HIV-1 Vif reduced the incorporation of APOBEC3G into HTLV-1 virions, resulting in the upregulation of the infectivity. (A) Expression of APOBEC3G in MT-2 cells and its incorporation into produced virions were reduced by HIV-1 Vif. Expression level of APOBEC3G was reduced in MT-2/Vif cells (lane 2, middle panel) as compared to MT-2/Mock cells (lane 1, middle panel). Incorporation of APOBEC3G into produced virions was also reduced in virions produced from MT-2/Vif cells (lane 2, bottom panel). Expression of Vif protein in MT-2/Vif cells was detected with anti-Vif mAb (top panel). (B) HIV Vif upregulated the infectivity of HTLV-1 produced from MT-2 cells. Infectivity of HTLV-1 virus produced from MT-2 cells was determined as described in Materials and Methods. Infectivity of viruses produced from MT-2/Vif cells was more than 4 times higher than that from MT-2/Mock cells. Four independent experiments gave similar results and the data was presented as the mean of these values. Values are presented as infectivity ratio relative to viruses from MT-2/Mock cells.