A luminal proteome of the endoplasmic reticulum and Golgi apparatus reveals a novel modulator of ER stress tolerance in African trypanosomes

African trypanosomes employ specialised mechanisms of membrane trafficking as a key strategy to persist in both the mammalian host and insect vector. Their survival and pathogenicity rely on the continuous synthesis and surface delivery of extremely abundant surface coat proteins, imposing an extraordinary biosynthetic burden on the secretory pathway. Despite this, the luminal proteome of the T. brucei endoplasmic reticulum (ER) and Golgi apparatus remains incompletely characterised. Here, we exploit TurboID proximity biotinylation, using the abundant ER chaperone BiP (Binding-immunoglobulin protein) as luminal bait to map the ER proteome in bloodstream and procyclic lifecycle stages of Trypanosoma brucei. Comparison with BiPN, a truncated secretory form of BiP that transits the Golgi, provides differential compartmental labelling, together identifying 366 (BiP) and 428 (BiPN) proximity partners respectively and encompassing established ER quality control machinery, secretory cargo, and Golgi proteins. Quantitative ranking of BiP labelling intensity identifies a cohort of candidate BiP interactors: the most strongly enriched is Tb927.5.1160, a protein sharing structural homology with the mammalian BiP nucleotide exchange inhibitor MANF (mesencephalic astrocyte-derived neurotrophic factor). Endogenous mNeonGreen tagging confirms ER localisation of TbMANF in both life cycle stages, and reciprocal manipulation of its abundance by RNAi and inducible expression produces opposing shifts in cellular sensitivity to ER stress. These data are consistent with a role in regulating BiP ATPase cycling in an organism that, unlike yeast and mammals, lacks a canonical unfolded protein response, making TbMANF the first candidate regulator of BiP activity identified in kinetoplastids. Finally, TurboID proximity labelling anchored at the inner face of the nuclear pore via NUP65 extends our endomembrane map to the inner nuclear membrane, identifying candidate proteins of this specialised ER-continuous domain. AUTHOR SUMMARYAfrican sleeping sickness is caused by Trypanosoma brucei, a parasite that survives in the mammalian bloodstream by constantly renewing its protective protein coat. To synthesise and export this surface coat, the parasite relies on two intracellular compartments, the endoplasmic reticulum (ER) and the Golgi apparatus, which function as a quality control and sorting factory for proteins entering the secretory pathway. However, the identity of the proteins that populate these compartments in blood-stage parasites, and that maintain functioning under stress conditions, has remained poorly mapped. Here, we used an enzyme-based proximity labelling strategy that identifies neighbouring proteins in live cells without disturbing their targeting signals, generating a comprehensive protein inventory of both compartments across the two main T. brucei lifecycle stages. Among the most strongly labelled proteins was Tb927.5.1160, a protein structurally related to a mammalian regulator of the master ER chaperone BiP. Reducing or increasing the abundance of Tb927.5.1160 in parasites produced opposite changes in ER stress tolerance, identifying it as a candidate modulator of ER homeostasis in a lineage that regulates protein quality control through mechanisms distinct from those operating in yeast or human cells. Together, our findings provide a new molecular resource for understanding how T. brucei sustains secretory pathway function under the biosynthetic demands of mammalian and insect host infection.