We present a comprehensive overview of the dependency of several Old World alphaviruses for the host protein G3BP. Based on their replication ability in G3BP-deleted cells, Old World alphaviruses can be categorized into two groups, being either resistant or sensitive to G3BP deletion. We observed that all sensitive viruses have an Arg residue at the P4 position of the cleavage site between nsP1 and nsP2 regions of the replicase precursor polyprotein (1/2 site), while a different residue is found at this site in viruses resistant to G3BP deletion. Swapping this residue between resistant and sensitive viruses also switches the G3BP deletion sensitivity. In the absence of G3BP, CHIKV replication is at the limit of detection. Substitution of P4 Arg-to-His partially rescues this defect. The P4 residue of the 1/2 site is known to play a regulatory role during processing at this site and we found that if processing is blocked, the influence of the P4 residue on the sensitivity to G3BP deletion is abolished. Immunofluorescence experiments with CHIKV replicase with manipulated processing indicate that synthesis of double-stranded RNA is defective in the absence of G3BP and suggest a role of G3BP during negative-strand RNA synthesis. This study provides a functional link between the host protein G3BP and the P4 residue of the 1/2 site for viral RNA replication of Old World alphaviruses. While this suggests a link between G3BP proteins and viral replicase polyprotein processing, we propose that G3BP proteins do not have a regulatory role during polyprotein processing.
IMPORTANCE Old World alphaviruses comprise several medically relevant viruses, including chikungunya virus and Ross River virus. Recurrent outbreaks and the lack of antivirals and vaccines demands ongoing research to fight the emergence of these infectious diseases. In this context, thorough investigation of virus-host interactions is critical. Here we highlight the importance of the host protein G3BP for several Old World alphaviruses. Our data strongly suggest that G3BP plays a crucial role for the activity of the viral replicase and thus, the amplification of the viral RNA genome. To our knowledge, the present work is the first to provide a functional link between the regulation of viral polyprotein processing and RNA replication and a host factor for alphaviruses. Moreover, the results of this study raise several questions about the fundamental regulatory mechanisms that dictate the activity of the viral replicase, thereby paving the way for future studies.