Membrane Protein Biochemistry

Our lab investigates how proteins and lipids work together to build, maintain, and safeguard cellular membranes, with a particular focus on ER-associated protein degradation (ERAD), the endoplasmic reticulum’s quality-control pathway that detects and eliminates aberrant membrane and secretory proteins.

The endoplasmic reticulum (ER) is the production and quality-control hub for many membrane and secreted proteins and the main site of lipid synthesis. Errors here can be highly disruptive. ERAD protects the ER by identifying faulty proteins, tagging them with ubiquitin, and directing them to the proteasome for destruction. The membrane adds unique challenges: proteins inside the ER lumen must be exported back across the membrane (retrotranslocation) to the cytosol for ubiquitin tagging, while integral membrane proteins must be extracted from the lipid bilayer before degradation.

We focus on two interconnected questions. First, how does ERAD recognize misfolded or misplaced membrane proteins and execute retrotranslocation and extraction? Second, how does the ER’s lipid composition tune ERAD, and how does ERAD, in turn, reshape membrane composition by degrading lipid‑metabolizing proteins? We tackle these using reconstituted systems assembled from defined lipids and purified proteins. This approach gives us precise control over key variables, allowing us to uncover mechanisms often obscured in cells. We also apply structural methods to visualize how ERAD components assemble and change conformation, and we perform experiments in yeast to validate mechanisms in living cells and explore their conservation across evolution.

Using these systems, we demonstrated that the ubiquitin ligase Hrd1 forms a gated retrotranslocation channel and that its activity is controlled by cycles of autoubiquitination and deubiquitination. We also discovered mechanisms allowing ERAD enzymes to attach ubiquitin to non‑lysine residues, such as serine and threonine. Furthermore, we found that the ubiquitin ligase Doa10 functions as a dislocase, collaborating with the ATP‑powered motor p97/Cdc48 to facilitate the extraction of membrane proteins from the bilayer. We continue to decipher the precise role of this dislocase activity within ERAD. More broadly, we found that multiple components of the ERAD ubiquitination machinery are exquisitely sensitive to lipids: some detect global membrane properties such as membrane packing, while others change their activity in response to specific lipid molecules such as cholesterol. We aim to establish general principles linking membrane biophysics to protein quality control and to test these insights in cells and organisms.

 

 

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 677770).