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The capsids of some nonenveloped viruses contain divalent metal ions coordinated by acidic amino acids. Banerjee et al. (p. 4737-4746) investigated the importance of calcium ion binding in the life cycle of the insect nodavirus, Flock House virus (FHV). Mutations that decreased Ca2+ content reduced FHV capsid stability and compromised virus infectivity by affecting the membrane disruption step during host cell entry. Calcium release during entry is essential for pH-dependent exposure of virus-encoded membrane-disrupting peptides inside endosomes and subsequent capsid disassembly. This work provides a mechanistic explanation for the role of metal ion binding in virus entry.
Zinc finger antiviral protein (ZAP) is an interferon-stimulated gene product with potent activity against Filoviridae, Retroviridae, and Togaviridae family members. Using Sindbis virus as a model, Law et al. (p. 4504-4512) uncovered a dominant-negative activity of a nonfunctional ZAP mutant. Antiviral activity of endogenous ZAP depends on homophilic interactions, which are rendered nonfunctional in the presence of mutant ZAP. Evaluation of ZAP mutants for antiviral and multimerization properties revealed a strict requirement for ZAP self-interaction to antagonize Sindbis virus replication. This new finding provides a biophysical explanation for the antiviral function of ZAP.
The herpes simplex virus type 1 (HSV-1) UL25 protein is essential for packaging full-length viral genomes into capsids. O'Hara et al. (p. 4252-4263) used structure-guided mutagenesis to define functional regions of UL25. Two UL25 mutant proteins support packaging of full-length genomes but do not allow egress of mature capsids from the nucleus. These results provide the first direct evidence that the UL25 protein is required for virus assembly following genome encapsidation.
The Epstein-Barr virus-encoded protein kinase (EBV-PK) is homologous to the cytomegalovirus UL97 kinase and mediates efficient viral egress from the nucleus of host cells. It is not known whether EBV-PK, like UL97, also confers susceptibility to acyclovir and ganciclovir. Meng et al. (p. 4524-4533) show that the nuclear egress defect of an EBV-PK mutant virus is rescued by simian virus 40 T/t antigens or lamin A/C silencing and that the EBV-PK mutant, but not an EBV thymidine kinase mutant, is resistant to both acyclovir and ganciclovir (p. 4534-4542). These results indicate that EBV-PK phosphorylation of lamin A/C is required for nuclear egress in some (but not all) cell lines and that EBV-PK activates acyclovir and ganciclovir.
The “golden mean” connotes the functionally and aesthetically pleasing middle between the extremes of excess and deficiency. Some double-stranded RNA viruses and retroviruses require correct ratios of structural and enzymatic proteins to ensure efficient particle assembly. Plant et al. (p. 4330-4340) demonstrate that this concept also applies to the severe acute respiratory syndrome-associated coronavirus (SARS-CoV), despite its having a very different genomic organization. SARS-CoV employs at least two posttranscriptional regulatory mechanisms to ensure synthesis of optimal ratios of virus-encoded proteins: programmed -1 ribosomal frameshifting and translational attenuation. These findings support the golden mean as a universal organizing principle of virology.