Figure 4From: Hemagglutinin-neuraminidase and fusion proteins of virulent Newcastle disease virus cooperatively disturb fusion–fission homeostasis to enhance mitochondrial function by activating the unfolded protein response of endoplasmic reticulum and mitochondrial stressSyncytia generated by HN and F of virulent NDV induce ER stress and UPRmt. A Representative images of F and HN co-operation that increased the interaction between ER and mitochondria in A549 cells. A549 cell were co-transfected with EGFP-Mito and DsRed-ER, together with both Flag-F and HA-HN plasmids, at the indicated time. At 0, 6, 12, 18 and 24 hpt, the coverslips were examined IFA. The plot profile of linear region of interest was analyzed using Image J. B Detection results of ER stress and UPRmt marker proteins in A549 cells through Western blot analysis. C Influence of the cleavage site motif of F on the activation of ER UPR and UPRmt. In A549 cells, wild-type HN plasmid was co-transfected with wild-type F, F112G, F115G, F117L, F112G+115G, F112G+117L, F115G+117L and F112G+115G+117L. At 36 hpt, the Western-blot samples were collected and analyzed. D A model pattern of activated F1 interacts with HN for the disturbance of mitochondrial fusion–fission homeostasis to enhance syncytium formation via the UPR of ER and UPRmt. After HN and F co-transfection, the non-fusogenically F protein precursor (F0) form is proteolytically cleaved into a disulfide-linked F1 + F2 heterodimer, to be fusogenically active, which is an essential for fusion by positioning the hydrophobic fusion peptide (FP) at the newly formed N-terminus of F1 [29, 30]. Then, the fusogenically active F protein and HN form an F + HN complex via the interaction between a stalk and a domain with two heptad repeat regions (HRA and HRB) [29, 30]. Once triggered, HRB completely refolds around HRA, thereby forming six stable HBs and a fusion pore [29, 30]. Apart from spatial distribution rearrangement, the UPR of ER and UPRmt is induced by activated F1 and HN to regulate and disturb mitochondrial fusion–fission homeostasis, which eventually produces hyper-fused and fragmented mitochondria. Eventually, nuclear-aggregated and hyper-fused mitochondria provide additional energy to enhance syncytia formation and membrane fusion.Back to article page