The binding assay is started by mixing samples 1 : 1 with 50 mM Na-HEPES, pH 8.0, 40 mM MgCl2, 1 mM EDTA, 1 mM DTT, 200 mM NaCl, 2 μM GTPγS, containing 0.1 μCi [35S]GTPγS (Dupont NEN, Boston, MA) per sample. Samples are started at 30-sec intervals and incubated for...