Abstract
Geminiviruses are a significant group of emergent plant DNA viruses causing devastating diseases in food crops worldwide, including the Southern United States, Central America and the Caribbean. Crop failure due to geminivirus-related disease can be as high as 100%. Improved global transportation has enhanced the spread of geminiviruses and their vectors, supporting the emergence of new, more virulent recombinant strains. With limited coding capacity, geminiviruses encode multifunctional proteins, including the AC2/C2 gene that plays a central role in the viral replication-cycle through suppression of host defenses and transcriptional regulation of the late viral genes. The AC2/C2 proteins encoded by mono- and bipartite geminiviruses and the curtovirus C2 can be considered virulence factors, and are known to interact with both basal and inducible systems. This review highlights the role of AC2/C2 in affecting the jasmonic acid and salicylic acid (JA and SA) pathways, the ubiquitin/proteasome system (UPS), and RNA silencing pathways. In addition to suppressing host defenses, AC2/C2 play a critical role in regulating expression of the coat protein during the viral life cycle. It is important that the timing of CP expression is regulated to ensure that ssDNA is converted to dsDNA early during an infection and is sequestered late in the infection. How AC2 interacts with host transcription factors to regulate CP expression is discussed along with how computational approaches can help identify critical host networks targeted by geminivirus AC2 proteins. Thus, the role of AC2/C2 in the viral life-cycle is to prevent the host from mounting an efficient defense response to geminivirus infection and to ensure maximal amplification and encapsidation of the viral genome.
Highlights
The Geminiviridae is a family of single-stranded DNA viruses that infect agricultural crops in tropical and sub-tropical regions worldwide, and are responsible for billions of dollars in annual losses contributing to famine and loss of life (Legg and Fauquet, 2004; Scholthof et al, 2011)
It was later discovered using tobacco cells that regulator of gene silencing calmodulin-like protein (rgs-CaM) is capable of interacting with other viral RNA silencing suppressors (RSS), including HC-Pro encoded by the potyvirus Turnip mosaic virus (TuMV) and Cucumber mosaic virus (CMV) 2b protein (Nakahara et al, 2012)
It is clear that the AC2/C2 proteins encoded by begomoviruses are multifunctional and have critical roles in both suppression of host immune responses and in regulation of the CP promoter
Summary
The Geminiviridae is a family of single-stranded DNA (ssDNA) viruses that infect agricultural crops in tropical and sub-tropical regions worldwide, and are responsible for billions of dollars in annual losses contributing to famine and loss of life (Legg and Fauquet, 2004; Scholthof et al, 2011). Genes that are expressed early, meaning prior to DNA synthesis, typically encode proteins necessary for replication of the viral genome and/or to modulate the host cell environment. The focus of this review is on the AC2 protein of begomoviruses and the C2 protein of monopartite begomoviruses and curtoviruses, and highlights the central role that AC2 plays in the viral replication cycle through suppression of host defenses and transcriptional regulation of the late viral genes. In bipartite begomoviruses the movement proteins BV1 (Nuclear shuttle protein, NSP) and BC1 (MP) are encoded by DNA-B. nuclear localization signal (NLS; amino acids 17 to 31), and a C terminal acidic region (amino acids 101 to 129) containing a transcriptional activation domain (TAD, amino acids 115 to 129) (Hartitz et al, 1999). AC2 localizes to sub-cellular compartments that correlate with known functions of the protein, which are to interact with plant nuclear and cytoplasmic proteins
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