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Abstract

In gram-negative bacteria, the transport of larger essential nutrients such as ferric siderophores or vitamin B12 depends on
TonB-dependent transporters. These transporters bind their substrates with high affinity and ensure transport by contacting
the Ton system, an energy transduction complex. The required energy obtained from the chemiosmotic gradient maintained
across the cytoplasmic membrane. TonB, in complex with ExbB and ExbD, transduces this energy causing conformational
rearrangements at the periplasmic face of outer membrane receptors and thereby facilitating passage of nutrients into the
periplasm. Despite decades of research, the stoichiometry, subunit organization, and mechanism of action of the membrane
proteins of the Ton system remain unclear. We copurified ExbB with ExbD as a ∼240 kDa protein-detergent complex, measured
by light scattering and by native gels. Quantitative Coomassie staining revealed a stoichiometry of ExbB4-ExbD2. Negative
stain electron microscopy and 2D analysis showed particles of ∼10 nm diameter in multiple structural states. Nanogold labeling
identified the position of the ExbD periplasmic domain. Random conical tilt was used to reconstruct the particles in three
structural states followed by sorting of the single particles and refinement of each state. The different states are interpreted by
coordinated structural rearrangements between the cytoplasmic domain and the periplasmic domain, concordant with in vivo
predictions.

Biography

Sofia Khalil was born on 9 August 1978 in Egypt. After graduation in Biochemistry Science in 1999, she conducted her master studies in the Biochemistry
department, Faculty of Science, Alexandria University in Egypt (2000-2004). She continued in her academic career by conducting her PhD studies at Chemistry
and Biochemistry department, Concordia University, Montreal, Canada (2006-2010). Her research interest focused on the protein-protein interaction networks in E.
coli. She had a postdoctoral position in the Microbiology and Immunology department, Faculty of medicine at Mc Gill University, Montreal, Canada (2011). Finally
she has worked in a lecturer position at Biochemistry department, Faculty of Science, Alexandria University in Egypt (2012 up to now).