X-ray crystallography of integral membrane protein complexes
The determination of structures of any protein is essential for the understanding protein function. Integral membrane proteins represent at least 30% of open reading frames in the genome and approximately 50% – 70% of pharmacological therapeutic targets. Despite their medical importance, they comprise only ~50 unique structures in the protein data base, as opposed to the >25,000 structures available for soluble proteins. The Iverson laboratory specializes in understanding the structure-function relationships in membrane proteins using x-ray crystallography as the primary tool. We are investigating several broad categories of membrane proteins including respiratory proteins, ion channels, and receptors.
Respiratory systems require integral membrane proteins to establish a transmembrane proton gradient used for ATP synthesis. Different organisms utilize different respiratory systems to help establish this electrochemical gradient, but the mechanisms are similar and the proteins mediating the process sometime use common architectures. During mitochondrial respiration, respiratory complex II converts succinate to fumarate in the Krebs cycle and passes the electrons to quinone molecules bound in the transmembrane region. I have previously determined the initial structure of a complex II homolog from E. coli and we are now using this information to direct further biochemical and structural studies. Some organisms with alternative respiratory systems can utilize nitric oxide (NO), a component of smog, as a respiratory metabolite. Pathogenic bacteria perform NO conversion to neutralize the NO secreted by host defense mechanisms and evade immunological responses. We are studying several enzymes that catalyze NO conversions to understand the chemistry of these reactions in biological systems.
Ion channels mediate all excitable processes in cells, and their hallmark characteristics include exquisite selectivity and rapid translocation of ions across the membrane. The open probability of ion channels physiologically can be modulated by a variety of different stimuli. The allure of structural studies on ion channels arises from the unrivaled information gained through the direct observation of interactions between the protein and the permeant ion. We are using structural techniques to understand the gating of ion channels and gain further insights into their selectivity.
Receptors are perhaps the most interesting membrane proteins to study using structural techniques. These proteins represent a very large number of drug targets, and elucidating the structures of receptors in their inactive and activated forms as well as in complex with the soluble proteins that initiate signal cascades can aid in structure-based drug design.
In addition to looking at structures in specific systems, research in the Iverson laboratory will work to develop techniques to make membrane protein crystallography more tractable.