The 5′-proximal region of Potato virus X RNA involves the potential cap-dependent “conformational element” for encapsidation

•Earlier we showed the crucial role of the PVX RNA 5′ cap structure for vRNP assembly.•The removal of the cap structure from PVX RNA does not prevent the vRNP assembly.•The efficiency of the vRNP assembly with decapped RNA was not decreased.•The cap structure by itself does not act as a signal for vRNP assembly.•We presume that capping creates a stable “conformational encapsidation signal”.

Filamentous helical Potato virus X (PVX) can be regarded as one of the well-studied viruses. Nevertheless, some aspects of the PVX assembly remained obscure. Previously, we have shown that the presence of a cap structure at the 5′ end of PVX RNA is indispensable for assembly of viral ribonucleoprotein (vRNP) particles varying in length. Here, most significantly, removal of the cap structure from previously capped PVX RNA did not affect the efficiency of decapped RNA molecules to be assembled into vRNP. This result provided evidence that the cap structure by itself does not act as a signal for initiation of vRNP assembly. These observations allowed to presume that the capping triggers some spatial changes in the 5'-proximal site of PVX RNA creating a “conformational encapsidation signal for vRNP assembly”, which is capable of triggering vRNP assembly in the absence of cap structure. Apparently, during capping the 5'-proximal segment of PVX RNA acquires a unique conformation which is stable to be retained even after cap removal.