This observation adds to existing evidence that M. tuberculosis Obg has an inherent specificity for guanine nucleotides, as do the Obg orthologues in C. crescentus [32], B. subtilis [13] and S. griseus [8]. To determine whether the overexpressed Obg can hydrolyze GTP, we incubated His10 -Obg with radiolabeled GTP ([γ-32P] GTP), and measured the release of phosphate (32Pi) after 3 hours. Figure 1C shows that His10-Obg readily hydrolyzes GTP, and STAT inhibitor that this hydrolysis is inhibited

by the addition of unlabeled GTP (5 mM), indicating that unlabeled GTP competes with labeled GTP for the enzyme. Addition of unlabeled ATP (5 mM) has no effect on the hydrolysis of labeled GTP (Figure 1C), indicating that Obg hydrolyzes specifically GTP. The effect of cold GTP in inhibiting the hydrolysis of radiolabeled GTP was not as pronounced as its effect in inhibition of GTP crosslinking (Compare Figure 1B and Figure 1C). This is most likely due to the differences in the positions of the radiolabeled

phosphates used in these two reactions. While the reaction mixture in the crosslinking experiment (Figure 1B) had 10 μCi (0.033 μM) of [α-32P] GTP, the reaction mixture in the hydrolysis experiment had 25 μCi (0.040 μM) of [γ-32P] GTP. In addition, the incubation times for these two experiments were different (1 h for GTP crosslinking vs. 3 h for GTP hydrolysis). Autophosphorylation PD-0332991 price of His10-Obg Autophosphorylation by GTP is a defining characteristic of eukaryotic GTP-binding proteins, e.g. Ras [33], and of prokaryotic GTP-binding proteins, including Era of E. coli [34] and Obg of B. subtilis (22). We therefore asked whether His10-Obg of M. tuberculosis is autophosphorylated by GTP. Figure 2A shows that purified His10-Obg from M. tuberculosis is autophosphorylated by [γ-32P] GTP, in a time-dependent manner. This autophosphorylation is fully dependent upon Mg2+ ions, since reactions conducted in the absence of MgCl2 in the buffer show almost zero phosphorylation activity (Figure 2B). By contrast, no autophosphorylation of His10-Obg occurs with [γ-32P] ATP, even after 60 min of incubation. Further, addition of unlabeled

ATP to the reaction mixture fails to produce any effect on His10-Obg phosphorylation with [γ-32P] GTP (Figure 2C). As expected, both unlabeled GTP HA-1077 cell line and GDP significantly affect the phosphorylation of [γ-32P] GTP from His10-Obg (Figure 2C), indicating that both molecules serve as competitors for the phosphorylation site. The eukaryotic Ras protein, which is encoded by the p21ras oncogene, controls cell proliferation, cell stress signaling and apoptosis. The autophosphorylaiton of Ras is independent of its GTPase activity [33], which means that GTP hydrolysis and GTP phosphorylation of Ras occur at two different sites. At present it is unclear whether GTP hydrolysis and GTP-mediated autophosphorylation are independent SBI-0206965 ic50 events for prokaryotic Obgs, and no one has identified a phsophorylation site on any Obg molecule.