Selective Interaction Between Chloroplast β-ATPase and TGB1L88 Retards Severe Symptoms Caused by Alternanthera mosaic virus Infection

Abstract

The multifunctional triple gene block protein 1 (TGB1) of the Potexvirus Alternanthera mosaic virus (AltMV) has been reported to have silencing suppressor, cell-to-cell movement, and helicase functions. Yeast two hybrid screening using an Arabidopsis thaliana cDNA library with TGB1 as bait, and co-purification with TGB1 inclusion bodies identified several host proteins which interact with AltMV TGB1. Host protein interactions with TGB1 were confirmed by biomolecular fluorescence complementation, which showed positive TGB1 interaction with mitochondrial ATP synthase delta′ chain subunit (ATP synthase delta′), light harvesting chlorophyll-protein complex I subunit A4 (LHCA4), chlorophyll a/b binding protein 1 (LHB1B2), chloroplast-localized IscA-like protein (ATCPISCA), and chloroplast β-ATPase. However, chloroplast β-ATPase interacts only with TGB1L88, and not with weak silencing suppressor TGB1P88. This selective interaction indicates that chloroplast β-ATPase is not required for AltMV movement and replication; however, TRV silencing of chloroplast β-ATPase in Nicotiana benthamiana induced severe tissue necrosis when plants were infected by AltMV TGB1L88 but not AltMV TGB1P88, suggesting that β-ATPase selectively responded to TGB1L88 to induce defense responses.

Alternanthera mosaic virus - an alternative ‘model’ potexvirus of broad relevance

“…3A). Similar crystalline aggregates have been observed in TGB1-containing fractions purified from AltMV-infected plants (Seo et al, 2014). The appearance of the crystals differs depending on the angle of the section, with crystals appearing to have parallel striations of two different periodicities, or hexagonal arrays of apparent tubular construction; TGB1 aggregates in the cytoplasm often appear to have ribosomes closely associated with the outsides of the crystals (Fig.…”

“…Partially purified and concentrated TGB1 inclusions could be prepared from the same tissue extract by following the potyvirus 'cytoplasmic inclusion pellet' fraction as described (Hammond and Lawson, 1988), centrifuging the pellet fraction over 50/60/70/80% sucrose step gradients, and collecting the material from the interfaces between each of the gradient steps; most of the TGB1 crystal-like aggregates were found at the 70%/80% sucrose interface and within the 80% layer (Seo et al, 2014).…”

“…Similarly, at present we do not know what differences affect TGB1 localization to cytoplasm versus the nucleus. It is notable that 2-D gel electrophoresis separated two TGB1 isoforms, presumably differing in charge due to different phosphorylation states (Seo et al, 2014). There are a total of seven predicted phosphorylation sites in TGB1 (one each of cAMP-activated protein kinase, casein kinase 2, and tyrosine kinase; and four protein kinase C sites), but we do not know which of these sites are actually phosphorylated, nor how phosphorylation contributes to movement between the cytoplasm and nucleus.…”

Chloroplast: the Trojan horse in plant–virus interaction

“…Many chloroplast proteins assisting in viral propagation are localized in the chloroplast or functionally related to the photosynthetic machinery (Abbink et al, 2002;Bhat et al, 2013;Qiao et al, 2009; Seo et al, 2014). Chloroplast unusual positioning protein1 (CHUP1)-P6 interaction aids the movement of the Cauliflower mosaic virus (CaMV) inclusion body inside the cell along the microfilament to reach PD (Harries et al, 2009).…”

“…The silencing of N. benthamiana photosystem I PSI-K, a chloroplast thylakoid protein that interacts in vitro with the CI protein of Plum pox virus (PPV) of the Potyviridae family, enhances PPV accumulation (Jim enez et al, 2006). TGB1L88 (TGB1 protein with strong silencing suppressor activity) of AltMV selectively interacts with chloroplast bATPase and elicits the defence response (Seo et al, 2014). However, the involvement of chloroplast proteins in antiviral defence may cause the organelle to compromise its photosynthetic efficiency, as evident in the case of HR-induced PCD in TMV-infected resistant tobacco plants.…”