Scientists discover a membrane transport system of bacteria

Researchers from the University Hospital of Bonn and the University of Bonn in collaboration with the University of York studied an important class of bacterial membrane transporters and their interaction with soluble substrate binding proteins. They found that transporter and its substrate binding protein adapt to each other rapidly and that these interactions are crucial for the working mechanism of the transporter system.

Membrane transport mechanisms are crucial for the survival of pathogenic bacteria as they, for instance, allow resistance against the host’s immune system. Thus, it is important to understand the underlying molecular mechanism of bacterial membrane transporters and ultimately to find ways to modulate or block them.

A membrane transport mechanism working like a molecular freight elevator

Pathogenic bacteria such as Haemophilus influenzae use tripartite ATP-independent periplasmic (TRAP) transporters to facilitate the uptake of sialic acid into the bacterial cell. Sialic acid is a small sugar molecule that is abundant in our tissues. H. influenzae bacteria can harvest the sialic acid and incorporate it into their own cell wall, thereby camouflaging themselves from the host’s immune system. The TRAP transporter comprises of two transmembrane domains that span the inner membrane of the bacterium and is supported by a substrate binding protein (SBP). Once the SBP encounters a sialic acid molecule, it undergoes a large conformational change, tightly binds the sialic acid and transports it to the TRAP transporter. In the next step, the transporter grabs the sialic acid from the SBP and translocates it across the membrane into the bacterial cell.

The TRAP transporter system can undergo distinct conformational changes

Each of the two proteins can adopt two different conformational states, the open- and closed state of the SBP and the inward- and outward-facing state of the transporter. The research team led by PD Dr. Gregor Hagelueken from the Institute of Structural Biology at the University Hospital of Bonn who is supported by the ImmunoSensation2 Cluster of Excellence and the Transdisciplinary Research Area (TRA) “Life & Health” suspected that there is a ”conformational coupling” between the TRAP transporter and its SBP, meaning that the closed state of the SBP selectively recognizes the inward-facing state of the transporter, while the open state of the SBP preferably interacts with the outward-facing state of the transporter. The team was able to lock both the transporter and its SBP in their conformational states by employing disulfide engineering and found that TRAP transporter and SBP could indeed recognize the conformational state of the other.
By using biophysical and structural biology experiments, as well as advanced microscopy techniques provided in collaboration with Prof. Dr. Ulrich Kubitscheck from the Clausius Institute of Physical and Theoretical Chemistry at the University of Bonn, it was possible to precisely observe the working mechanism of TRAP transporter and the SBP. The researchers incorporated the TRAP transporter into artificial membranes and employed single-molecule fluorescence microscopy to observe the interaction between TRAP transporter and SBP in real time, giving insights into the working principle of the transporter system.
The interesting study was recently published in Nature Communications and sheds light on the structural properties and therefore the working mechanism of SBP-dependent bacterial membrane transport systems.