Distinct regions of RPB11 are required for heterodimerization with RPB3 in human and yeast RNA polymerase II

In Saccharomyces cerevisiae, RNA polymerase II assembly is probably initiated by the formation of the RPB3–RPB11 heterodimer. RPB3 is encoded by a single copy gene in the yeast, mouse and human genomes. The RPB11 gene is also unique in yeast and mouse, but in humans a gene family has been identified that potentially encodes several RPB11 proteins differing mainly in their C-terminal regions. We compared the abilities of both yeast and human proteins to heterodimerize. We show that the yeast RPB3/RPB11 heterodimer critically depends on the presence of the C-terminal region of RPB11. In contrast, the human heterodimer tolerates significant changes in RPB11 C-terminus, allowing two human RPB11 variants to heterodimerize with the same efficiency with RPB3. In keeping with this observation, the interactions between the conserved N-terminal ‘α-motifs’ is much more important for heterodimerization of the human subunits than for those in yeast. These data indicate that the heterodimerization interfaces have been modified during the course of evolution to allow a recent diversification of the human RPB11 subunits that remains compatible with heterodimerization with RPB3

Apolipoprotein E interacts with hepatitis C virus nonstructural protein 5A and determines assembly of infectious particles

Chronic hepatitis C virus (HCV) infection is a major cause of liver disease worldwide. Restriction of HCV infection to human hepatocytes suggests that liver-specific host factors play a role in the viral life cycle. Using a yeast-two-hybrid system, we identified apolipoprotein E (apoE) as a liver-derived host factor specifically interacting with HCV nonstructural protein 5A (NS5A) but not with other viral proteins. The relevance of apoE–NS5A interaction for viral infection was confirmed by co-immunoprecipitation and co-localization studies of apoE and NS5A in an infectious HCV cell culture model system. Silencing apoE expression resulted in marked inhibition of infectious particle production without affecting viral entry and replication. Analysis of particle production in liver-derived cells with silenced apoE expression showed impairment of infectious particle assembly and release. The functional relevance of the apoE–NS5A interaction for production of viral particles was supported by loss or decrease of apoE–NS5A binding in assembly-defective viral mutants. Conclusion: These results suggest that recruitment of apoE by NS5A is important for viral assembly and release of infectious viral particles. These findings have important implications for understanding the HCV life cycle and the development of novel antiviral strategies targeting HCV–lipoprotein interaction