Setting Up Shop: The Formation and Function of the Viral Factories of Cauliflower mosaic virus

Abstract

Similar to cells, viruses often compartmentalize specific functions such as genome replication or particle assembly. Viral compartments may contain host organelle membranes or they may be mainly composed of viral proteins. These compartments are often termed: inclusion bodies (IBs), viroplasms or viral factories. The same virus may form more than one type of IB, each with different functions, as illustrated by the plant pararetrovirus, Cauliflower mosaic virus (CaMV). CaMV forms two distinct types of IBs in infected plant cells, those composed mainly of the viral proteins P2 (which are responsible for transmission of CaMV by insect vectors) and P6 (required for viral intra-and inter-cellular infection), respectively. P6 IBs are the major focus of this review. Much of our understanding of the formation and function of P6 IBs comes from the analyses of their major protein component, P6. Over time, the interactions and functions of P6 have been gradually elucidated. Coupled with new technologies, such as fluorescence microscopy with fluorophore-tagged viral proteins, these data complement earlier work and provide a clearer picture of P6 IB formation. As the activities and interactions of the viral proteins have gradually been determined, the functions of P6 IBs have become clearer. This review integrates the current state of knowledge on the formation and function of P6 IBs to produce a coherent model for the activities mediated by these sophisticated virus-manufacturing machines.

Cauliflower mosaic virus (CaMV) Biology, Management, and Relevance to GM Plant Detection for Sustainable Organic Agriculture

“…The viral DNA is transcribed via the two promoters (P19S and P35S) into two long messenger RNAs (mRNAs), the 19S mRNA that encodes protein P6 and the 35S mRNA that encodes the other six proteins. Translation of the 19S mRNA results in the production of protein P6, which aggregates in numerous cytoplasmic virus factories (Harries et al, 2009;Angel et al, 2013;Rodriguez et al, 2014;Schoelz et al, 2016;Schoelz and Leisner, 2017), where the translation of other viral proteins will take place: the movement protein P1 (required for cell-to-cell movement), the helper component P2 (required for aphid transmission), the virus-associated protein P3, the coat protein P4, and the reverse transcriptase P5. Protein P7 has never been detected in planta and can be deleted by mutagenesis without any effect on virus infection or transmission (Dixon et al, 1986;Wurch et al, 1990).…”

Cauliflower Mosaic Virus TAV, a Plant Virus Protein That Functions like Ribonuclease H1 and is Cytotoxic to Glioma Cells

“…We further characterized the role of TAV by searching for possible physical interactions of CaMV viral proteins with human proteins. e possible interactors of TAV in the natural host cells, in plants, have been previously reported [14,[33][34][35][36][37]. A clear similarity with human proteins is shown for some of the plant interactors, suggesting that human analogous can conserve the interaction with TAV.…”

“…Among CaMV proteins analyzed, TAV is the only one sharing significant similarity with a human protein (see Table S1 in the Supplementary Material for comprehensive analysis). TAV (UniprotKB: P03558) is a transactivator/ viroplasmin protein (P6) involved in the translation of polycistronic viral DNA and its domains have been previously described [14,31,32]. Figure 1(a) shows the sequence alignment between target (TAV) and the hybrid domain of the human RNase H that we used as template (Protein Data Bank code: 3BSU) and the comparison between their secondary structures as predicted with PSIPRED (for both target and template) or derived from the structure (for the template).…”

“…In the cytoplasm, TAV is translated from the 19S RNA and aggregates in small inclusion bodies, where it transactivates translation of all other viral proteins from the 35S RNA [11]. TAV is recognized as a multifunctional effector interacting with a broad array of host proteins and either initiates the innate immunity reaction in a nonpermissive host or interferes with it in a permissive host [12][13][14]. TAV is composed of 520 amino acids that form an alpha-helical motif and is the least conserved protein within the CaMV genome.…”

Ribosome Shunting, Polycistronic Translation, and Evasion of Antiviral Defenses in Plant Pararetroviruses and Beyond

“…It is puzzling why RTSV would need to evolve a sophisticated ribosome shunt mechanism by enlarging its genomic RNA leader sequence and making it a major obstacle for scanning ribosomes ( Pooggin et al, 2012 ). In RTBV (by analogy with CaMV), the highly-structured leader region, shunted over during translation initiation, contains the putative RNA packaging signal, a purine-rich sequence exposed on top of the large stem-loop structure for interaction with viral CP ( Guerra-Peraza et al, 2000 ; Figures 1A,B ): the viral CP binding to the packaging signal, preserved by shunting ribosomes from being melted, would allow to divert the pgRNA from translation to packaging into a previrion, followed by reverse transcription ( Pooggin et al, 1999 ; Ryabova et al, 2002 ; Schoelz and Leisner, 2017 ; references therein). It would be interesting to explore if RTSV had co-evolved a similar mechanism for sorting its genomic RNA for translation and packaging and thereby coordinating its replication cycle with RTBV in co-infected rice cells.…”