The Streptococcus agalactiae Ribose Binding Protein B (RbsB) Mediates Quorum Sensing Signal Uptake via Interaction with Autoinducer-2 Signals

Understanding aquatic pathogen in sediments or aquacultural water is crucial to protect public health from soilborne and waterborne diseases. Quorum sensing (QS) was increasingly reported in biological wastewater treatment processes because of their inherent roles in biofilm development, bacterial aggregation and so on. The widely QS signals was Antoinducer-2 (AI-2), primarily involved to allow the possibility of interspecies communication. However, the cellular components that mediate the response of Streptococcus agalactiae to AI-2 have not been fully characterized. Analysis of the complete genome sequence of S. agalactiae indicated that its RbsB protein has similarity to Escherichia coli LsrB and Aggregatibacter actinomycetemcomitans RbsB proteins that bind AI-2. We hypothesized that RbsB protein mediates quorum sensing signal uptake via interaction with AI-2. To evaluate the regulatory effect of RbsB on QS system, the recombinant plasmid pGEX-6p-1-RbsB was constructed and RbsB protein was purified with GST-tag. To further elucidate the role of RbsB protein binding to DPD (AI-2 precursor dihydroxypentanedione), the systematically throughput circular dichroism (CD) spectroscopy, isothermal titration calorimetry200 (ITC200) and molecular docking methods were employed. The high expression of soluble RbsB protein with molecular weight of 33 kDa was obtained. The thermodynamics results (ΔH <0, ΔS < 0, ΔG *lt; 0) with ITC determination indicated that the binding process between DPD and RbsB was exothermic and spontaneous, with hydrogen bonds and van der Waals forces as the main binding forces. Obviously, DPD can be more easily combined with RbsB in a dose-dependent manner, suggesting that RbsB was changed in the microenvironment of DPD when the DPD concentration was between 0.8–1.0 mmolL−1 and reaching the maximum binding amount. According to molecular docking, 3 hydrophobic residues involved in DPD and RbsB protein stable binding were be found, and also hydrogen bonding plays a key role in the formation of the new complex. RbsB efficiently inhibited V. harveyi bioluminescence induced by both S. agalactiae AI-2 and V. harveyi AI-2 in a dose-dependent manner. However, our results suggest that RbsB may play a role in the response of S. agalactiace to AI-2.