Sugarcane streak mosaic virus P1 protein inhibits unfolded protein response through direct suppression of bZIP60U splicing.

The unfolded protein response (UPR) is a cell-designated strategy that maintains the balance of protein folding in the endoplasmic reticulum (ER). UPR features a network of signal transduction pathways that reprogram the transcription, mRNA translation, and protein post-translational modification to relieve the ER stresses from unfolded/misfolded proteins. Infection with plant viruses can induce the UPR, and activated UPR often promotes plant viral infections in turn. However, the mechanism used by plant viruses to balance UPR and achieve robust infection remain largely unknown. In this study, P1SCSMV was identified as a virus-encoded RNA silencing suppressor (VSR). Heterologous overexpression of P1SCSMV via potato virus X (PVX) was found lead to programmed cell death (PCD) in Nicotiana benthamiana. Furthermore, P1SCSMV was also found to inhibit the PVX infection-triggered UPR by downregulating UPR-related genes and directly induced the distortion and collapse of the ER polygonal meshes on PVX-P1SCSMV infected N. benthamiana. Moreover, self-interaction, VSR activity, UPR inhibition, and cell death phenotype of P1SCSMV were also found to be dependent on its bipartite nuclear localization signal (NLS) (251RKRKLFPRIPLK262). P1SCSMV was found to directly bind to the stem-loop region of NbbZIP60U via its NLS and inhibit the UPR pathways, ultimately resulting in a PCD phenotype in PVX-P1SCSMV infected N. benthamiana leaves. This study also revealed the balancing role of potyviruses encoded P1SCSMV in the UPR pathway to achieve robust viral infection. This may represent a novel virulence strategy for plant viruses. Author summary: Unfolded protein response (UPR) is a cell designated strategy that maintains the balance of protein folding in the endoplasmic reticulum (ER). UPR contains a network of signal transduction pathways that reprogram the transcription, mRNA translation, and protein post-translational modification to relieve the ER stresses from unfolded/misfolded proteins. Plant viruses' infection induce the UPR, and activated UPR often promotes plant viral infection in turn. However, it is unclear how plant viruses balance the UPR to achieve robust infection. In this study, P1SCSMV was identified as a virus-encoded RNA silencing suppressor (VSR). P1SCSMV was found to inhibit PVX infection-triggered UPR by directly binding to the stem-loop region of NbbZIP60U with its bipartite nuclear localization signal (NLS) and inhibits the UPR pathways, thus leading to the distortion and collapse of the ER polygonal meshes and the cell-death phenotype on PVX-P1SCSMV infected N. benthamiana. The self-interaction, VSR activity, UPR inhibition, and cell-death phenotype of P1SCSMV are dependent on its NLS (251RKRKLFPRIPLK262). This study reveals the balancing role to the UPR pathway by potyviruses encoded P1SCSMV, which enriched the multi-functionality of the viral-encoded VSRs. This may represent a novel virulence strategy by plant viruses. [ABSTRACT FROM AUTHOR]