Structural and Mechanistic Studies of Membrane Protein Biogenesis and Quality Control at the Endoplasmic Reticulum

Author: Nguyen, Vy Ngoc Mai

Year: 2025

Degree: Dissertation (Ph.D.)

Advisor: Voorhees, Rebecca M.

Committee Members: Clemons, William M.; Rees, Douglas C.; Bjorkman, Pamela J.; Voorhees, Rebecca M.

Option: Biochemistry and Molecular Biophysics

DOI: 10.7907/ar35-w658

Abstract

Membrane proteins make up around 30% of the human proteome and carry out essential functions in the cell, including but not limited to signaling, cell adhesion, and metabolic transport. To maintain homeostasis, the eukaryotic cell has evolved complex pathways for membrane protein biogenesis and quality control, as they contain hydrophobic transmembrane domains (TMDs) that can easily aggregate in the aqueous environment of the cytosol causing cellular damage. At the endoplasmic reticulum (ER), the ER membrane protein complex (EMC) co-translationally inserts the first TMD of multipass membrane protein and post-translationally inserts tail-anchored membrane proteins. In this thesis, we are able to show how the EMC can coordinate with the other protein machineries of the multipass translocon (such as the back-of-Sec61/BOS complex) at the ER to accommodate the insertion of diverse multipass membrane protein substrates, depending on the biophysical properties of their N-terminal soluble domain. We also structurally characterize the EMC•BOS holocomplex, highlighting its spatial relation with the other biogenesis factors at the multipass translocon. In addition, this thesis also explores how a novel quality control factor TXNDC15 functions in ER-associated degradation (ERAD) to facilitate degradation of unassembled membrane proteins. We found that TXNDC15 works with the E3 ligase MARCHF6 and recognizes a set of membrane proteins with exposed hydrophobic domain in the ER lumen. Lastly, we were able to also link TXNDC15’s quality control function to its implication in ciliopathies, such as Meckel-Gruber syndrome.

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