A trefoil knot mimic and multiple pore states define the mechanism of a split-shell Tc toxin.

Cryo-EM structures of the split-shell Yersinia entomophaga YenTc reveal a trefoil knot mimic and three pore states, including an alkaline pH lollipop state.

• Why it matters: Split-shell Tc toxins are structurally distinct from well-studied fused-shell forms, leaving their injection mechanism and pore dynamics unclear.
• What they did: The authors solved cryoelectron microscopy structures of YenTc in multiple conformations at 2.6–3.7 Å, showing YenChi2’s role in shell expression and N-terminal stabilization, plus trefoil-knot-mimic and pore-state transitions.
• The result: They identify conventional, inverted, and alkaline-induced lollipop pore states, with the lollipop state involving full dissociation of the YenA1–YenChi2 subcomplex, and find all three states also in Photorhabdus luminescens PTC3’s TcA pentamer.

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