SFB 449 III: Structural and functional aspects of bacterial solute receptors (TP B 14)

ATP-binding cassette (ABC) proteins exist in all living organisms and form one of the largest superfamilies. ABC transporters share a common architectural organization comprising two hydrophobic transmembrane domains (TMDs) that form the translocation pathway and two hydrophilic nucleotide binding (ABC) domains (NBDs) that hydrolyze ATP. Prokaryotic ABC transporters that mediate the uptake of a large variety of nutrients at the expense of ATP are essential for cell growth and survival. In contrast to ABC exporters, import systems require an additional component, a specific extracellular solute receptor (also known as substrate binding protein) for function. The histidine ABC transport system of Salmonella typhimurium is of one of the best-studied import systems. In order to map putative contact sites of HisJ and LAO with the membrane-integral subunits HisQM in detail, we will use a pepscan approach. In a comparative effort we will also perform a similar set of experiments with an arginine transporter (YqiXYZ) from the thermophilic bacterium Geobacillus stearothermophilus. The crystal structure of the receptor YqiX will also be determined. In acidophilic microorganisms solute receptors of ABC import systems are exposed to low pH and thus, need to be stable under these conditions. We have solved the structure of a maltose binding protein from the thermoacidophilic bacterium Alicyclobacillus acidocaldarius that binds its substrate at pH values ranging from 2 to 7. Compared to related proteins from mesophilic organisms the protein has fewer charged residues which is compensated by an increase in polar residues. Moreover, the protein has a highly positive surface charge. These features appear to be correlated with acidostability. We will test this hypothesis by mutational analysis. Promising candidates will then be crystallized. Furthermore, in order to explore whether these features reflect a more general phenomenon we will solve the crystal structure of a sulfate receptor from the same organism.