Bacterial multidrug resistance (mdr) is a significant problem in the treatment of most infectious diseases. This multidrug resistance is caused by the over-expression of drug efflux pumps that are located in the lipid bilayer of bacteria. An important class of mdr pumps in bacteria are the Small Multidrug Resistance (SMR) family of transporters. These transporters translocate hydrophobic cations through a coupling mechanism through the cell membrane using energy derived from H+ gradients. Recently, our laboratory has over-expressed, purified, and crystallized a full-length member of the SMR family. Our objective is to discover the molecular structural components that are involved in the translocation of multiple drug molecules through the cell membrane by SMR transporters and to understand the general transport mechanisms that confer the multidrug phenotype. A high-resolution atomic structure of a bacterial multidrug resistance SMR transporter will serve as an excellent model for other 12-TM antiporters that are involved in sugar, ion, amino acid, inorganic, and organic permeation through the membrane. An x-ray crystal structure of an SMR transporter could also provide structural information that will be useful for understanding more complicated yet homologous mammalian transporters. Our objectives are: 1. Over-expression and purification of SMR transporters and their homologs.2. Crystallization and x-ray data collection of SMR transporters.3. X-ray structure determination and refinement of SMR transporters.4. Structural studies of SMR transporters concerning substrate translocation.5. Structural studies of SMR transporters concerning substrate recognition.