Analysis of chimeric chemoreceptors in Bacillus subtilis reveals a role for CheD in the function of the McpC HAMP domain

Comparative proteomics reveals stress responses of Vibrio parahaemolyticus biofilm on different surfaces: Internal adaptation and external adjustment

Vibrio parahaemolyticus is a kind of gram-negative marine pathogen, which usually adheres to stainless steel (SS), glass (GS) and other abiotic surfaces in aquaculture and food processing in the form of biofilm and causes the spread of gastrointestinal illness. However, the deeply survival adaptation mechanism of V. parahaemolyticus biofilm cells on these contact surface remained unclear. Here, proteomics was used to investigated the physiological response of the V. parahaemolyticus biofilms cells to different abiotic surfaces (SS, GS and polystyrene (PS)). In addition, the effect of contact materials on the physical-chemical properties of biofilms are also characterized. Results showed that the expression of proteins of biofilm cells established on the SS surface were mainly related to the alleviation of metal ion stress and toxicity. The up-regulated proteins in the biofilm cells formed on the GS surface were mainly involved in the biological processes of sugar uptake, protein synthesis and bacterial chemotaxis. Meanwhile, the significantly expressed proteins in the biofilm cells formed on the PS surface were mainly involved in the cellular physiological activity of aromatic compound metabolism, osmotic stress and nutrient transport. All functional proteins mentioned above were closely related to the interaction characteristics of the contact surface and biofilm. This study provided an in-depth comparison of V. parahaemolyticus biofilm formation on these three abiotic surfaces, and presented a model in first time for the adaptation behavior of biofilm cells on different surfaces as affected by metal ion stress, nutrition, osmotic stress, and sugar utilization, which could facilitate an efficient control strategy for biofilm formation in industrial field.

Elucidation of the multiple roles of CheD in Bacillus subtilis chemotaxis

Chemotaxis by Bacillus subtilis requires the CheD protein for proper function. In a cheD mutant when McpB was the sole chemoreceptor in B. subtilis, chemotaxis to asparagine was quite good. When McpC was the sole chemoreceptor in a cheD mutant, chemotaxis to proline was very poor. The reason for the difference between the chemoreceptors is because CheD deamidates Q609 in McpC and does not deamidate McpB. When mcpC-Q609E is expressed as the sole chemoreceptor in a cheD background, chemotaxis is almost fully restored. Concomitantly, in vitro McpC activates the CheA kinase poorly, whereas McpC-Q609E activates it much more. Moreover, CheD, which activates chemoreceptors, binds better to McpC-Q609E compared with unmodified McpC. Using hydroxyl radical susceptibility in the presence or absence of CheD, the most likely sites of CheD binding were the modification sites where CheD, CheB and CheR carry out their catalytic activities. Thus, CheD appears to have two separate roles in B. subtilis chemotaxis - to bind to chemoreceptors to activate them as part of the CheC/CheD/CheYp adaptation system and to deamidate selected residues to activate the chemoreceptors and enable them to mediate amino acid chemotaxis.

Cellular stoichiometry of the chemotaxis proteins in Bacillus subtilis

The chemoreceptor-CheA kinase-CheW coupling protein complex, with ancillary associated proteins, is at the heart of chemotactic signal transduction in bacteria. The goal of this work was to determine the cellular stoichiometry of the chemotaxis signaling proteins in Bacillus subtilis. Quantitative immunoblotting was used to determine the total number of chemotaxis proteins in a single cell of B. subtilis. Significantly higher levels of chemoreceptors and much lower levels of CheA kinase were measured in B. subtilis than in Escherichia coli. The resulting cellular ratio of chemoreceptor dimers per CheA dimer in B. subtilis is roughly 23.0 ± 4.5 compared to 3.4 ± 0.8 receptor dimers per CheA dimer observed in E. coli, but the ratios of the coupling protein CheW to the CheA dimer are nearly identical in the two organisms. The ratios of CheB to CheR in B. subtilis are also very similar, although the overall levels of modification enzymes are higher. When the potential binding partners of CheD are deleted, the levels of CheD drop significantly. This finding suggests that B. subtilis selectively degrades excess chemotaxis proteins to maintain optimum ratios. Finally, the two cytoplasmic receptors were observed to localize among the other receptors at the cell poles and appear to participate in the chemoreceptor complex. These results suggest that there are many novel features of B. subtilis chemotaxis compared with the mechanism in E. coli, but they are built on a common core.

The three adaptation systems of Bacillus subtilis chemotaxis

Adaptation has a crucial role in the gradient-sensing mechanism that underlies bacterial chemotaxis. The Escherichia coli chemotaxis pathway uses a single adaptation system involving reversible receptor methylation. In Bacillus subtilis, the chemotaxis pathway seems to use three adaptation systems. One involves reversible receptor methylation, although quite differently than in E. coli. The other two involve CheC, CheD and CheV, which are chemotaxis proteins not found in E. coli. Remarkably, no one system is absolutely required for adaptation or is independently capable of generating adaptation. In this review, we discuss these three novel adaptation systems in B. subtilis and propose a model for their integration.

Mutational analysis of the connector segment in the HAMP domain of Tsr, the Escherichia coli serine chemoreceptor

HAMP domains are approximately 50-residue motifs, found in many bacterial signaling proteins, that consist of two amphiphilic helices joined by a nonhelical connector segment. The HAMP domain of Tsr, the serine chemoreceptor of Escherichia coli, receives transmembrane input signals from the periplasmic serine binding domain and in turn modulates output signals from the Tsr kinase control domain to elicit chemotactic responses. We created random amino acid replacements at each of the 14 connector residues of Tsr-HAMP to identify those that are critical for Tsr function. In all, we surveyed 179 connector missense mutants and identified three critical residues (G235, L237, and I241) at which most replacements destroyed Tsr function and another important residue (G245) at which most replacements impaired Tsr function. The region surrounding G245 tolerated 1-residue deletions and insertions of up to 10 glycines, suggesting a role as a relatively nonspecific, flexible linker. The critical connector residues are consistent with a structural model of the Tsr-HAMP domain based on the solution structure of an isolated thermophile HAMP domain (M. Hulko, F. Berndt, M. Gruber, J. U. Linder, V. Truffault, A. Schultz, J. Martin, J. E. Schultz, A. N. Lupas, and M. Coles, Cell 126:929-940, 2006) in which G235 defines a critical turn at the C terminus of the first helix and L237 and I241 pack against the helices, perhaps to stabilize alternative HAMP signaling conformations. Most I241 lesions locked Tsr signal output in the kinase-on mode, implying that this residue is responsible mainly for stabilizing the kinase-off signaling state. In contrast, lesions at L237 resulted in a variety of aberrant output patterns, suggesting a role in toggling output between signaling states.

The CheC Phosphatase Regulates Chemotactic Adaptation through CheD

The bacterial chemotaxis system is one of the most extensively studied signal transduction systems in biology. The response regulator CheY controls flagellar rotation and is phosphorylated by the CheA histidine kinase to its active form. CheC is a CheY-P phosphatase, and this activity is enhanced in a CheC-CheD heterodimer. CheC is also critical for chemotactic adaptation, the return to the prestimulus system state despite persistent attractant concentrations. Here, CheC point mutants were examined in Bacillus subtilis for in vivo complementation and in vitro activity. The mutants were identified separating the three known abilities of CheC: CheD binding, CheY-P binding, and CheY-P phosphatase activity. Remarkably, the phosphatase ability was not as critical to the in vivo function of CheC as the ability to bind both CheY-P and CheD. Additionally, it was confirmed that CheY-P increases the affinity of CheC for CheD, the later of which is known to be necessary for receptor activation of CheA. These data suggest a model of CheC as a CheY-P-induced regulator of CheD. Here, CheY-P would cause CheC to sequester CheD from the chemoreceptors, inducing adaptation of the chemotaxis system. This model represents the first plausible means for feedback from the output of the system, CheY-P, to the receptors.

Structural and Functional Studies of the HAMP Domain of EnvZ, an Osmosensing Transmembrane Histidine Kinase in Escherichia coli

The HAMP domain plays an essential role in signal transduction not only in histidine kinase but also in a number of other signal-transducing receptor proteins. Here we expressed the EnvZ HAMP domain (Arg(180)-Thr(235)) with the R218K mutation (termed L(RK)) or with L(RK) connected with domain A (Arg(180)-Arg(289)) (termed LA(RK)) of EnvZ, an osmosensing transmembrane histidine kinase in Escherichia coli, by fusing it with protein S. The L(RK) and LA(RK) proteins were purified after removing protein S. The CD analysis of the isolated L protein revealed that it consists of a random structure or is unstructured. This suggests that the EnvZ HAMP domain by itself is unable to form a stable structure and that this structural fragility may be important for its role in signal transduction. Interestingly the substitution of Ala(193) in the EnvZ HAMP domain with valine or leucine in Tez1A1, a chimeric protein of Tar and EnvZ, caused a constitutive OmpC phenotype. The CD analysis of LA(RK)(A193L) revealed that this mutated HAMP domain possesses considerable secondary structures and that the thermostability of this entire LA(RK)(A193L) became substantially lower than that of LA(RK) or just domain A, indicating that the structure of the HAMP domain with the A193L mutation affects the stability of downstream domain A. This results in cooperative thermodenaturation of domain A with the mutated HAMP domain. These results are discussed in light of the recently solved NMR structure of the HAMP domain from a thermophilic bacterium (Hulko, M., Berndt, F., Gruber, M., Linder, J. U., Truffault, V., Schultz, A., Martin, J., Schultz, J. E., Lupas, A. N., and Coles, M. (2006) Cell 126, 929-940).

A receptor-modifying deamidase in complex with a signaling phosphatase reveals reciprocal regulation

Signal transduction underlying bacterial chemotaxis involves excitatory phosphorylation and feedback control through deamidation and methylation of sensory receptors. The structure of a complex between the signal-terminating phosphatase, CheC, and the receptor-modifying deamidase, CheD, reveals how CheC mimics receptor substrates to inhibit CheD and how CheD stimulates CheC phosphatase activity. CheD resembles other cysteine deamidases from bacterial pathogens that inactivate host Rho-GTPases. CheD not only deamidates receptor glutamine residues contained within a conserved structural motif but also hydrolyzes glutamyl-methyl-esters at select regulatory positions. Substituting Gln into the receptor motif of CheC turns the inhibitor into a CheD substrate. Phospho-CheY, the intracellular signal and CheC target, stabilizes the CheC:CheD complex and reduces availability of CheD. A point mutation that dissociates CheC from CheD impairs chemotaxis in vivo. Thus, CheC incorporates an element of an upstream receptor to influence both its own effect on receptor output and that of its binding partner, CheD.

The influence of acetyl phosphate on DspA signalling in the Cyanobacterium Synechocystis sp. PCC6803

BACKGROUND

The dspA (hik33) gene, coding for a putative sensory histidine kinase, is conserved in plastids (ycf26) and cyanobacteria. It has been linked with a number of different stress responses in cyanobacteria.

RESULTS

We constructed an insertional mutant of dspA (ycf26) in Synechocystis 6803. We found little phenotypic effect during nitrogen starvation. However, when the mutation was combined with deletion of the pta gene coding for phosphotransacetylase, a more significant phenotype was observed. Under nitrogen starvation, the pta/dspA double mutant degrades its phycobilisomes less than the wild type and still has about half of its chlorophyll-protein complexes.

CONCLUSION

Our data indicates that acetyl-phosphate-dependent phosphorylation of response regulator(s) overlaps with DspA-dependent signalling of the degradation of chlorophyll-protein complexes (and to a lesser extent phycobilisomes) in Synechocystis 6803.