A novel Arabidopsis acetyltransferase interacts with the geminivirus movement protein NSP

Lysine acetylation and the bromodomain: a new partnership for signaling

Lysine acetylation has been shown to occur in many protein targets, including core histones, about 40 transcription factors and over 30 other proteins. This modification is reversible in vivo, with its specificity and level being largely controlled by signal-dependent association of substrates with acetyltransferases and deacetylases. Like other covalent modifications, lysine acetylation exerts its effects through "loss-of-function" and "gain-of-function" mechanisms. Among the latter, lysine acetylation generates specific docking sites for bromodomain proteins. For example, bromodomains of Gcn5, PCAF, TAF1 and CBP are able to recognize acetyllysine residues in histones, HIV Tat, p53, c-Myb or MyoD. In addition to the acetyllysine moiety, the flanking sequences also contribute to efficient recognition. The relationship between acetyllysine and bromodomains is reminiscent of the specific recognition of phosphorylated residues by phospho-specific binding modules such as SH2 domains and 14-3-3 proteins. Therefore, lysine acetylation forges a novel signaling partnership with bromodomains to govern the temporal and spatial regulation of protein functions in vivo.

Interaction of the movement protein NSP and the Arabidopsis acetyltransferase AtNSI is necessary for Cabbage leaf curl geminivirus infection and pathogenicity

DNA viruses can modulate the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in order to support virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus DNA genome was recently suggested by the findings that the viral movement protein NSP, which shuttles the viral genome between the nucleus and the cytoplasm, interacts with a novel Arabidopsis acetyltransferase, AtNSI, and the increased expression of AtNSI enhances susceptibility to Cabbage leaf curl virus infection. To further investigate the interaction of NSP and AtNSI and to establish the importance of this interaction in virus infections, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSP mutants that were impaired in their ability to bind AtNSI. These mutants identified a 38-amino-acid region of NSP, to which no function had so far been assigned, as being necessary for NSP-AtNSI interaction. Three NSP missense mutants were analyzed in detail and were found to be comparable to wild-type NSP in their levels of accumulation, nucleocytoplasmic shuttling, DNA binding, and cooperative interaction with the viral cell-to-cell movement protein MP. Despite this, Cabbage leaf curl virus that expressed each mutated NSP was defective in its ability to infect Arabidopsis, exhibiting lower levels of infectivity than the wild-type virus, and delayed systemic spread of the virus and attenuated disease symptoms. Our data demonstrate the importance of the interaction of NSP with AtNSI for virus infection and pathogenicity.

Interaction of the movement protein NSP and the Arabidopsis acetyltransferase AtNSI is necessary for Cabbage leaf curl geminivirus infection and pathogenicity

DNA viruses can modulate the activity of cellular acetyltransferases to regulate virus gene expression and to affect cell cycle progression in order to support virus replication. A role for protein acetylation in regulating the nuclear export of the bipartite geminivirus DNA genome was recently suggested by the findings that the viral movement protein NSP, which shuttles the viral genome between the nucleus and the cytoplasm, interacts with a novel Arabidopsis acetyltransferase, AtNSI, and the increased expression of AtNSI enhances susceptibility to Cabbage leaf curl virus infection. To further investigate the interaction of NSP and AtNSI and to establish the importance of this interaction in virus infections, we used a reverse yeast two-hybrid selection and deletion analysis to identify NSP mutants that were impaired in their ability to bind AtNSI. These mutants identified a 38-amino-acid region of NSP, to which no function had so far been assigned, as being necessary for NSP-AtNSI interaction. Three NSP missense mutants were analyzed in detail and were found to be comparable to wild-type NSP in their levels of accumulation, nucleocytoplasmic shuttling, DNA binding, and cooperative interaction with the viral cell-to-cell movement protein MP. Despite this, Cabbage leaf curl virus that expressed each mutated NSP was defective in its ability to infect Arabidopsis, exhibiting lower levels of infectivity than the wild-type virus, and delayed systemic spread of the virus and attenuated disease symptoms. Our data demonstrate the importance of the interaction of NSP with AtNSI for virus infection and pathogenicity.

Roles for host factors in plant viral pathogenicity

The simple, obligate nature of viruses requires them to usurp or divert cellular resources, including host factors, away from their normal functions. The characterization of host proteins, membranes, and nucleic acids that are implicated in viral infection cycles, together with other recent discoveries, is providing fundamental clues about the molecular bases of viral susceptibility. As viruses invade susceptible plants, they create conditions that favor systemic infections by suppressing multiple layers of innate host defenses. When viruses meddle in these defense mechanisms, which are interlinked with basic cellular functions, phenotypic changes can result that contribute to disease symptoms.

Reprogramming plant gene expression: a prerequisite to geminivirus DNA replication

SUMMARY Geminiviruses constitute a large family of plant-infecting viruses with small, single-stranded DNA genomes that replicate through double-stranded intermediates. Because of their limited coding capacity, geminiviruses supply only the factors required to initiate their replication and use plant nuclear DNA polymerases to amplify their genomes. Many geminiviruses replicate in differentiated cells that no longer contain detectable levels of host DNA polymerases and associated factors. To overcome this barrier, geminiviruses induce the accumulation of DNA replication machinery in mature plant cells by reprogramming host gene expression. The mammalian DNA tumour viruses activate host genes required for DNA replication by binding to the retinoblastoma protein, a negative regulator of cell cycle progression, and relieving repression through the E2F family of transcription factors. In this review, we discuss recent experiments showing that geminiviruses also modulate components of the retinoblastoma/E2F transcription regulatory network to induce quiescent plant cells to re-enter the cell cycle and regain the capacity to support high levels of DNA replication. Regulation of the cell division cycle and its integration with developmental pathways is complex, with many factors, including hormones, sucrose and environmental signals, controlling re-entry into the plant cell cycle. Geminivirus interactions with these regulatory networks are likely to determine if and where they can replicate their genomes in different plant tissues and hosts.

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.

Getting the message across: how do plant cells exchange macromolecular complexes?

A major pathway for macromolecular exchange in plants involves plasmodesmata (PD), the small pores that connect adjoining cells. This article considers the nature of macromolecular complexes (MCs) that pass through PD and the pathways and mechanisms that guide them to the PD pore. Recent cell-biological studies have identified proteins involved in the directional trafficking of MCs to PD, and yeast two-hybrid studies have isolated novel host proteins that interact with viral movement proteins. Collectively, these studies are yielding important clues in the search for components that compose the plant intercellular MC trafficking pathway. Here, they are placed in the context of a functional model that links the cytoskeleton, chaperones and secretory pathway in the intercellular trafficking of MCs.