There are also reports that the ubiquitin ligase machinery is vital for regulating host immunity to infection. The ubiquitin ligase machinery is central to ubiquitin tagging misfolded proteins and targeting them for degradation by cellular proteasomes. Chaperones within the context of the cellular quality control machinery enable misfolded or aggregated proteins to be refolded ( Tyedmers et al., 2010) or targeted for degradation by cellular proteases ( Bukau et al., 2006). Among these are the heat shock protein (Hsp) 40, 70, 90, and 100 families of protein chaperones which are highly conserved across eukaryotes and are vital factors in the quality control of cellular proteins and protein complexes contributing to a wide range of cellular processes ( Mayer, 2010). The second cellular alteration mentioned above is the recruitment of host proteins, including cellular chaperones, to membrane bound sites required for virus replication and cell-to-cell movement. Such extensive rearrangement of host membrane compartments are a hallmark of (+) strand RNA virus infection and the specific structures produces by various virus species have been reviewed in prior publications and will not be explored in depth here ( Heath et al., 2001 Netherton et al., 2007 Wileman, 2007 Verchot, 2011). Typically these various membrane bound virus factories are induced by non-structural viral proteins and serve to concentrate replication proteins, viral genomes, and host proteins needed for efficient virus replication. Recent research indicates that many plant infecting (+) strand RNA create microenvironments that are sometimes referred to as miniorganelles and these can range in size from vesicles or invaginations along organelle membranes to slightly larger virus factories. The term viroplasm was first used to describe such perinuclear virus factories produced by large DNA viruses and some (+) strand RNA viruses such as poxvirus and poliovirus. At the electron microscopic level viroplasms are large virus factories that are amorphous structures containing virion particles, viral RNAs, and non-structural proteins, but typically exclude organelles. With regard to changes in membrane architecture, viruses typically create membrane bound environments, called virus factories, to protect replication and assembly complexes from cellular defenses. Two types of cellular alterations that are essential for (+) strand RNA virus replication and cell-to-cell movement include: (1) discrete and well characterized changes in the endomembrane architecture, and (2) the recruitment of host factors into viral protein containing complexes. Positive-strand RNA viruses are among the largest group of viruses infecting plants worldwide and contribute to some of the most critical issues in agriculture. Such chaperones and co-factors are potential targets for antiviral defense. In addition, TGB3 stimulates SKP1 which is a co-factor in proteasomal degradation of cellular proteins. Up-regulating factors involved in protein folding may be essential to handling the load of viral proteins translated along the ER. The potexvirus TGB3 protein stimulates expression of ER resident chaperones via the bZIP60 transcription factor. The pomovirus TGB2 interacts with RME-8 in the endosome. Calreticulin also resides in the plasmodesmata and plays a role in calcium sequestration as well as glycoprotein folding. TMV relies on calreticulin to promote virus intercellular transport. There are also examples of plant viruses that rely on chaperone systems in the endoplasmic reticulum (ER) to support cell-to-cell movement. There are several co-chaperones, including Yjd1, RME-8, and Hsp40 that associate with the bromovirus replication complex, pomovirus TGB2, and tospovirus Nsm movement proteins. During plant virus infection, Hsp70 and Hsp90 proteins play central roles in the formation of membrane-bound replication complexes for certain members of the tombusvirus, tobamovirus, potyvirus, dianthovirus, potexvirus, and carmovirus genus. Their activities often depend upon co-chaperones and ATP hydrolysis. They are system components that drive substrate protein folding, complex assembly or disaggregation. For most chaperones discussed, their primary role in the cell is to ensure protein quality control. This article examines the key cellular chaperones and discusses evidence that these factors are diverted from their cellular functions to play alternative roles in virus infection. Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK, USAĬellular chaperones and folding enzymes play central roles in the formation of positive-strand and negative-strand RNA virus infection.
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