Translational activation of encapsidated potato virus X RNA by coat protein phosphorylation

Previously we showed that encapsidated potato virus X (PVX) RNA is nontrans tatable in vitro, but can be converted into a translatable form after bindi ng to PVX particles of PVX-coded movement protein, the product of the first gene of triple gene block (TGBp1). Here we report that a similar effect oc curs via in situ phosphorylation of the PVX coat protein (CP) by Ser/Thr pr otein kinase (PK) C, the mixture of casein kinases I and II or by cytoplasm ic PK(s) from Nicotiana glutinosa leaves. Immunochemical analyses indicated that phosphorylation induced conformational changes in PVX CP. The N-termi nal region of the PVX CR rich in Ser and Thr residues, is exposed at the vi rion surface and can be removed by treatment with trypsin. We showed that ( i) trypsin treatment removed the bulk of P-32-radioactivity from in situ ph osphorylated PVX CP, (ii) PVX containing N-terminally truncated CP (PVX-Ptd ) failed to be translationally activated by phosphorylation, and (iii) the specific infectivity of PVX-Ptd was reduced. However, the PVX-Ptd RNA remai ned intact and PVX-Ptd could be translationally activated by the PVX MP TGB p1, We hypothesize that phosphorylation of the parental PVX by cytoplasmic PK(s) in vivo renders PVX RNA translatable in primary inoculated cells, whe reas translational activation of the progeny virions destined for plasmodes mata trafficking is triggered by TGBp1. (C) 2001 Academic Press.