Tombusvirus replication depends on Sec39p endoplasmic reticulum-associated transport protein.

Positive-stranded RNA viruses subvert subcellular membranes to built viral replicases complexes (VRCs) in infected cells. Tombusviruses use peroxisomal membranes for the assembly of their VRCs and they can efficiently switch to the endoplasmic reticulum membrane in the absence of peroxisomes. In this paper, we show that the ER-resident Sec39p vesicular transport protein is critical for the formation of active VRCs in yeast model host. Repression of Sec39p expression in yeast or in plants resulted in greatly reduced tombusvirus accumulation. Moreover, the purified tombusvirus replicase from Sec39p-depleted yeast cells showed low in vitro activity. Also, tombusvirus RNA replication was poor in cell-free extracts or in isolated ER membranes from yeast with repressed Sec39p expression. The tombusvirus p33 replication protein was mislocalized to the ER when Sec39p was depleted in yeast. Overall, Sec39p is the first peroxisomal biogenesis protein characterized that is critical for tombusvirus replication in yeast and plants.