The ubiquitin-proteasome system (UPS) regulates eukaryotic protein quality control and signaling. Over the past four decades, our understanding of mechanisms underlying such regulation has been well-established for many cytoplasmic and nuclear proteins. This deep knowledge serves as the foundation for targeted protein degradation (TPD), a major drug discovery platform that rewires the ubiquitination machinery to selectively degrade disease-associated proteins. Despite these advances, our understanding of mechanistic and structural details of membrane protein regulation by the UPS remains limited. Yet membrane proteins constitute one-third of all human proteins and are the targets of more than 70% of drugs. Dysregulation in their biosynthesis or function can lead to various diseases, such as Alzheimer's disease and cardiovascular diseases. While many recent structural studies have elucidated factors mediating membrane protein biogenesis, much less is known about the mechanisms by which membrane proteins are subjected to ubiquitination. Therefore, it is important to fill this gap in understanding ubiquitin-mediated regulation of membrane proteins.
The specificity of the UPS is dictated by E3 ligases, which bind to substrates and execute ubiquitination. Glycoprotein 78 (gp78) is a master regulator of membrane protein lifecycle: (1) gp78 interacts with the ER Membrane Complex (EMC), a biogenesis factor responsible for high-fidelity structure formation of membrane proteins; (2) gp78 closely associates with multiple key factors involved in ER-associated degradation, the pathway that mediates membrane protein degradation. These suggest a gp78-centred membrane protein quality control (gp78-MPQC) hub that connects biogenesis to degradation and regulates the dynamic balance between the birth and death of membrane proteins.
However, how gp78 coordinates with EMC in detection of misfolded proteins; how the membrane-embedded gp78 ligase ubiquitinates its substrates; and how the actions of gp78 are coordinated with quality control machineries remain largely unknown.
This study integrated structural biology and biochemistry, cell biology, and chemical biology to systematically dissect the molecular mechanisms by which the gp78-MPQC hub mediates substrate detection, recognition, and ubiquitination. The mechanisms elucidated in this project will advance our understanding of how cells perform the difficult task of MPQC and pave the way for pharmacological approaches targeting membrane proteins.
