Browsing by Subject "Plant virology"

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  • De, Swarnalok (Helsingin yliopisto, 2019)
    Potyviral helper component proteinase (HCPro) is a quintessential example of a multifunctional viral protein. Its name comes from two of its earliest identified functions- ‘Helper Component’ involved in aphid-mediated plant-plant transmission of the virus, and cysteine proteinase responsible for its self-cleavage from the rest of the viral polyprotein. HCPro’s best-studied function is its ability to suppress RNA silencing. One of the factors underlying the multifunctionality of HCPro is its ability to interact with a wide range of host factors causing perturbations in several cellular pathways. In this study, interaction of HCPro with the host proteins S-adenosyl-L-methionine synthase (SAMS), S-adenosyl-L homocysteine hydrolase (SAHH), ARGONAUTE 1 (AGO1) and VARICOSE (VCS) was addressed and implications of these interactions in potato virus A (PVA; genus Potyvirus) infection, and potato virus X (PVX)-PVA mixed infection were studied. In this study, HCPro was found to interact with the host methionine cycle enzymes SAMS and SAHH and to inhibit SAMS activity. Disruption of the methionine cycle promoted PVA infection. Methionine cycle plays a crucial role in the smooth running of RNA silencing by providing S-adenosyl methionine (SAM) for methyltransferase hua enhancer 1 (HEN1). Small RNA (sRNA) duplexes, which are methylated by HEN1, are stable and capable to act in RNA silencing, whereas, the unmethylated sRNAs are polyuridylated and targeted to degradation. A blockage in sRNA methylation via HCPro-mediated methionine cycle disruption was proposed to act as a circuit breaker of RNA silencing pathway for the benefit of PVA infection. SAHH was also found to be involved in PVX-PVA synergism. Blockage of the methionine cycle at SAHH, coupled with synergism-specific downregulation of closely associated glutathione (GSH) biosynthesis pathway enhanced PVX genomic RNA accumulation and subgenomic RNA expression. Moreover, depletion of cellular antioxidant GSH was suggested to be the reason behind induction of severe oxidative stress during potex–potyvirus mixed infection. In another line of this study, formation of HCPro-associated high molecular weight (HMW) complexes and their functions were studied. Interaction between HCPro and a WD40 domain containing scaffolding protein VCS was shown to be crucial for formation and stability of HCPro-associated HMW complexes. Importance of HCPro-VCS interaction in governing the assembly of PVA-induced granules (PGs) was demonstrated. This study reinforced the correlation between the PGs and RNA silencing suppression. Interestingly, HCPro, AGO1, VPg and CI were detected in the ribosome-associated HMW-complexes. Association of AGO1 with ribosomes may indicate occurrence of RISC-mediated translational repression as an additional defense mechanism against PVA infection. While, presence of HCPro, VPg and CI therein suggested a putative mechanism by which HCPro derived ribosome-associated HMW complexes might participate in relieving PVA translational repression. Accordingly, co-operation between HCPro, VCS and VPg was shown to act in favor of active PVA translation. Intriguingly, importance of HCPro-VCS interaction was also found to be important in PVA encapsidation. In conclusion this study provides evidence for interaction between HCPro and host proteins SAMS, SAHH, VCS and AGO1 in planta. Furthermore, importance of these interactions are demonstrated to play crucial role in governing various viral processes during PVA infection.