The phosphorylation state of poly(A)-binding protein specifies its bindingto poly(A) RNA and its interaction with eukaryotic initiation factor (eIF)4F, eIFiso4F, and eIF4B

The poly(A)-binding protein (PABP) interacts with the eukaryotic initiation factor (eIF) 4G (or eIFiso4G), the large subunit of eIF4F (or eIFiso4F) to promote translation initiation. In plants, PABP also interacts with eIF4B, a factor that assists eIF4F function. PABP is a phosphoprotein, although t he function of its phosphorylation has not been previously investigated. In this study, we have purified the phosphorylated and hypophosphorylated iso forms of PABP from wheat to examine whether its phosphorylation state affec ts its binding to poly(A) RNA and its interaction with eIF4G, eIFiso4G, or elF4B, Phosphorylated PABP exhibited cooperative binding to poly(A) RNA eve n under non-stoichiometric binding conditions, whereas multiple molecules o f hypophosphorylated PABP bound to poly(A) RNA only after free poly(A) RNA was no longer available. Together, phosphorylated and hypophosphorylated PA BP exhibited synergistic binding. eIF4B interacted with PABP in a phosphory lation state-specific manner; native eIF4B increased the RNA binding activi ty specifically of phosphorylated PABP and was greater than 14-fold more ef fective than was recombinant eIF4B, whereas eIF4F promoted the cooperative binding of hypophosphorylated PABP, These data suggest that the phosphoryla tion state of PABP specifies the type of binding to poly(A) RNA and its int eraction with its partner proteins.