Rabbit hemorrhagic disease virus (RHDV) is an important member of the Caliciviridae family and cannot be propagated in vitro , which has greatly impeded progress of investigating its replication mechanism. Construction of an RHDV replicon system has recently provided a platform for exploring RHDV replication in host cells. Here, aided by this replicon system and using two-step affinity purification, we purified the RHDV replicase and identified its associated host factors. We identified rabbit nucleolin (NCL) as a physical link required for the formation of RHDV replication complexes (RCs), by mediating the interaction between other host proteins and the viral RNA replicase, RNA-dependent RNA polymerase (RdRp). We found that RHDV RdRp uses an amino acid (aa) region spanning residues 448–478 to directly interact with NCL’s RNA-recognition motif 2. We also found that the viral p16 protein uses a highly conserved region ( 35 Cys–Ile–Arg–Ala 38 or CIRA motif) to specifically bind the N-terminal region of NCL (aa 1–110) and that RHDV p23 uses a specific domain (aa 90–145) to bind NCL’s  RNA-recognition motif 1. Disrupting these protein–protein interactions severely weakened viral replication. Furthermore, NCL overexpression or knockdown significantly increased or severely impaired, respectively, RHDV replication. Collectively, these results indicate that the host protein NCL is essential for RHDV replication and plays a key role in the formation of RHDV RCs. The mechanisms by which NCL promotes viral replicase assembly reported here shed light on viral RC biogenesis and may inform antiviral therapies.

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