Figure 5.

SOS2 kinase activity is activated by SOS3 in a Ca²⁺-dependent manner. (A) SOS2 phosphorylation of peptide substrates is activated by SOS3. Oligopeptides p1, p2, and p3 were incubated in the presence of [γ-³²P] ATP and Ca²⁺ in a kinase buffer with GST-SOS2, and with or without GST-SOS3. γ-³²P-incorporation was measured by scintillation counting. (B) SOS3 activation of SOS2 is Ca²⁺-dependent. Oligopeptide p3 was phosphorylated by GST-SOS2 in the presence of GST-SOS3, and with or without free Ca²⁺. Error bars represent the standard deviation (n = 3).