Figure 1.

PS-mediated packaging mechanisms. (A) High-affinity binding mechanism. Selective packaging of the genomic RNA (gRNA) is mediated by high-affinity binding between the capsid protein (CP) and the packaging signal (PS). In this scenario the PS is one small RNA element (and/or sequence) present only in the gRNA. (B) PS allosteric mechanism. Binding of the CP to the PS is an allosteric switch that changes the conformation of the CP from one with weak CP-CP interactions (CPi) to one strong interactions (CPa). (C) Multiple PS-like elements. There is one PS and a series of PS-like elements distributed across the gRNA. The interaction between the PS and the CP (a) are stronger than between the PS-like elements and the CP (b). Unlike the first two scenarios, all these clusters contribute to the nucleation of virion assembly. In all cases the unstructured tail represents the CP N-terminal domain. For most (+)ssRNA viruses the N-terminal domain of the CP is extremely basic and is required for RNA/CP interactions and RNA packaging.