A Complex of Htm1 and the Oxidoreductase Pdi1 Accelerates Degradation of Misfolded Glycoproteins.

A quality control system in the endoplasmic reticulum (ER) efficiently discriminates polypeptides that are in the process of productive folding from conformers that are trapped in an aberrant state. Only the latter are transported into the cytoplasm and degraded in a process termed ER-associated protein degradation (ERAD). In the ER, an enzymatic cascade generates a specific N-glycan structure of seven mannosyl and two N-acetylglucosamine residues (Man7GlcNAc2) on misfolded glycoproteins to facilitate their disposal. We show that a complex encompassing the yeast lectin-like protein Htm1 and the oxidoreductase Pdi1 converts Man8GlcNAc2 on glycoproteins into the Man7GlcNAc2 signal. In vitro the Htm1-Pdi1 complex processes both unfolded and native proteins albeit with a preference for the former. In vivo, elevated expression of HTM1 causes glycan trimming on misfolded and folded proteins, but only degradation of the non-native species is accelerated. Thus, modification with a Man7GlcNAc2 structure does not inevitably commit a protein for ER-associated protein degradation. The function of Htm1 in ERAD relies on its association with Pdi1, which appears to regulate the access to substrates. Our data support a model in which the balanced activities of Pdi1 and Htm1 are crucial determinants for the efficient removal of misfolded secretory glycoproteins.

Usa1 functions as a scaffold of the HRD-ubiquitin ligase.

Protein quality control in the endoplasmic reticulum is of central importance for cellular homeostasis in eukaryotes. Crucial for this process is the HRD-ubiquitin ligase (HMG-CoA reductase degradation), which singles out terminally misfolded proteins and routes them for degradation to cytoplasmic 26S-proteasomes. Certain functions of this enzyme complex are allocated to defined subunits. However, it remains unclear how these components act in a concerted manner. Here, we show that Usa1 functions as a major scaffold protein of the HRD-ligase. For the turnover of soluble substrates, Der1 binding to the C terminus of Usa1 is required. The N terminus of Usa1 associates with Hrd1 and thus bridges Der1 to Hrd1. Strikingly, the Usa1 N terminus also induces oligomerization of the HRD complex, which is an exclusive prerequisite for the degradation of membrane proteins. Our data demonstrate that scaffold proteins are required to adapt ubiquitin ligase activities toward different classes of substrates.

Ubx2 links the Cdc48 complex to ER-associated protein degradation.

Endoplasmic reticulum (ER)-associated protein degradation requires the dislocation of selected substrates from the ER to the cytosol for proteolysis via the ubiquitin-proteasome system. The AAA ATPase Cdc48 (known as p97 or VCP in mammals) has a crucial, but poorly understood role in this transport step. Here, we show that Ubx2 (Sel1) mediates interaction of the Cdc48 complex with the ER membrane-bound ubiquitin ligases Hrd1 (Der3) and Doa10. The membrane protein Ubx2 contains a UBX domain that interacts with Cdc48 and an additional UBA domain. Absence of Ubx2 abrogates breakdown of ER proteins but also that of a cytosolic protein, which is ubiquitinated by Doa10. Intriguingly, our results suggest that recruitment of Cdc48 by Ubx2 is essential for turnover of both ER and non-ER substrates, whereas the UBA domain of Ubx2 is specifically required for ER proteins only. Thus, a complex comprising the AAA ATPase, a ubiquitin ligase and the recruitment factor Ubx2 has a central role in ER-associated proteolysis.

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.

A lack of SUMO conjugation affects cNLS-dependent nuclear protein import in yeast.

Yeast SUMO (Smt3) and its mammalian ortholog SUMO-1 are ubiquitin-like proteins that can reversibly be conjugated to other proteins. Among the substrates for SUMO modification in vertebrates are RanGAP1 and RanBP2/Nup358, two proteins previously implicated in nucleocytoplasmic transport. Sumoylated RanGAP1 binds to the nuclear pore complex via RanBP2/Nup358, a giant nucleoporin, which was recently reported to act as a SUMO E3 ligase on some nuclear substrates. However, no direct evidence for a role of the SUMO system in nuclear transport has been obtained so far. By the use of conditional yeast mutants, we examined nuclear protein import in vivo. We show here that cNLS-dependent protein import is impaired in mutants with defective Ulp1 and Uba2, two enzymes involved in the SUMO conjugation reaction. In contrast, other transport pathways such as rgNLS-mediated protein import and mRNA export are not affected. Furthermore, we find that the yeast importin-alpha subunit Srp1 accumulates in the nucleus of ulp1 and uba2 strains but not the importin-beta subunit Kap95, indicating that a lack of Srp1 export might impair cNLS import. In summary, our results provide evidence that SUMO modification in yeast, as has been suspected for vertebrates, plays an important role in nucleocytoplasmic trafficking.