In the past few years a number of bacterial transport proteins have been shown to act as co-sensors for signal transduction pathways. This process generally occurs via a protein-protein interaction between the membrane bound sensor domain, which binds specific substrates, and the signalling domain, which transfers the signal information into the cytoplasm of the cell.
In this paper by Dintner et al., as well as in previously published studies, it has been shown that in the absence of the transporter component these signal transduction pathways are rendered inactive. This is due to signalling activation being entirely dependent on a sensory transporter sensing its specific substrate. All currently known examples of these systems are involved in resistance to antibiotics and the role of a transporter in signalling is conserved.
The system used in this paper to investigate this phenomenon in greater detail was the BceRS-BceAB system from the Bacillus subtilis bacterium, which confers resistance against the antibiotic bacitracin. The BceRS component of the system is a two-component regulatory system (TCS) and the signal transduction domain, whereas BceAB is an ABC transporter and the sensor domain. It is not known exactly how BceRS-BceAB confers resistance to bacitracin, however it is possible that it’s sequestered into the cytoplasm via the BceAB ABC transporter. Bacitracin is known to inhibit both cell wall and peptidoglycan synthesis in bacteria.
It has previously been shown that BceS, the histidine kinase component, is unable to detect the presence of bacitracin without BceAB, the ABC transporter component. This therefore lead to the assumption that BceAB is the sensory part of the system.
Initial experiments showed clear interactions between BceS and BceB or BceAB, however BceA was not observed to interact with any components of the TCS (BceS, BceR or BceRS). Dintner et al., also showed that BceR production in the absence of BceS resulted in a lack of interaction with the transporter (BceA, BceB or BceAB). This lead to the conclusion that BceS and BceAB form a scaffold that allows BceR to interact with the complex. Addition of the bacitracin antibiotic did not appear to have an effect on complex formation.
Following on from these discoveries the group wanted to identify whether BceAB, the ABC transporter, interacted directly with the substrate bacitracin or not. They investigated this via surface plasmon resonance (SPR) spectroscopy. This technique uses light diffracted off the underside of a surface containing the molecule of interest to create a spectrum. The change in this spectrum as a substrate is added to the surface, and possibly binds the molecule of interest, can be measured accurately along with the association and dissociation rates.
Unfortunately the BceAB complex was unstable under the SPR conditions and so BceB alone was used in the studies. Zn2+-bacitracin, the active form of the antibiotic, was used as the substrate along with the peptide nisin as a nonsubstrate control. The KD of Zn2+-bacitracin under steady state was calculated to be 60nM, whilst nisin showed no binding to the BceB. Interestingly the absence of Zn2+ prevented bacitracin binding BceB, giving further evidence of the specificity of BceB to the active peptide, Zn2+-bacitracin. The data obtained from these experiments show that the transporter, BceAB, binds free Zn2+-bacitracin specifically and with high affinity.
Dintner et al., conclude by stating that they have proposed a “working model for the mechanism of signal transduction within Bce-like models”. Bce-like systems “represent widely spread resistance determinants against peptide antibiotics in Firmicutes bacteria” and therefore make this study important in the war against antibiotic resistance.
Source: Dintner et al., (2014). A sensory complex consisting of an ATP-binding cassette transporter and a two-component regulatory system controls bacitracin resistance in Bacillus subtilis. The Journal of Biological Chemistry, 289(40)27899-910.