SHIP downregulates FcγR-induced activation of Erk. (A) SHIP^+/+ and SHIP^–/– macrophages were activated by clustering Fcγ receptors for the indicated time points. Protein-matched lysates were probed with anti-phospho–ERK and reprobed with antibody against actin (bottom panel). The graph shown below is a quantitative estimate of Erk phosphorylation and represents the mean and SEM of values obtained from 3 independent experiments. Data were analyzed by Student t test. *P ≤ .05. □ indicates SHIP^+/+; ▴, SHIP^–/–. (B) Erk phosphorylation was likewise measured in stable transfectants overexpressing vector alone, wild-type SHIP, or catalytic-deficient D675A SHIP.