The parasite that causes sleeping sickness has a remarkable trick for staying invisible to the immune system. It carries the largest gene family in nature — more than 2,600 versions of a surface coat protein — and it switches between them constantly, like a spy cycling through disguises. But the mechanism behind how it chooses which disguise to wear, while keeping all the other options locked away, has eluded biologists for four decades. A team at the University of York thinks they finally have the answer, and it turns out the parasite is running a molecular paper shredder inside its own cell.
The work, posted as a preprint to bioRxiv, centers on a protein called ESB2 — Expression-Site-Body protein 2 — that Joana Correia Faria's lab at York identified as an active RNA endonuclease. ESB2 sits in the parasite's expression site body, the compartment where virulence genes are actively read, and it destroys the transcripts of every gene the parasite isn't currently using. The ones it needs stay intact. Everything else gets shredded in real time. The team showed this by depleting ESB2 in Trypanosoma brucei, the parasite that causes human African trypanosomiasis, and watching ESAG transcripts — the genes it normally keeps quiet — accumulate up to elevenfold.
The finding resolves what Faria had privately called a cold case since her postdoc years. "The mystery of how this parasite manages the asymmetric expression of its genetic manual has been a cold case in the back of my mind since my days as a postdoc," she said in a statement. "To finally solve it now, as the first major output of my own lab here at York, is incredibly rewarding." Correia Faria is a Sir Henry Dale Fellow and lecturer in York's Department of Biology.
The numbers involved are striking even by parasitology standards. VSG mRNA and its encoded protein together represent five to ten percent of everything in a bloodstream-form trypanosome — roughly ten million molecules on the cell surface alone. The genes it keeps silent, the ESAGs, are transcribed from the same genetic operon at more than 140-fold lower levels, despite being in the same transcriptional unit. The parasite essentially runs two programs simultaneously on the same stretch of DNA, then post-transcriptionally destroys one of them. ESB2 is the executioner.
What makes ESB2 unusual, and potentially useful for drug development, is its structural resemblance to a human protein called SMG6. SMG6 is a core component of nonsense-mediated decay — the system human cells use to find and destroy faulty messenger RNA before it produces truncated, potentially harmful proteins. ESB2 performs an analogous function in the parasite, except it's targeting virulence transcripts rather than defective ones. The structural homology suggests the parasite co-opted a conserved eukaryotic RNA surveillance architecture and pointed it at a different job.
"The closest experimentally determined structural homologue occurred in human SMG6," the paper notes — a critical component of nonsense-mediated decay. ESB2 recruitment to the expression site body follows a cascade initiated by VEX2 — the protein that anchors the whole system to the expression site — which recruits ESAP1, which recruits ESB3, which then recruits ESB2. The researchers mapped this hierarchy using a combination of genetic tools and super-resolution microscopy.
This is not purely a basic-science result. Sleeping sickness — human African trypanosomiasis, caused by T. brucei — has declined dramatically thanks to fexinidazole, the first all-oral drug added to WHO guidelines in 2024 as first-line for T.b. rhodesiense cases, and a broader treatment push that brought reported cases below 1,000 per year by 2018, down from nearly 40,000 in 1998, according to the WHO). But the disease is not gone, and existing drugs have real limitations. Fexinidazole requires multiple days of dosing and doesn't work at all stages. The remaining cases are concentrated in some of the most remote parts of sub-Saharan Africa, where diagnostics and treatment access remain inadequate.
The T.b. gambiense form accounts for 92 percent of reported cases and causes a chronic illness that can take months to manifest symptoms; the faster-acting T.b. rhodesiense accounts for 8 percent. Both are transmitted by tsetse flies and both depend, at least in part, on antigenic variation — the VSG switching system — to maintain chronic infection. If you could disrupt the parasite's ability to selectively destroy unwanted VSG transcripts, you might break the switching mechanism. That is a speculative therapeutic angle, not a drug target, but it's the kind of connection that makes parasitologists pay attention.
"ESB2, ESB3 and ESAP1 depletion led to a striking and specific upregulation of all ESAG transcripts from the active VSG-ES," the paper reports — language that signals the team knows they've found something the field will want to interrogate. The research was funded by the Wellcome Trust, the BBSRC, and the European Research Council, among other bodies. It has not yet entered animal model testing for therapeutic purposes.
The broader lesson from the paper, if there is one, is that even a well-studied parasite can harbor fundamental mechanisms that went undetected because the right tools weren't available. Trypanosomes have been under scientific scrutiny since the early 20th century. Their antigenic variation system was described in detail decades ago. What was missing was the molecular explanation for how the asymmetry was maintained at the RNA level — and that explanation turns out to involve a protein that looks like part of the cell's own quality-control machinery, repurposed as an instrument of immune evasion. The parasite didn't invent a new molecular tool. It borrowed one from us and pointed it somewhere unexpected.
