A protein-proximity screen reveals Ebola virus co-opts the mRNA decapping complex through the scaffold protein EDC4.

The interaction of host and Ebola virus (EBOV) proteins is required for establishing infection of the cell. To identify protein binding partners, a proximity-dependent protein interaction screen was performed for six EBOV proteins. Hits were computationally mapped onto a human protein-protein interactome and then annotated with viral proteins to reveal known and previously undescribed EBOV-host protein interactions and processes. Importantly, this approach efficiently arranged proteins into functional complexes associated with single viral proteins. Focused characterization of interactions between EBOV VP35 and the mRNA decapping complex demonstrated that VP35 binds the scaffold protein EDC4 through the C-terminal subdomain, with each protein found associated in EBOV-infected cells. Mechanistically, depletion of three components of the complex each similarly inhibited viral replication by reducing early viral RNA synthesis. Overall, we demonstrate successful identification of EBOV protein interaction with entire cellular machines, providing a deeper understanding of replication mechanism for therapeutic intervention.

A Novel Proximity Biotinylation Assay Based on the Self-Associating Split GFP1-10/11.

Proximity biotinylation was developed to detect physiologically relevant protein-protein interactions in living cells. In this method, the protein of interest is tagged with a promiscuous biotin ligase, such as BioID or BioID2, which produces activated biotin that reacts with nearby proteins; these proteins can subsequently be purified and identified by mass spectrometry. Here we report a novel modification of this technique by combining it with a self-associating split-GFP system in which we exploit the high-affinity interaction between GFP1-10 and GFP11 to recruit BioID2 to the protein of interest. As a test case, we fused GFP11 to clathrin light chain (CLTB) and BioID2 to GFP1-10. Co-expression of GFP11-CLTB and BioID2-GFP1-10 yielded a green fluorescent complex that co-localized with clathrin heavy chain. To facilitate removal of non-specifically biotinylated proteins, we generated an inducible cell line expressing BioID2-GFP1-10. Proximity biotinylation in this cell line with GFP11-CLTB yielded a higher percentage of biologically relevant interactions than direct fusion of BioID2 to CLTB. Thus, this system can be used to monitor expression and localization of BioID bait proteins and to identify protein-protein interactions.

Zika virus NS5 localizes at centrosomes during cell division.

Zika virus (ZIKV) nonstructural protein 5 (NS5) plays a critical role in viral RNA replication and mediates key virus-host cell interactions. As with other flavivirus NS5 proteins, ZIKV NS5 is primarily found in the nucleus. We previously reported that the NS5 protein of dengue virus, another flavivirus, localized to centrosomes during cell division. Here we show that ZIKV NS5 also relocalizes from the nucleus to centrosomes during mitosis. In infected cells with supernumerary centrosomes, NS5 was present at all centrosomes. Transient expression of NS5 in uninfected cells confirmed that centrosomal localization was independent of other viral proteins. Live-cell imaging demonstrated that NS5-GFP accumulated at centrosomes shortly after break down of nuclear membrane and remained there through mitosis. Cells expressing NS5-GFP took longer to complete mitosis than control cells. Finally, an analysis of ZIKV NS5 binding partners revealed several centrosomal proteins, providing potential direct links between NS5 and centrosomes.

Role of heparan sulfate in entry and exit of Ross River virus glycoprotein-pseudotyped retroviral vectors.

Variants of Ross River virus (RRV) that bind to heparan sulfate (HS) were previously selected by serial passaging in cell culture. To explore the effects of mutations that convey HS utilization, we pseudotyped Moloney murine leukemia virus (MoMLV), with the RRV envelope. We substituted amino-acid residues 216 and 218 on RRV-E2-envelope glycoprotein with basic amino-acid residues, because these mutations confer affinity for HS upon RRV. However, T216R-RRV- and N218R-RRV-pseudotyped viruses possessed lower transduction titers, and we demonstrated that HS-affinity impeded release of pseudotyped virus from producer cells. Addition of heparinase to HS-expressing target cells reduces the transduction efficiency of the T216R-RRV- and N218R-RRV-pseudotyped viruses, whereas no such effect is seen in cells lacking HS. Under appropriate conditions, these T216R-RRV- and N218R-RRV-pseudotyped viruses have enhanced capacities for transducing HS-expressing cells. General principles concerning viral adaptation to the use of attachment factors and design of pseudotyped viral vectors are discussed.