Enterovirus 71 (EV71) is a picornavirus that can cause severe neurological complications in children. Like other picornaviruses, the genomic RNA of EV71 contains a long 59 untranslated region (UTR). Cellular proteins interact with the EV71 59 UTR, and these interactions are important for virus replication. Using an RNA pull-down assay and proteomics approaches, this study identified the heterogeneous nuclear ribonucleoprotein K (hnRNP K) as one of the EV71 59 UTR-associated proteins. The interaction between hnRNP K and the 59 UTR was further confirmed by mapping the interaction regions to stem-loops I-II and IV in the 59 UTR. During EV71 infection, hnRNP K was enriched in the cytoplasm where virus replication occurs, whereas hnRNP K was localized in the nucleus in mock-infected cells. Viral yields were found to be significantly lower in hnRNP K knockdown cells and viral RNA synthesis was delayed in hnRNP K knockdown cells in comparison with negative-control cells treated with small interfering RNA. These results suggest that hnRNP K interacts with the EV71 59 UTR and participates in virus replication.
Cardiotoxins are small proteins that are found in the venoms of snakes from the Elapidae family. These toxins are known to bind to and disrupt the organization, integrity, and function of the cell membrane. Most of the well-studied cardiotoxins cause depolarization of membrane potentials and/or lysis of red cells. In contrast, CTX V from Naja naja atra displays poor hemolytic activity but is proficient at inducing aggregation and fusion of sphingomyelin vesicles [Chien et al. (1991) J. Biol. Chem. 266, 3252-3259]. To determine whether the unique activity of this CTX is attributable to its tertiary structure, the solution structure of CTX V was determined by NMR methods. On the basis of these studies, this cardiotoxin has the same general topology as other members of the family, and thus its unusual properties do not arise from any gross structural differences that are detectable by solution NMR methods. Molecular dynamics calculations indicate that residues 36-50 show concerted fluctuations. On the basis of sequence similarity, we postulate that residues 30-34 are important in determining the specificity of cardiotoxins for fusion versus lysis of vesicles.
CTP synthase (CTPS) forms compartmentalized filaments in response to substrate availability and environmental nutrient status. However, the physiological role of filaments and mechanisms for filament assembly are not well understood. Here, we provide evidence that CTPS forms filaments in response to histidine influx during glutamine starvation. Tetramer conformation-based filament formation restricts CTPS enzymatic activity during nutrient deprivation. CTPS protein levels remain stable in the presence of histidine during nutrient deprivation, followed by rapid cell growth after stress relief. We demonstrate that filament formation is controlled by methylation and that histidine promotes re-methylation of homocysteine by donating one-carbon intermediates to the cytosolic folate cycle. Furthermore, we find that starvation stress and glutamine deficiency activate the GCN2/ATF4/MTHFD2 axis, which coordinates CTPS filament formation. CTPS filament formation induced by histidine-mediated methylation may be a strategy used by cancer cells to maintain homeostasis and ensure a growth advantage in adverse environments.
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