Recruiting partners for destruction

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Two groups have shown that the recruitment of Cdc48 and ubiquitin ligases to the endoplasmic reticulum (ER) by Ubx2 is required for ER-associated protein degradation.

Endoplasmic reticulum (ER)-associated protein degradation (ERAD) is a stringent quality-control mechanism that removes misfolded proteins from the ER through a process that requires ubiquitin ligases and Cdc48. However, how the activities of these proteins are coupled remains unclear. Two groups now provide insights into the mechanisms that regulate ERAD in yeast, by showing that the UBX domain protein Ubx2 recruits Cdc48 to the ER, and mediates the interaction of Cdc48 with ER-membrane-bound ubiquitin ligases.


GFP fluorescence of cells expressing GFP-Ubx2. Image courtesy of Alexander Buchberger, Max Planck Institute of Biochemistry, Martinsried, Germany. Image reproduced, with permission, from Nature Cell Biology © (2005) Macmillan Magazines.

Although recognition of misfolded polypeptides takes place in the ER lumen, their proteolysis occurs in the cytosol, through a transport process known as dislocation. ERAD substrates are tagged by ubiquitin ligases for recognition, and dislocation occurs with the help of Cdc48, which is required to release substrates from the ER membrane. In an attempt to understand how Cdc48 is integrated into the ERAD pathway, Neuber and colleagues showed that Ubx2 links Cdc48 to ERAD. In a complementary report, Schuberth and Buchberger, who were interested in identifying the specific functions of yeast Ubx2, confirmed that Ubx2 recruits Cdc48 to the ER.

Indeed, both groups showed that Ubx2 localizes in the ER and behaves like an integral ER membrane protein in membrane-fractionation experiments. Studies that compared wild-type cells and cells lacking Ubx2, combined with immunoprecipitation experiments, provided evidence that Ubx2 not only contributes significantly to the recruitment of Cdc48 to the ER membrane and, more specifically, to proteins that are destined for degradation, but also provided the link between Cdc48 and the membrane-bound E3 ubiquitin ligases Hrd1 and Doa10. Furthermore, analysis of different types of ERAD substrates indicated that Ubx2 is involved in the turnover of substrates in both Hrd1- and Doa10-dependent ERAD pathways.

But which domains of Ubx2 are important for these interactions? To address this question, Neuber et al. studied the N terminus of Ubx2 and found that this region of the protein is required for the breakdown of ER proteins, but is dispensable for the degradation of soluble cytoplasmic or nuclear proteins. On the other hand, Schuberth and Buchberger focused on the C terminus and, by using a truncated variant that lacked the UBX domain, they showed that this domain is essential for the recruitment of Cdc48.

So, Ubx2 is crucial for the interaction of Cdc48 with ERAD substrates and ERAD ubiquitin ligases in yeast, and it has a pivotal role in coordinating the action of these components within the ERAD machinery. Ubx2 is known to have a mammalian homologue, but whether its role is evolutionarily conserved remains to be seen.

Ekat Kritikou

References

Neuber, O. et al. Ubx2 links the Cdc48 complex to ER-associated protein degradation. Nature Cell Biol. 7, 993–998 (2005) | Article | PubMed |
Schuberth, C. & Buchberger, A. Membrane-bound Ubx2 recruits Cdc48 to ubiquitin ligases and their substrates to ensure efficient ER-associated protein degradation. Nature Cell Biol. 7, 999–1006 (2005) | Article | PubMed |
Meusser, B. et al. ERAD: the long road to destruction. Nature Cell Biol. 7, 766–772 (2005) | Article | PubMed |