Structural insights into viral genome replication by the Severe fever with thrombocytopenia syndrome virus L protein

The large (L) protein of severe fever with thrombocytopenia syndrome virus (SFTSV) plays a key role in catalyzing viral genome replication and transcription. Nonetheless, conformational changes of SFTSV L protein in complex with viral RNA, during viral genome replication and transcription are unclear. Cryo-electron microscopy, in addition to in vitro biochemical and cell-based mini-replicon assays, was used to study structural changes of the SFTSV L-protein during viral genome replication, from pre-initiation to late-stage elongation. The study reveals that the L protein undergoes several conformational changes upon binding 5' RNA in a hook-like conformation. During early-elongation, the 5' and 3' RNA form a distal duplex structure and the template RNA 3' terminus is positioned to feed on the L protein core, particularly in the RNA dependent RNA polymerase active site. The 5' and 3' RNA distal duplex structure however dissolves as the L protein moves from early-elongation to late-stage elongation. The L-protein stalls at different stages of elongation and the L protein core opens to accommodate a 10bp product template duplex. In late-stage elongation, the template duplex splits and moves towards a designated 3' secondary binding site. The template and progeny RNA exit through template and progeny exit channels respectively.

(LML)