Competitive effect of lentiviral Gag protein on the kinase activity of PRP4. (A) PRP4 protein incorporating a V5 epitope tag immunoprecipitated from transfected 293T cells phosphorylates SF2 protein fused to glutathione-Sepharose beads (lane 1). Increasing molar excess of MBP protein (lanes 2, 3, and 4) or MBP-HIV-2 Gag protein (lanes 5, 6, and 7) are titrated against a constant molar amount of PRP4. (B) PRP4 protein phosphorylates SF2 alone (lane 1) and when a 20× molar excess of MBP is added (lane 2). MBP-SIV Gag protein is titrated against PRP4 protein at 5, 10, 20, and 30× the amount of PRP4 (lanes 3, 4, 5, and 6). (C) PRP4 phosphorylates SF2 (lane 1) and when 20× molar excess of MBP protein is added (lane 2). MBP-HTLV-1 Gag is titrated against the phosphorylation at 5, 10, and 20× the molar excess of PRP4 (lanes 3, 4, and 5). The data in each graph to the right represent the mean phosphorylation activity of three independent kinase assays for each lentivirus. Data were analyzed with NIH Image (A) and the Packard Instant Imager (B and C).