To respond appropriately to different environmental changes, bacteria have evolved two-component signal transduction systems (TCSTs), which are absent in mammals (including human beings). A typical TCST consisted of a sensor kinase (histidine kinase, HK) and a response regulator (RR). HK is capable of autophosphorylation in response to an environmental signal, while RR interacts with the phosphorylated HK. Membrane protein AgrC is a sensor kinase of a TCST from Staphylococcus aureus. Illumination of signal transduction mechanism is of great significance to solve the problem of bacterial resistance. At present, the main bottleneck of membrane protein is the difficulties in obtaining large quantities of sufficiently pure and functional protein. The target protein was overexpressed in Escherichia coli, solubilized from cell membranes and purified in detergent micelles. This series of steps tend to lead to destabilizations of membrane protein and loss of function. In this study, AgrC was incorporated into liposomes by a detergent-mediated method. For standard incorporation, protein solution was added to detergent-lipid suspension containing lipid at 2.5 mmol/L at a lipid-to-protein ratio of 300 (mol/mol). For slow detergent removal, successive additions of small amounts of beads at Bio-bead-to-detergent ratios of 2 (w/w) will allow the removal of the detergent, resulting in formation of proteoliposome. The structure, morphology and average diameter of liposomes and proteoliposomes were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS), respectively. Sucrose density gradient centrifugation was employed to separate proteoliposomes. The result showed efficiency of protein incorporation and liposomes recovery reached 80% and 90%, respectively. Thiol reagent labeling test showed the cytoplasmic domain of AgrC was almost exclusively oriented towards the inside of the liposome vesicles. In vitro phosphorylation experiments showed that kinase activity of AgrC in proteoliposomes was significantly higher than in detergent micelles. Proteoliposomes could be stored for two weeks with little loss of function. Preparation of proteoliposome not only solves the instability problem of membrane proteins, but also provides a new approach of the study of membrane protein structure, function and signal transduction mechanism in vitro.
Background: Bacillus amyloliquefaciens Q-426 can secrete numerous cyclic lipopeptides that have antifungal and antitumor activities. ComQXPA is a common quorum sensing (QS) system in Bacillus species. Most B. amyloliquefaciens strains are encoding the QS gene cluster comQXPA, however, the biological function of the ComQXPA system in B. amyloliquefaciens has not been well studied. In this study, we identified the comQXPA gene locus and the chemical structure of ComXQ-426 in B. amyloliquefaciens Q-426, and explored the function of ComXQ-426 in regulating lipopeptide production.Results: We identified and analyzed the comQXPA locus in Q-426. The full length of the comQXPA gene cluster was 4,014 bp, including 912 bp of comQ, 165 bp of comX, 2292 bp of comP, and 645 bp of comA. The comQXPA locus belongs to group B, as comQ and comX overlap by only one base pair. ComXQ-426 consists of six amino acids (GGDWKY) that contain a modified tryptophan residue. The antifungal activity of Q426ΔcomX was significantly affected, and almost no antifungal activity was observed, while the antifungal activity of strain Q426ΔcomX /comQX was restored to the same level as that of the wild-type strain. When the ComXQ-426 was added to the culture medium at a final concentration of 8 μg/L at the early stage of the log-phase, the antifungal activity of the wild-type strain Q-426 was significantly improved. Knocking out the comX gene did not affect the growth of the bacteria, however, the strain Q426ΔcomX lost its swimming ability, was unable to form colonies when spread on a solid surface, and could not form biofilms on the interface between the gas and liquid medium.Conclusions: Disruption of the ComPA signaling pathway in the Q-426 strain resulted in significant effects on bacillomycin D production, morphology, and motility.
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