Bacterial Membranes

Bacterial Membranes Unit

Cécile Wandersman
Iron is essential for many metabolic pathways. Yet, iron is not readily available due to its low solubility in aerobic conditions, or to its tight association to iron carrier proteins or to heme in hemoproteins. Bacteria have developed several iron acquisition sytems, one of which requiring the secretion of hemophores, a family of proteins discovered in our laboratory. These proteins capture free heme or extract heme from heme carrier proteins (owing to their higher affinity for heme), and return it to hemophore-specific outer membrane receptors. This new Heme Acquisition System (HAS) was found in Serratia marcescens, but is also present in Erwinia carotovora, Pseudomonas aeruginosa, Pseudomonas fluorescens, Yersinia enterocolitica and Yersinia pestis.
Our studies cover the regulation of gene expression as well as the elucidation of the mecanism of action of each of the protein of the HAS system. Fixation of holo-HasA on the receptor (HasR) is the signal allowing a specific sigma and anti-sigma to regulate hasRADEB operon expression induction. Hemophore fixation to the receptor is mediated by two peptides and does not require energy, while heme transport as well as hemophore release are both energy dependent. This energy derives from the proton motive force through the complex of three proteins TonB, ExbB and ExbD. HasA secretion requires an ABC exporter.
Our team has contributed to study these transporters that consist of three cell envelope proteins, one of them belonging to ABC ATPase family. We showed that the ABC protein has several interaction sites with its substrate that we are currently trying to characterize.
We are also working in two other projects. One is the study of a new two component system (YgiX/YgiY) involved in regulation of several Escherichia coli functions such as motility, biofilm formation and iron acquisition. The sensor YgiY has a potential iron binding motive : EXXE and regulates YgiX. The other, is related to the study of ABC proteins of unknown functions. We identified 4 such ABC proteins in E. coli (Uup, YbiT, YjjK and YheS) and showed that 3 are involved in the competitiveness of bacteria and in the dynamics of their mobile genetic elements