Figure 1.

p56^Lck prevents CD4 endocytosis independently of the kinase activation state. (A) Rates of CD4 internalization were calculated based on the percentage of the total cell-associated radioactivity incorporated after 5 min at 37°C by using iodinated anti-CD4 antibodies as described in Materials and Methods. (B) Kinase activity of p56^Lck mutants was assessed in a kinase assay monitoring phosphorylation of enolase as a substrate. As a control of the reaction, 2 μM PP2 was used to inhibit the reaction. Data are representative of three independent experiments. (C) Internalization of CD4 in 293T cells expressing p56^Lck mutants by using a FACS-based assay as described in Materials and Methods. Data are means ± SE of four independent experiments. (D) p56^Lck association with CD4 in 293T cells cotransfected with CD4 and p56^Lck mutants. Data are representative of three independent experiments.