ION TRANSPORTER

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In this experiment we aim at determining the structure of a Na+/K+ symporter. This bacterial membrane protein co-transports sodium and potassium ions; potassium ions are pumped across a membrane against their chemical gradient by coupling their movement to the favorable movement in the same direction of sodium ions. In the bacteria the role of this protein is to keep a high concentration of K+ inside the cell and to control cell turgor. Symport is a widespread process used for the pumping of ions or metabolites across the membrane. The structure of Lactose permease (a H+/lactose symporter) has provided a good general three-dimensional model for metabolite symport. In fact its structure is very similar to the structure of a related protein, the glycerol-3-phosphate transporter, which is a phosphate/sugar antiporter. The study we are proposing will provide structural and mechanistic information that will be of general relevance for the study of ion symport. For ion symport there are some studies pointing to an alternative mechanism to the variable-accessibility model demonstrated by lactose permease and glycerol-3-phosphate transporter. In this alternative mechanism binding of ion A will alter the affinity for ion B at another binding site without changing the accessibility from bulk solution. Proteins using this mechanism will share some structural characteristics with ion channels, in particular the existence of an ion pore. In fact it has been proposed that these symporters will have the same general architecture present in the KcsA potassium channel, more specifically, a repeat of the (TM helix-pore helix-TM helix) motif. We hope that the determination of the structure of the Na+/K+ symporter will help in the definition of transport mechanism.