Authors

J Correia Faria¹;  ¹ Biology Department & York Biomedical Research Institute, University of York, UK

Discussion

African trypanosomes are ‘masters of disguise'. They rely on a vast genetic repertoire (>2,600 genes and pseudogenes) encoding for their variant surface glycoprotein (VSG) to undergo antigenic variation and successfully evade the host immune response. Their ability to express a single VSG at any given time, known as monogenic expression, is imperative for successful antigenic variation; yet mechanisms governing this complex process are not fully understood in any eukaryote. In Trypanosoma brucei bloodstream-form, the single active-VSG is transcribed by RNA-Polymerase I within a dedicated sub-nuclear compartment designated expression-site body (ESB). To date, only two proteins have been shown to specifically localise to the ESB: ESB1, a stage-specific transcriptional activator; and VSG-exclusion-2 (VEX2), a large and enigmatic RNA:DNA helicase that is critical for VSG monogenic expression. Further, VEX2 integrates transcription and splicing, whereby the active-VSG gene establishes a stable inter-chromosomal interaction with a locus involved in trans-splicing, therefore enabling high levels of VSG expression.
We sought to define the spatial interactome within the ESB using TurboID-mediated proximity labelling combined with quantitative LC-MS/MS analysis. We found new components of the SL-array and NUFIP bodies, several ESB-enriched proteins, but most notably, three novel ESB-specific components. Moreover, these three novel factors are compartmentalised at the ESB in a VEX2-dependent manner, two are stage-specific and likely to be involved in splicing and RNA decay.
Transcriptomic analysis following the depletion of either of these three factors shows a specific role in the regulation of expression-site-associated-genes (ESAGs) expression. Notably, it has long been known that ESAGs upstream of VSGs, despite co-transcription in the same polycistron, yield far less abundant transcripts, and no factor responsible for this differential control has previously been identified.