The flagella of E. coli move the bacterium in one of two ways. When they spin counterclockwise, the cell is propelled forward in a straight line. When they spin clockwise, the bacterium tumbles in place, ultimately pointing in some new, random direction, ready for another straight-line run. The search for food is a major trigger for this tumbling and running, and the direction of flagellar spin is regulated in part by methyl-accepting chemotaxis proteins (MCPs). The extracellular domain of an MCP is a chemosensor, which responds to changing concentrations of its target molecule by a shape change. The signal embodied by that shape change is transduced first through a membrane-spanning domain, then to one or more small linkers called HAMPs, and finally to a kinase control module, which can turn on, or turn off, a kinase, which then either does, or doesn’t, phosphorylate another protein. The ratio of the phosphorylated to unphosphorylated protein is the final determinant of the direction of flagellar rotation, and hence the movement of the bacterium. The HAMPs are not merely passive links in this chain, they relay the signal from the chemosensor to the control module. A Little Switch: Alternative Domain Conformations Control Bacterial Flagella Rotation Direction. (2013) PLoS Biol 11(2): e1001480. doi:10.1371/journal.pbio.1001480
HAMP Domain Conformers That Propagate Opposite Signals in Bacterial Chemoreceptors. (2013) PLoS Biol 11(2): e1001479. doi:10.1371/journal.pbio.1001479
HAMP domains are signal relay modules in >26,000 receptors of bacteria, eukaryotes, and archaea that mediate processes involved in chemotaxis, pathogenesis, and biofilm formation. We identify two HAMP conformations distinguished by a four- to two-helix packing transition at the C-termini that send opposing signals in bacterial chemoreceptors. Crystal structures of signal-locked mutants establish the observed structure-to-function relationships. Pulsed dipolar electron spin resonance spectroscopy of spin-labeled soluble receptors active in cells verify that the crystallographically defined HAMP conformers are maintained in the receptors and influence the structure and activity of downstream domains accordingly. Mutation of HR2, a key residue for setting the HAMP conformation and generating an inhibitory signal, shifts HAMP structure and receptor output to an activating state. Another HR2 variant displays an inverted response with respect to ligand and demonstrates the fine energetic balance between “on” and “off” conformers. A DExG motif found in membrane proximal HAMP domains is shown to be critical for responses to extracellular ligand. Our findings directly correlate in vivo signaling with HAMP structure, stability, and dynamics to establish a comprehensive model for HAMP-mediated signal relay that consolidates existing views on how conformational signals propagate in receptors. Moreover, we have developed a rational means to manipulate HAMP structure and function that may prove useful in the engineering of bacterial taxis responses.