FIG. 6.

Activation of the MEK/ERK kinase pathway by T-cell-tropic or macrophagetropic HIV-1 or anti-CD4 antibody depends on the presence of cytoplasmic domain of CD4 receptor. A2.01/CD4.401 cells expressing truncated CD4 receptors were left untreated (control) or preincubated on ice for 60 min with SDF-1 (5 μg/ml), anti-CD4 antibody (5 μg/ml), gp120 IIIB (1 μg/ml), or purified NL4-3 or Ba-L (5 RT cpm/cell) and then incubated at 37°C for 5 min before lysis. Proteins (50 μg) were resolved by SDS-PAGE and analyzed as described in the legend to Fig. 3. The positions of phosphorylated forms of MEK1/2 (MEK-P) and ERK1/2 (ERK1/2-P) as well as total MEK and ERK are indicated. Sizes are indicated in kilodaltons.