The cap-independent translational regulation of NbRabGAP1 in Nicotiana benthamiana during BaMV infection

While cap-dependent translation is the eukaryotic norm, alternative pathways enable selective protein synthesis during stress. We describe a cap-independent mechanism regulating NbRabGAP1, a host factor essential for bamboo mosaic virus (BaMV) movement in Nicotiana benthamiana. The 5' untranslated region (5' UTR, 553-nt) of NbRabGAP1 contains three upstream open reading frames (uORFs) that typically hinder ribosome scanning. Using bicistronic reporter assays, we identified a functional internal ribosome entry site (IRES) within nucleotides 61-150. This IRES features a predicted stable Y-shaped RNA structure and a purine-rich R-motif. We demonstrate that NbPAB8, a poly(A)-binding protein, specifically binds this R-motif to enhance IRES-mediated translation over 2.5-fold. During BaMV infection, NbPAB8 expression is transiently upregulated, coinciding with maintained NbRabGAP1 levels despite potential repression of global cap-dependent translation. This represents a regulatory switch where viral infection triggers a PAB8-dependent IRES mechanism to sustain the vesicle trafficking machinery required for viral movement. Our findings reveal how viral stress rewires host translational control through specific RNA-protein interactions, ensuring the synthesis of critical host factors via IRES-mediated pathways. HighlightDuring bamboo mosaic virus infection, NbPAB8 binds a purine-rich IRES in the NbRabGAP1 5' UTR, bypassing global translation repression to maintain vesicle trafficking essential for viral movement.