[Translational regulation of potato virus X RNA-coat protein complexes: the key role of a coat protein N-terminal peptide].

The efficiency of in vitro translation of potato virus X (PVX) RNA within vRNP complexes assembled from genomic RNA and viral CP was examined. The vRNP particles contain the 5'-proximal RNA segments encapsidated by helically arranged CP head-like portions heterogeneous in length and the CP-free RNA tail. Translation of RNA is completely repressed upon incubation with PVX CP and is accompanied by vRNP particles production. By contrast, translation is activated in vRNPs in vitro assembled using two CP forms, differing in the principals of their N-terminal peptides modification. The N-terminal peptide of PVX CP represents the major phosphorylation site(s) for Thr/Ser-specific protein kinases. It was shown that: (i) CP phosphorylation results in a translational activation of vRNP; (ii) removal of N-terminal peptide from CP abolished activation and CP retains the translation repressing ability. It was suggested that substitution of Ser/Thr residues by non-phosphorylated Ala/Gly in N-terminal peptide of the mutant CP will led to a complete inhibition of vRNP translation. However, opposite results were obtained in our experiments: (i) RNA of such mutant virus (PVX-ST) was efficiently translated within the virions; (ii) RNA of a wild-type (wt) PVX also efficiently translated in mixedly assembled vRNP "wt PVX RNA + PVX-ST CP"; (iii) opposite result (repression of translation) was obtained with "mixed" vRNP (PVX-ST RNA + wtPVX CP). Therefore, the N-terminal peptide located at the surface of the particle and of the particles plays a key role in translation activation of the RNA encapsidated in vRNP and native virions.