Transport of ions across cellular membranes and signalling processes relating to ion availability.

Research

Research in my laboratory is focused on membrane proteins involved in transport of ions across cellular membranes and in signalling processes relating to ion availability. Studies of membrane transporters/channels have had a great impact on our understanding of human disease and drug design; indeed ion channels are, at present, the third biggest target class in drug discovery and about 30% of current clinically marketed drugs are targeted against these proteins.

The philosophy of my research is to use simple and well-studied bacterial model organisms and a multidisciplinary approach to shed more light on two ubiquitous families of ion transporters, both of which are conserved from bacteria to man: the anion transporters from the SLC26/SulP family (TC 2.A.53) and the ammonium transporter from the Amt/Rh family (TC 1.A.11).

The SLC26/SulP family of anion transporters

The ubiquitous SLC26/SulP family is a recently discovered gene family of highly versatile anion transporters with intriguing roles in normal physiology and human pathophysiology. Proteins within the SLC26/SulP family exhibit a wide variety of functions transporting anions ranging from halides to carboxylic acids. The molecular basis for this diversity, however, is poorly understood and structural information is extremely limited. A model of the transmembrane region containing 12 transmembrane helices has been published for the Synechococcus SLC26 protein BicA, and it has been proposed that this topology may apply across the family. The only high resolution structural data available to date is for the cytoplasmic Sulfate Transporter and Anti-Sigma Factor Antagonist (STAS) domain, which is fused to the C-terminus of the transmembrane domain. While the amount of information on eukaryotic members of the SCL26/SulP family is steadily increasing, the role of the prokaryotic homologues remains unclear. Therefore, the two major objectives of this project are:

1.To obtain essential information on the structure of the SLC26/SulP proteins.

2.To investigate the physiological role of the SulP proteins in bacteria.

Ultimately we seek to develop a model system to investigate questions of structure and function relating to the SLC26/SulP proteins.

The Amt/Rh family of ammonium transporters/channels

Ammonium transport is facilitated by the Amt proteins, found in all domains of life from bacteria to man. In animals they are represented by the Rhesus (Rh) proteins, which have been implicated in ammonium homeostasis. The predominant view has long been that these proteins act as secondary active transporters for NH₄⁺ but this has been challenged by the resolution of the X-ray structure of the Escherichia coli channel AmtB that revealed a predominantly hydrophobic channel, suggesting a high energy barrier for the conduction of NH₄⁺. Therefore, the exact mechanism of ammonium transport is intensely debated. The AmtB protein from E. coli is the best system to investigate questions of structure, function and signal transduction relating to Amt proteins. Therefore, we sought to use a multi-faceted approach calling upon expertise in genetics, biochemistry, biophysics and molecular dynamic simulations to gain valuable structural and functional information on the E. coli AmtB protein

Teaching

Publications

1. Compton EL, Karinou E, Naismith JH, Gabel F, Javelle A.  (2011) Low resolution structure of a bacterial SLC26 transporter reveals a dimeric stoichiometry and mobile intracellular domains. J Biol Chem.  286:27058-27067.
2. Javelle A, Lupo D, Ripoche P, Fulford T, Merrick M, and Winkler FK (2008) Substrate binding, deprotonation and selectivity at the periplasmic entrance of the ammonia channel AmtB from E. coli. Proc Natl Acad Sci U S A. 105, 5040-5045.
3. Javelle A, Lupo D, Li XD, Merrick M, Chami M, Ripoche P, and Winkler FK. (2007) Structural and mechanistic aspects of Amt/Rh proteins. J Struct Biol. 158, 472-481.
4. Javelle A, Lupo D, Zheng L, Li XD, Winkler FK and Merrick M. (2006) An unusual twin-His structure in the pore of ammonia channels is essential for substrate conduction. J Biol Chem. 281, 39492-39498
5. Javelle A, Thomas G, Marini AM, Kramer R, Merrick M. (2005) In vivo functional characterisation of the E. coli ammonium channel AmtB: evidence for metabolic coupling of AmtB to glutamine synthetase. Biochem J. 390, 215-222.
6. Javelle A, Severi E, Thornton J, Merrick M, (2004) Ammonium sensing in E.coli : The role of the ammonium transporter AmtB and AmtB-GlnK complex formation.J Biol Chem. 279, 8530-8538.