Gram-negative pathogens secrete effector proteins into human cells to modulate normal cellular processes and establish a bacterial replication niche. Shigella and pathogenic Escherichia coli possess homologous effector kinases, OspG and NleH1/2, respectively. Upon translocation, OspG but not NleH binds to ubiquitin and a subset of E2~Ub conjugates, which was shown to activate its kinase activity. Here we show that OspG, having a minimal kinase fold, acquired a novel mechanism of regulation of its activity. Binding of the E2~Ub conjugate to OspG not only stimulates its kinase activity but also increases its optimal temperature for activity to match the human body temperature and stabilizes its labile C-terminal domain. The melting temperature (T_{m}) of OspG alone is only 31 °C, as compared to 41 °C to NleH1/2 homologs. In the presence of E2~Ub, the T_{m} of OspG increases to ~42 °C, while Ub by itself increases the T_{m} to 39 °C. Moreover, OspG alone displays maximal activity at 26 °C, while in the presence of E2~Ub, maximal activity occurs at ~42 °C. Using NMR and molecular dynamics calculations, we have identified the C-terminal lobe and, in particular, the C-terminal helix, as the key elements responsible for lower thermal stability of OspG as compared to homologous effector kinases.
【저자키워드】 molecular dynamics, Shigella flexneri, activation mechanism, effector kinase, OspG, temperature stability.,