Viral cross-linking and solid-phase purification enables discovery of ribonucleoprotein complexes on incoming RNA virus genomes

The initial interactions between incoming, pre-replicated RNA virus genomes and host protein factors are important in infection and immunity. Yet there are no current methods to study these crucial events. We established VIR-CLASP (VIRal Cross-Linking And Solid-phase Purification) to identify the primary viral RNA-host protein interactions. First, host cells are infected with 4SU-labeled RNA viruses and irradiated with 365 nm light to crosslink 4SU-labeled viral genomes and interacting proteins from host or virus. The cross-linked RBPs are purified by solid-phase reversible immobilization (SPRI) beads with protein denaturing buffers, and then identified by proteomics. With VIR-CLASP, only the incoming viral genomes are labeled with 4SU, so cross-linking events specifically occur between proteins and pre-replicated viral genomic RNA. Since solid-phase purification under protein-denaturing conditions is used to pull-down total RNA and cross-linked RBPs, this facilitates investigation of potentially all RNA viruses, regardless of RNA sequence. Preparation of 4SU-labeled virus takes [~]7 days and VIR-CLASP takes 1 day.