Ronald Kaback, MD

Professor of Physiology, Microbiology and Molecular Genetics, UCLA and Howard Hughes Medical Institute

Dr. Kaback overarching interest is in molecular mechanisms of cellular transport. The lactose permease of Escherichia coli (LacY), a particularly well-studied paradigm for ion gradient driven active transport proteins, is solely responsible for all translocation reactions catalyzed by the galactoside transport system in E. coli. Like many active transport proteins, LacY couples the free energy released from downhill translocation of protons in response to a proton electrochemical gradient to drive the energetically uphill stoichiometric accumulation of D-galactopyranosides. Dr. Kaback has solved an X-ray structure of Lac in an inward-facing conformation, confirming many conclusions derived from various biochemical and biophysical studies. The molecule is composed of N- and C-terminal domains, each with six transmembrane helices, symmetrically positioned within the molecule, and many of the helices are irregularly shaped. A large internal water-filled cavity is exposed to the cytoplasm, and sugar is bound at the two-fold axis of symmetry at the apex of the hydrophilic cavity and in the approximate middle of the molecule. Access to the sugar-binding sites completely blocked from the outer side of the molecule. By combining a large body of experimental data derived from systematic studies of site-directed mutants, residues involved in substrate binding and proton translocation have been identified. Based on the functional properties of the mutants and the X-ray structure, a working model for the mechanism involving alternating access of the binding site to either side of the membrane has been postulated. Recent experimental results obtained using site-directed alkylation of cysteine-replacement mutants, isothermal calorimetry, site-directed fluorescence and spin labeling provide strong supporting evidence for the model. Dr. Kaback has been a consultant for CURE: DDRCC members in cell biology and transport and has interacted especially with Drs. George Sachs and Ernest Wright. Collaborations and Past Core Utilization are not listed because Dr. Kaback is a new CURE: DDRCC member.