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Multidrug transporters(MDR),which mediate export of a broad range of chemically unrelatedsubstrates across the membranes,are the most important pumps for multidrug resistance.MATE(multidrug and toxic compound extrusion)family is the last identified and characterized MDRgroup.The first crystal structure of NorM,a member of MATE family,was determined withRb+/Cs+ binding to the protein.NorM uses Na+ gradients to extrude the substrates.In order tounderstand the structure and function of NorM in complex with Na+ ions,we carried out MDsimulation for wild-type and mutants of NorM with and without Cs+/Na+ binding.We alsostudied Energy-coupling factor(ECF)transporter RibU which is responsible for uptake ofmicronutrients in prokaryotes.Our result and analysis revealed a critically important gatingmechanism,in which part of loop5(L5)(eleven residues,missing in the crystal structure)betweenTM5 and TM6 is dynamically flexible and serves as a gate.Specifically,the L5 opens a largecavity accessible to riboflavin from the extracellular space in Apo-RibU and closes the cavity uponriboflavin binding through hydrophobic packing with riboflavin.Thus,L5 is proposed to be the gatefor riboflavin binding.