Pre-initiation and elongation structures of full-length La Crosse virus polymerase reveal functionally important conformational changes

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ABSTRACT Bunyavirales is an order of segmented negative stranded RNA viruses comprising several life-threatening pathogens such as Lassa fever virus ( Arenaviridae ), Rift Valley Fever virus ( Phenuiviridae ) and La Crosse virus (LACV, Peribunyaviridae ) against which neither specific treatment nor licenced vaccine is available. Replication and transcription of Bunyavirales genome constitute essential reactions of their viral cycle that are catalysed by the virally encoded RNA-dependent RNA polymerase or L protein. Here we describe the complete high-resolution cryo-EM structure of the full-length (FL) LACV-L protein. It reveals the presence of key C-terminal domains, notably the cap-binding domain that undergoes large movements related to its role in transcription initiation and a zinc-binding domain that displays a fold not previously observed. We capture the structure of LACV-L FL in two functionally relevant states, pre-initiation and elongation, that reveal large conformational changes inherent to its function. We uncover the coordinated movement of the polymerase priming loop, lid domain and C-terminal region required for the establishment of a ten-base-pair template-product RNA duplex before strand separation into respective exit tunnels. The revealed structural details and dynamics of functional elements will be instrumental for structure-based development of compounds that inhibit RNA synthesis by the polymerase.

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