Dissecting the quality control pathways at the ER translocon

The National Institute of General Medical Sciences, part of the National Institutes of Health within the Department of Health and Human Services, awarded Texas Tech University Health Sciences Center $492,625 under CFDA 93.859 (Biomedical Research and Research Training) on September 1, 2026, to support research on quality control pathways governing protein translocation and folding in the endoplasmic reticulum.

The research focuses on understanding molecular mechanisms regulating protein movement through the SEC61 translocon, a cellular channel critical for the biogenesis of secretory and membrane proteins comprising approximately 30 percent of the human proteome. The investigator previously identified a complex linking the ER stress sensor IRE1A, the SEC61 translocon, and its partner protein SEC63, and demonstrated that SEC63 inhibits IRE1A by promoting chaperone recruitment. Under this project grant, work will define how the SEC61/SEC63 complex regulates IRE1A signaling during ER stress, including structural analysis of recombinant IRE1A/SEC61/SEC63 complexes using cryo-electron microscopy and investigation of how this complex protects pancreatic beta cells from ER stress-induced apoptosis. The research will also examine how SEC63 alleviates translocation pauses by promoting chaperone binding to nascent chains, coupling efficient translocation with proper protein folding.

Performance is based in Lubbock, Texas. The award period runs from September 1, 2026, through May 31, 2031. The assistance type is Project Grant.

Abstract

My research program focuses on understanding the quality control mechanisms that govern protein translocation and folding in the endoplasmic reticulum (ER). The ER is a vital cellular organelle that provides a specialized environment for folding secretory and membrane proteins, which comprise ~30% of the human proteome. Most of these proteins enter the ER CO-translationally through the SEC61 translocon. Disruptions in SEC61-mediated translocation compromise cellular health and are linked to diseases such as polycystic liver disease, type 2 diabetes, and neurodegenerative disorders. Despite its critical role, the molecular mechanisms regulating protein translocation, folding, and quality control at the SEC61 translocon remain poorly defined. Our group was the first to uncover a direct connection between protein translocation and the unfolded protein response (UPR). We identified a novel complex consisting of the ER stress sensor IRE1Α, the SEC61 translocon, and its partner protein SEC63. IRE1Α is a transmembrane kinase and rnase that mediates UPR signaling to mitigate ER stress, but its hyperactivation can lead to apoptosis. We demonstrated that SEC63 inhibits IRE1Α oligomerization and rnase activity by facilitating BIP chaperone recruitment. Building on this discovery, project 1 will define how the SEC61/SEC63 complex regulates IRE1Α signaling during ER stress. Our preliminary data include a newly developed mammalian expression system for producing recombinant IRE1Α/SEC61/SEC63 complexes, which will be structurally analyzed using cryo-EM. We will also investigate how this complex protects pancreatic beta cells from ER stress-induced cell death. Beyond regulating IRE1Α, we found that SEC63 alleviates translocation pauses for specific substrates by promoting BIP binding to nascent chains, thereby coupling efficient translocation with proper folding. Under BIP deficiency, non-translocated proteins (ntps) accumulate at the SEC61 translocon, compromising cellular fitness. Our preliminary studies identified UBXN4 as a novel SEC61-interacting protein that facilitates the removal of these ntps. Project 2 will investigate how UBXN4 dislodges ntps without destabilizing the translocon and how IT targets them for proteasomal degradation. Ribosome stalling at the SEC61 translocon presents another challenge to ER protein synthesis. We recently demonstrated that the ribosome-associated quality control (RQC) pathway ubiquitinates nascent chains on stalled ribosomes at the ER membrane. Unexpectedly, contrary to prevailing models, we found that this ubiquitination is not required for nascent chain release. Project 3 will identify the molecular players responsible for resolving ribosome stalling at the translocon. Together, these studies will elucidate key quality control pathways at the SEC61 translocon, advancing our understanding of ER protein homeostasis and offering new insights for therapeutic strategies targeting human diseases linked to translocation defects.