Salmonella entericaSerovar Typhimurium BipA Exhibits Two Distinct Ribosome Binding Modes

Abstract

BipA is a highly conserved prokaryotic GTPase that functions to influence numerous cellular processes in bacteria. In Escherichia coli and Salmonella enterica serovar Typhimurium, BipA has been implicated in controlling bacterial motility, modulating attachment and effacement processes, and upregulating the expression of virulence genes and is also responsible for avoidance of host defense mechanisms. In addition, BipA is thought to be involved in bacterial stress responses, such as those associated with virulence, temperature, and symbiosis. Thus, BipA is necessary for securing bacterial survival and successful invasion of the host. Steadystate kinetic analysis and pelleting assays were used to assess the GTPase and ribosome-binding properties of S. enterica BipA. Under normal bacterial growth, BipA associates with the ribosome in the GTP-bound state. However, using sucrose density gradients, we demonstrate that the association of BipA and the ribosome is altered under stress conditions in bacteria similar to those experienced during virulence. The data show that this differential binding is brought about by the presence of ppGpp, an alarmone that signals the onset of stress-related events in bacteria.

Structure of BipA in GTP form bound to the ratcheted ribosome

“…Consistent with these observations, our structure of GDPCP-BipA bound to the ribosome reveals an activated state with catalytic histidine (His78) positioned within interaction distance to the SRL (Fig. 3B), further corroborating BipA as a classic trGTPase (5,12). In addition to the SRL and the trGTPase factor, efficient activation of trGTPases also requires other GAC components.…”

“…Consistent with this notion, BipA is able to bind to 70S ribosome in a GTP-dependent manner and its GTPase activity is enhanced in the presence of ribosomes, a characteristic feature of classical trGTPase factors (5,12). Salmonella enterica BipA has been shown to interact with either 70S ribosomes or 30S subunits depending on the relative abundance of GTP and of the stress alarmone guanosine-3′,5′-bisdiphosphate (ppGpp), respectively (12). In addition, a recent study links BipA to ribosome biogenesis because bipA gene deletion results in perturbed 50S subunit processing and assembly, particularly at low temperatures (13).…”

“…Thus, BipA likely functions as an elongation factor as well. Consistent with this notion, BipA is able to bind to 70S ribosome in a GTP-dependent manner and its GTPase activity is enhanced in the presence of ribosomes, a characteristic feature of classical trGTPase factors (5,12). Salmonella enterica BipA has been shown to interact with either 70S ribosomes or 30S subunits depending on the relative abundance of GTP and of the stress alarmone guanosine-3′,5′-bisdiphosphate (ppGpp), respectively (12).…”

“…Alike, ppGpp binding to IF2 interfers with binding to its interaction partners (41). This finding could provide a structural insight for the observation that BipA associates with either the 70S ribosome or the 30S subunit depending on the relative intracellular abundance of GTP and ppGpp (12).…”

“…In case of trGTPases, this partner is the ribosome. An intriguing feature of BipA is that it has been reported to exhibit different modes of ribosome binding when in complex with ppGpp or GTP, namely binding to 30S subunits and 70S ribosomes, respectively (5,12). Therefore, we obtained the structures of BipA in complex with GDPCP (a nonhydrolysable GTP analog), GDP, and ppGpp by using the cocrystallization method (Table S1).…”