Activation of Tomato bushy stunt virus RNA-dependent RNA polymerase by cellular heat shock protein 70 is enhanced by phospholipids In vitro

Similar to other positive-strand RNA viruses, tombusviruses are replicated by the membrane-bound viral replicase complex (VRC). The VRC consists of the p92 virus-coded RNA-dependent RNA polymerase (RdRp), the viral p33 RNA chaperone, and several co-opted host proteins. In order to become a functional RdRp after its translation, the p92 replication protein should be incorporated into the VRC, followed by its activation. We have previously shown in a cell-free yeast extract-based assay that the activation of the Tomato bushy stunt virus (TBSV) RdRp requires a soluble host factor(s). In this article, we identify the cellular heat shock protein 70 (Hsp70) as the co-opted host factor required for the activation of an N-terminally truncated recombinant TBSV RdRp. In addition, small-molecule-based blocking of Hsp70 function inhibits RNA synthesis by the tombusvirus RdRp in vitro. Furthermore, we show that neutral phospholipids, namely, phosphatidylethanolamine (PE) and phosphatidylcholine (PC), enhance RdRp activation in vitro. In contrast, phosphatidylglycerol (PG) shows a strong and dominant inhibitory effect on in vitro RdRp activation. We also demonstrate that PE and PC stimulate RdRp-viral plus-strand RNA [(+)RNA] interaction, while PG inhibits the binding of the viral RNA to the RdRp. Based on the stimulatory versus inhibitory roles of various phospholipids in tombusvirus RdRp activation, we propose that the lipid composition of targeted subcellular membranes might be utilized by tombusviruses to regulate new VRC assembly during the course of infection.