The kinase activity of Csk is decreased in HIV-infected patients. (A) Csk protein tyrosine kinase was immunoprecipitated from lysates containing equal total protein as measured by the bicinchoninic acid method, from uninfected individuals and from asymptomatic HIV⁺ patients. The immune complexes were washed, incubated with c-Src C-terminal peptide [Thr-Ser-Thr-Glu-Pro-Gln-Tyr-Gln-Pro-Gly-Glu-Asn-Leu] and 10 μCi [γ³²P]ATP at 37°C for 15 minutes, centrifuged, and the supernatant blotted onto phosphocellulose paper and washed with 10% phosphoric acid. After drying, the blotted phosphocellulose paper was tested for ³²P-containing c-Src peptide by scintillation counting. The results represent the relative Csk kinase activity of uninfected control and HIV⁺ patient after subtracting the background control (immune-complex without lysate; 350 cpm). (B) Western immunoblotting of 50-kD Csk protein for (A). Antibody control consists of immune complex without lysate. These results are representative of three independent experiments.