In human parainfluenza virus type 2 (hPIV-2)-infected cells, anti-phosphoprotein (P)-specific monoclonal antibody (MAb) densely stained the perinuclear regions of infected cells throughout infection, indicating that the P protein was localized exclusively in the cell cytoplasm. By contrast, antigens recognized by MAbs directed against the P-V-common domain of hPIV-2 were located predominantly in the cytoplasm, but in some hPIV-2-infected cells they were also found in the nuclei, suggesting that a fraction of hPIV-2V protein is localized there. hPIV-2 V protein expressed from a cDNA clone was localized in the nuclei of transfected cells. By using indirect immunofluorescence analyses, we examined the intracellular localization of various sequentially deleted V proteins, to determine the nuclear localization signals (NLS) of the V protein. Two noncontiguous regions in the V protein were required for nuclear localization and retention, since deletion of these regions [region I (aa 1–46) and region II (aa 175–196)] resulted in cytoplasmic localization. Both regions resulted in nuclear localization independently. A nucleoplasmin-like NLS was identified in region II but no consensus targeting sequence could be found in region I. When NP protein was co-expressed with V protein or the N-terminal fragment (aa 1–46) of V protein, a fraction of the NP protein was translocated into cell nuclei.

HPIV-2 V protein interferes with interaction between FTH1 and NCOA4 and inhibits NCOA4-mediated ferritin degradation, leading to the inhibition of iron release to the cytoplasm. This iron homeostasis modulation allows infected cells to avoid apoptotic cell death, resulting in effective growth of hPIV-2.

We have isolated new MAbs directed against the human fusion regulatory protein 1 (FRP-1; CD98) molecule using human FRP-1-expressing L929 cells as antigens. The biological activities, and in particular the human immunodeficiency virus (HIV)-mediated fusion regulatory activity of seven anti-FRP-1/CD98 MAbs were analysed using the U937/gp160 cell line, which is a CD4+ U937 cell line expressing HIV gp160. Two MAbs induced multi-nucleated giant cell formation in U937/gp160 cells and the other five MAbs showed no fusion-inducing ability. However, four of these MAbs suppressed multinucleated giant cell formation of U937/gp160 cells induced by the activating anti-FRP-1 MAbs. Interestingly, five of the MAbs induced multi-nucleated giant cells in peripheral blood monocytes and one MAb showing fusion-inducing ability in U937/gp160 cells suppressed multinucleated giant cell formation of monocytes induced by anti-FRP-1 MAbs. Furthermore, four of the anti-FRP-1 MAbs suppressed cell fusion of Jurkat/gp160 cells, which are Jurkat cells expressing HIV gp160. Thus, FRP-1/CD98 is capable of either activating or inhibiting HIV-mediated cell fusion depending on whether an enhancing or inhibiting antibody is used, indicating that FRP-1/CD98 is a multipotential molecule. Thus, HIV-mediated cell fusion can be regulated by modification of the FRP-1 system. Furthermore, the present study demonstrates that the FRP-1 and FRP-2 systems are interdependent.