The SNARE Protein Syp71 Is Essential for Turnip Mosaic Virus Infection by Mediating Fusion of Virus-Induced Vesicles with Chloroplasts

Taiyun Wei,Aiming Wang,Changwei Zhang,Hélène Sanfaçon,Xilin Hou,Peter D Nagy

doi:10.1371/journal.ppat.1003378

Abstract

All positive-strand RNA viruses induce the biogenesis of cytoplasmic membrane-bound virus factories for viral genome multiplication. We have previously demonstrated that upon plant potyvirus infection, the potyviral 6K2 integral membrane protein induces the formation of ER-derived replication vesicles that subsequently target chloroplasts for robust genome replication. Here, we report that following the trafficking of the Turnip mosaic potyvirus (TuMV) 6K2 vesicles to chloroplasts, 6K2 vesicles accumulate at the chloroplasts to form chloroplast-bound elongated tubular structures followed by chloroplast aggregation. A functional actomyosin motility system is required for this process. As vesicle trafficking and fusion in planta are facilitated by a superfamily of proteins known as SNAREs (soluble N-ethylmaleimide-sensitive-factor attachment protein receptors), we screened ER-localized SNARES or SNARE-like proteins for their possible involvement in TuMV infection. We identified Syp71 and Vap27-1 that colocalize with the chloroplast-bound 6K2 complex. Knockdown of their expression using a Tobacco rattle virus (TRV)-based virus-induced gene silencing vector showed that Syp71 but not Vap27-1 is essential for TuMV infection. In Syp71-downregulated plant cells, the formation of 6K2-induced chloroplast-bound elongated tubular structures and chloroplast aggregates is inhibited and virus accumulation is significantly reduced, but the trafficking of the 6K2 vesicles from the ER to chloroplast is not affected. Taken together, these data suggest that Syp71 is a host factor essential for successful virus infection by mediating the fusion of the virus-induced vesicles with chloroplasts during TuMV infection.

Highlights

The host endomembrane system directly contributes to the formation of virus-induced membrane-bound virus factory for positive-strand RNA virus replication [1,2]
Genome-wide screens for host factors affecting Tomato bushy stunt virus (TBSV) replication in yeast, has led to the identification of seven ESCRT proteins involved in the assembly of membrane-bound replicase complexes through interaction with the viral integral membrane protein p33 [4,5,6]
We have demonstrated that upon potyvirus infection, the potyviral 6K2 integral membrane protein is responsible for inducing the formation of the endoplasmic reticulum (ER)-derived replication vesicles that subsequently target chloroplasts for robust genome replication

Summary

Introduction

The host endomembrane system directly contributes to the formation of virus-induced membrane-bound virus factory for positive-strand RNA virus replication [1,2]. Genome-wide screens for host factors affecting Tomato bushy stunt virus (TBSV) replication in yeast, has led to the identification of seven ESCRT (endosomal sorting complex required for transport) proteins involved in the assembly of membrane-bound replicase complexes through interaction with the viral integral membrane protein p33 [4,5,6]. The second 6,000-molecular-weight protein (designated 6K2 or 6K) of the family Potyviridae, the largest and most agriculturally important family of plant positive-strand RNA viruses, is an integral membrane protein that induces the formation of 6K2containing membranous vesicles at ER exit sites [10,11]. The 6K2-induced vesicles subsequently target chloroplasts to form chloroplast-6K2 complexes that harbour the virus factory for potyvirus replication [12]

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Conclusion