FIG. 4.

Activation of Ras by hSos1 truncation mutants. (A) COS1 cells were cotransfected with HRasWT (0.2 μg) and the indicated hSos1 truncation mutants. Serum-starved cells were labeled with [³²P]orthophosphate, and the guanine nucleotide content of HRasWT was analyzed as described in Materials and Methods. GTP and GDP markers are labeled on the right. (B) Quantitation of GTP/(GDP + GTP) percentages. Results are the averages of four independent experiments. Error bars represent the standard deviations.