Delineating stages of Aquaporin biogenesis

Secondary Supervisor(s): Professor Roslyn Bill, Dr Philip Kitchen

University of Registration: Aston University

BBSRC Research Themes: Understanding the Rules of Life (Structural Biology)

Project Outline

Membrane proteins are essential for cellular function and are key targets for drug development. However, our understanding of their biogenesis; the process of folding and membrane insertion during synthesis, remains limited - particularly in humans. The aquaporin (AQP) family of proteins, responsible for water transport across cellular membranes, depends on proper folding and localization for their function. Misfolding of AQPs can lead to severe conditions, including genetic chronic brain edema (AQP4), nephrogenic diabetes insipidus (AQP2), and pulmonary arterial hypertension (AQP1).

AQP biogenesis occurs at the endoplasmic reticulum (ER), where proteins are synthesized by ribosomes and fold into the membrane in a cotranslational manner. This intricate process requires a coordinated effort of various factors that guide ribosome docking and membrane integration via the Sec61 translocation machinery and other factors. If successfully folded, AQPs are trafficked from the ER to the plasma membrane. When misfolding occurs, through mutation or dysregulation of synthesis, the protein may be defective or depleted, leading to disease.

This exciting project will use original methodologies to investigate the biogenesis of AQPs at the ER during synthesis, aiming to pinpoint when and why misfolding occurs. A key focus will be to understand cotranslational folding pathways of AQPs, with special attention to how lipid composition at the ER and throughout the secretory pathway influences protein stability and trafficking.

The student will first build on existing research to purify membrane protein ribosome-bound nascent chains (RNCs) of AQPs directly from the human ER. This will be achieved using innovative copolymer technologies that preserve the native lipid and protein environment around truncated versions of AQPs while attached to the ribosome, representing snapshots of cotranslational folding intermediates. While this technology is still in its infancy, it has enormous capability to drive membrane protein folding research in a new direction.

The project will explore differences in folding pathways between AQP1 and AQP4, which have been shown to fold through different trajectories, as well determining the effects of pathogenic variants of each AQP on folding. The influence of specific lipids and membrane insertion machineries on the folding of AQPs will also be explored.

Lipidomic analysis will be used to understand how the membrane environment around nascent AQP chains changes during synthesis, and observations will be tested using ‘cell-free’ translation reactions with artificial lipid mimics.

Proteomics will be employed to identify potential interactors with translation machinery, chaperone proteins or quality control factors during AQP biogenesis. The project will also explore how and where AQPs assemble into functional tetramers within the membrane, and to gain an understanding of the functional relevance of this tetramerization.

Throughout, the student will use a range of cell biology and biochemical techniques to provide insights into the molecular mechanisms of folding and trafficking, including immunoblotting, cell culture and imaging, cell-free translation systems, limited proteolysis, gel-shift assays, chemical crosslinking and FRET.

This research will greatly enhance our understanding of de novo AQP biogenesis and ER quality control, made possible through the generation of RNCs. By pinpointing the factors that promote successful folding and identifying the conditions that lead to misfolding, this project will pave the way for new advancements in membrane protein biogenesis research. These insights will drive breakthroughs in drug development, offering potential therapeutic applications for diseases associated with AQP dysfunction.

References

Pellowe et al, 10.1101/2024.09.19.613857.

Kitchen et al, 10.1016/j.cell.2020.03.037.

Passchier et al, 10.1093/brain/awad146.

Pellowe et al, 10.1021/acs.biochem.0c00423.

Lu et al, doi.org/10.1091/mbc.11.9.2973.