The members of the genus Alphavirus are positive-sense RNA viruses, which are predominantly transmitted to vertebrates by a mosquito vector. Alphavirus disease in humans can be severely debilitating, and depending on the particular viral species, infection may result in encephalitis and possibly death. In recent years, alphaviruses have received significant attention from public health authorities as a consequence of the dramatic emergence of chikungunya virus in the Indian Ocean islands and the Caribbean. Currently, no safe, approved or effective vaccine or antiviral intervention exists for human alphavirus infection. The molecular biology of alphavirus RNA synthesis has been well studied in a few species of the genus and represents a general target for antiviral drug development. This review describes what is currently understood about the regulation of alphavirus RNA synthesis, the roles of the viral non-structural proteins in this process and the functions of cis-acting RNA elements in replication, and points to open questions within the field.

Arthropod-borne viruses, such as the members of genus , are a significant concern to global public health. As obligate intracellular pathogens, RNA viruses must interact with the host cell machinery to establish, and complete, their viral lifecycles. Despite considerable efforts to define the host/pathogen interactions essential for alphaviral replication, an unbiased and inclusive assessment of alphaviral RNA:protein interactions has not been undertaken. Moreover, the biological and molecular importance of these interactions, in the full context of their molecular function as RNA-binding proteins, has not been fully realized. The data presented here introduces a robust viral RNA:protein discovery method to elucidate the Sindbis virus (SINV) RNA:Protein host interface. Cross-Link Assisted mRNP Purification (CLAMP) assessment reveals an extensive array of host/pathogen interactions centered on the viral RNAs (vRNAs). After prioritization of the host proteins associated with the vRNAs, we identified the site of Protein:vRNA interaction via a CLIP-seq approach and assessed the consequences of the RNA:protein binding event of hnRNP K, hnRNP I, and hnRNP M in regards to viral infection. Herein we demonstrate that mutation of the prioritized hnRNP:vRNA interaction sites effectively disrupted the hnRNP:vRNA interaction. Correlating with disrupted hnRNP:vRNA binding, SINV growth kinetics were reduced relative to wild type parental viral infections in a vertebrate and invertebrate tissue culture models of infection. The molecular mechanism leading to reduced viral growth kinetics were found to be dysregulated structural gene expression. Collectively, this study further defines the scope and importance of the alphavirus host/pathogen vRNA:protein interactions. Members of the genus Alphavirus are widely recognized for their potential to cause severe disease. Despite this recognition, there are no antiviral therapeutics, or safe and effective vaccines, currently available to treat alphaviral infection. Alphaviruses utilize the host cell machinery to efficiently establish and complete their viral lifecycle. However, the extent, and importance, of host/pathogen RNA:protein interactions is woefully under characterized. The efforts detailed in this study fulfill this critical gap; and the significance of this research is three-fold. First, the data presented here fundamentally expands the scope and understanding of alphavirus host/pathogen interactions. Secondly, this study identifies the site of interactions for several prioritized interactions and defines the contribution of the RNA:protein interaction at the molecular level. Finally, these studies build a strategy by which the importance of given host/pathogen interactions may be assessed, in the future, using a mouse model of infection.

Alphaviruses are arthropod-borne viruses that represent a significant threat to public health at a global level. While the formation of alphaviral nucleocapsid cores, consisting of cargo nucleic acid and the viral capsid protein, is an essential molecular process of infection, the precise interactions between the two partners are ill-defined. A CLIP-seq approach was used to screen for candidate sites of interaction between the viral Capsid protein and genomic RNA of Sindbis virus (SINV), a model alphavirus. The data presented in this report indicates that the SINV capsid protein binds to specific viral RNA sequences in the cytoplasm of infected cells, but its interaction with genomic RNA in mature extracellular viral particles is largely non-specific in terms of nucleotide sequence. Mutational analyses of the cytoplasmic viral RNA-capsid interaction sites revealed a functional role for capsid binding early in infection. Interaction site mutants exhibited decreased viral growth kinetics; however, this defect was not a function of decreased particle production. Rather mutation of the cytoplasmic capsid-RNA interaction sites negatively affected the functional capacity of the incoming viral genomic RNAs leading to decreased infectivity. Furthermore, cytoplasmic capsid interaction site mutants are attenuated in a murine model of neurotropic alphavirus infection. Collectively, the findings of this study indicate that the identified cytoplasmic interactions of the viral capsid protein and genomic RNA, while not essential for particle formation, are necessary for genomic RNA function early during infection. This previously unappreciated role of capsid protein during the alphaviral replication cycle also constitutes a novel virulence determinant.

Sindbis virus subgenomic mRNA is efficiently translated in infected vertebrate cells whereas host translation is shut-off. Deletions in the 5′ UTR of the subgenomic mRNA were made to investigate its role in viral gene expression. Deletion of nucleotides 1-10 and 11-20 caused a small plaque phenotype, reduced levels of subgenomic mRNA and structural proteins, and increased expression of nonstructural proteins. Whereas deletion 1-10 virus inhibited cellular protein synthesis, deletion 11-20 did so inefficiently. A large plaque revertant of deletion 11-20, possessing a duplication of the subgenomic promoter region, produced subgenomic mRNA at WT levels and restored inhibition of host protein synthesis. Further analysis of the mutant and revertant 5′UTR sequences showed the ability to shut-off host cell translation correlated with the efficiency of translation of subgenomic mRNA. We propose that the translational efficiency and quantity of the subgenomic mRNA play a role in inhibition of host cell translation.

The identification of host / pathogen interactions is essential to both understanding the molecular biology of infection and developing rational intervention strategies to overcome disease. Alphaviruses, such as Sindbis virus, Chikungunya virus, and Venezuelan Equine Encephalitis virus are medically relevant positive-sense RNA viruses. As such, they must interface with the host machinery to complete their infectious lifecycles. Nonetheless, exhaustive RNA:Protein interaction discovery approaches have not been reported for any alphavirus species. Thus, the breadth and evolutionary conservation of host interactions on alphaviral RNA function remains a critical gap in the field. Herein we describe the application of the Cross-Link Assisted mRNP Purification (CLAMP) strategy to identify conserved alphaviral interactions. Through comparative analyses, conserved alphaviral host / pathogen interactions were identified. Approximately 100 unique host proteins were identified as a result of these analyses. Ontological assessments reveal enriched Molecular Functions and Biological Processes relevant to alphaviral infection. Specifically, as anticipated, Poly (A) RNA Binding proteins are significantly enriched in virus specific CLAMP data sets. Moreover, host proteins involved in the regulation of mRNA stability, proteasome mediated degradation, and a number of 14-3-3 proteins were identified. Importantly, these data expand the understanding of alphaviral host / pathogen interactions by identifying conserved interactants.