants defective in autophagocytosis (aut mutants), using a rapid colony screening procedure.

The Active Sites of the Eukaryotic 20 S Proteasome and Their Involvement in Subunit Precursor Processing

The 26 S proteasome is the central protease involved in ubiquitin-mediated protein degradation and fulfills vital regulatory functions in eukaryotes. The proteolytic core of the complex is the 20 S proteasome, a cylindrical particle with two outer rings each made of 7 different α-type subunits and two inner rings made of 7 different β-type subunits. In the archaebacterial 20 S proteasome ancestor proteolytically active sites reside in the 14 uniform β-subunits. Their N-terminal threonine residues, released by precursor processing, perform the nucleophilic attack for peptide bond hydrolysis. By directed mutational analysis of 20 S proteasomal β-type proteins of Saccharomyces cerevisiae, we identified three active site-carrying subunits responsible for different peptidolytic activities as follows: Pre3 for post-glutamyl hydrolyzing, Pup1 for trypsin-like, and Pre2 for chymotrypsin-like activity. Double mutants harboring only trypsin-like or chymotrypsin-like activity were viable. Mutation of two potentially active site threonine residues in the Pre4 subunit excluded its catalytic involvement in any of the three peptidase activities. The generation of different, incompletely processed forms of the Pre4 precursor in active site mutants suggested that maturation of non-active proteasomal β-type subunits is exerted by active subunits and occurs in the fully assembled particle. Thistrans-acting proteolytic activity might also account for processing intermediates of the active site mutated Pre2 subunit, which was unable to undergo autocatalytic maturation.

For whom the bell tolls: protein quality control of the endoplasmic reticulum and the ubiquitin–proteasome connection

The surveillance of the structural fidelity of the proteome is of utmost importance to all cells. The endoplasmic reticulum (ER) is the organelle responsible for proper folding and delivery of proteins to the secretory pathway. It contains a sophisticated protein proofreading and elimination mechanism. Failure of this machinery leads to disease and, finally, to cell death. Elimination of misfolded proteins requires retrograde transport across the ER membrane and depends on the central cytoplasmic proteolytic machinery involved in cellular regulation: the ubiquitin–proteasome system. The basics of this process as well as recent advances in the field are reviewed.

Retrograde protein translocation: ERADication of secretory proteins in health and disease

Eukaryotic cells have a complex degradation machinery that eliminates misfolded or unassembled secretory proteins from the endoplasmic reticulum (ER). The proteins are retained in an ER/pre-Golgi compartment and then hydrolysed by the cytosolic ubiquitin-proteasome system. This requires retrograde translocation of proteins from the ER back to the cytoplasm, which is mediated by Sec61, the central component of the ER protein-import channel. This proteolytic pathway prevents a potentially lethal aggregation of secretory proteins; however, several viruses misuse it to escape detection, and bacterial and plant toxins might also exploit it. Underactive or overactive ER degradation machinery contributes to the pathogenesis of several severe human diseases.

Proteinase yscE, the yeast proteasome/multicatalytic-multifunctional proteinase: mutants unravel its function in stress induced proteolysis and uncover its necessity for cell survival.

Proteinase yscE is the yeast equivalent of the proteasome, a multicatalytic‐multifunctional proteinase found in higher eukaryotic cells. We have isolated three mutants affecting the proteolytic activity of proteinase yscE. The mutants show a specific reduction in the activity of the complex against peptide substrates with hydrophobic amino acids at the cleavage site and define two complementation groups, PRE1 and PRE2. The PRE1 gene was cloned and shown to be essential. The deduced amino acid sequence encoded by the PRE1 gene reveals weak, but significant similarities to proteasome subunits of other organisms. Two‐dimensional gel electrophoresis identified the yeast proteasome to be composed of 14 different subunits. Comparison of these 14 subunits with the translation product obtained from PRE1 mRNA synthesized in vitro demonstrated that PRE1 encodes the 22.6 kd subunit (numbered 11) of the yeast proteasome. Diploids homozygous for pre1–1 are defective in sporulation. Strains carrying the pre1–1 mutation show enhanced sensitivity to stresses such as incorporation of the amino acid analogue canavanine into proteins or a combination of poor growth medium and elevated temperature. Under these stress conditions pre1–1 mutant cells exhibit decreased protein degradation and accumulate ubiquitin‐protein conjugates.

Der1, a novel protein specifically required for endoplasmic reticulum degradation in yeast.

The endoplasmic reticulum (ER) of the yeast Saccharomyces cerevisiae contains of proteolytic system able to selectively degrade misfolded lumenal secretory proteins. For examination of the components involved in this degradation process, mutants were isolated. They could be divided into four complementation groups. The mutations led to stabilization of two different substrates for this process. The mutant classes were called ‘der’ for ‘degradation in the ER’. DER1 was cloned by complementation of the der1–2 mutation. The DER1 gene codes for a novel, hydrophobic protein, that is localized to the ER. Deletion of DER1 abolished degradation of the substrate proteins. The function of the Der1 protein seems to be specifically required for the degradation process associated with the ER. The depletion of Der1 from cells causes neither detectable growth phenotypes nor a general accumulation of unfolded proteins in the ER. In DER1‐deleted cells, a substrate protein for ER degradation is retained in the ER by the same mechanism which also retains lumenal ER residents. This suggests that DER1 acts in a process that directly removes protein from the folding environment of the ER.

Proteasomes: destruction as a programme

Proteasomes are large multi-subunit protease complexes that selectively degrade intracellular proteins. Most of the proteins removed by these proteases are tagged for destruction by ubiquitination. Proteasomes have a role to play in controlling cellular processes, such as metabolism and the cell cycle, through signal-mediated proteolysis of key enzymes and regulatory proteins. They also operate in the stress response, by removing abnormal proteins, and in the immune response, by generating antigenic peptides.