Besides its effects on immune response, activation of MAPK/ ERK pathway has direct consequences on viral replication as assessed by in vitro experiments. It is now well documented that MAPK/ERK pathway inhibition with U0126 or PD098059 deeply impairs the replication of numerous RNA viruses including hRSV ( [50] ; and for review see [31] ). Similarly, we show in this report that MAPK/ERK pathway inhibition prevents hPIV3 protein expression in infected cells as assessed by hPIV3-HN detection. In influenza virus infected cells, membrane accumulation of influenza virus hemagglutinin (HA) induces lipid-rafts clustering that leads to MAPK/ERK pathway activation and nuclear export of viral ribonucleoprotein complexes to achieve A and B) hPIV3-C 90-195 and hPIV3-C 1-89 were tested for their ability to modulate IFN-a/b or EGF signaling. (A) As described in Figure 2 , HEK-293T cells were transfected with pISRE-Luc and pRL-CMV to determine the activation level of IFN-a/b signaling pathway. Cells were co-transfected with expression plasmids encoding 36FLAG-tagged full-length hPIV3-C (C FL ) or fragments. 24 h after transfection, 1000 IU/ml of recombinant IFN-b were added. After 24 h, relative luciferase activity was determined. Experiments were performed in triplicates, and data represent means6SD. (B) As described in Figure 2 , cells were transfected with pFA2-Elk1, pGal4-UAS-Luc and pRL-CMV to determine the activation level of MAPK/ERK signaling pathway. Cells were co-transfected with expression plasmids encoding full-length hPIV3-C (C FL ) or fragments. 12 h later, cells were starved during 6 h and stimulated with EGF at a final concentration of 100 ng/ml. After 24 h, relative luciferase activity was determined. Experiments were performed in triplicate, and data represent means6SD. As a control, relative expression levels of C FL , C 90-195 and C 1-89 were determined by western blot analysis (upper right panel). (C) Full-length hPIV3-C, hPIV3-C 90-195 or hPIV3-C 1-89 was expressed in fusion downstream of the red fluorescent protein Cherry to determine subcellular localization in HEK-293T cells. 24 h after transfection, cells were fixed with PFA, permeabilized and labeled with DAPI to stain nuclei. Single confocal sections show Cherrytagged protein fluorescence in red and DAPI staining in blue. doi:10.1371/journal.ppat.1000587.g008 viral particles assembly [51] [52] [53] . Because hPIV3 replication cycle is only cytoplasmic, mechanisms involved are necessarily distinct. A possible link between MAPK/ERK pathway and hPIV3 protein expression lies in the fact that among downstream targets of this pathway are essential factors of cellular translational machinery.