Model on the role of ORP and VAP proteins and MCSs in tombusvirus replicase assembly.

<p>The ORP proteins, such as the yeast Osh6p, are recruited via binding to the tombusviral p33 replication protein and the ER-bound Scs2p VAP to MCS, formed between the ER and peroxisomal membranes. The ORPs then facilitate the enrichment of sterols in the peroxisomal membrane (or mitochondrial membrane in case of CIRV), forming sterol-rich microdomains needed for the formation of virus-induced spherules. These spherules contain the VRCs performing viral RNA synthesis. Note that the MCSs are not part of the VRCs based on the difference in BiFC localization of p33:Ssa1p Hsp70 complex and p33:Scs2p, which suggests that different subpopulations of p33 are involved in each interaction. We have previously shown that the Ssa1p is present in the active VRCs.