STRUCTURE DETERMINATION OF ION CHANNEL PROTEINS

Cornell University, Ithaca NY

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. OmpF porins are cation-selective aqueous channels found in the outer membrane (OM) of Gram-negative bacteria. Their main function is to facilitate the translocation of small hydrophilic solutes across the OM. One unique feature of the OmpF porin is an extracellular loop that folds into the interior of the pore, creating the constriction zone. It has been suggested that charge asymmetry in the constriction zone could give rise to a strong electric field and aid in permeation of dipolar solutes. Computational studies have suggested an intriguing ion-conducting property of OmpF, in which cations and anions follow separated pathways. However, this feature has yet to be directly observed experimentally. We are, therefore, employing x-ray crystallography in order to study the cation and anion selective pathways along the OmpF pore. In addition to its unique ion conduction properties, the OmpF channel is also the main gateway that allows antibiotic molecules to permeate Gram-negative bacterial cells. With the emergence of antibiotic resistance by virtue of decreased membrane permeability, it is necessary to find ways to increase antibiotic translocation across OmpF porin. Therefore, in a second subproject, we have crystallized OmpF in complex with various antibiotics. For the first time, we are able to experimentally visualize protein-drug interactions at the molecular level. Therefore, results of this work will give insights into the design of antibiotics with improved diffusional characteristics and target specificity. The ionotropic glutamate receptor ion channels (iGluRs) mediate excitatory responses at the vast majority of synapses in the brain and spinal cord. The binding of neurotransmitter molecules to the ligand-binding domain (LBD) of these receptors drives the opening of the transmembrane pore. These receptors assemble as tetramers. However, previous crystal structures have revealed only dimer complexes. Therefore, in this third project, we have solved the crystal structure of a novel GluR2 LBD dimer-of-dimers.