The active transport of a large number of metabolites across the cytoplasmic membrane of Gram negative bacteria is accomplished by specific transport systems which consist of three components: a ligand, a periplasmic binding protein, and an associated membrane-protein complex. Various periplasmic binding proteins share a similar two-domain hinged structure yet have high specificity for their respective ligands. Upon binding the ligand (to a site in the cleft between domains), the protein undergoes a conformation change which closes the cleft, resulting in a compact globular structure. This conformational change appears to be a prerequisite for the recognition of the liganded protein by its corresponding membrane-bound protein components, which results in translocation of the ligand across the membrane. The detailed molecular mechanisms of these processes remain to be understood, and promise to be a fruitful field for investigation by NMR. The L-glutamine transport system of E. coli provides an excellent model for a detailed investigation of the structural and dynamic properties involved in active transport. The essential component of this transport system is glutamine-binding protein (GlnBP). An effort is currently under way to assign the resonances of GlnBP, a prerequisite to determining its solution structure. The size of this protein (226 amino acids; 25 kDa) causes extensive overlap of NMR signals, requiring the use of triple-resonance 3D experiments. Several such experiments have been performed on both uniformly- and site-specifically (13C,15N)-labeled GlnBP using a Bruker DMX500 spectrometer. A few additional experiments, requiring proton decoupling and pulsed field gradients, may be necessary to complete the backbone assignment. Once this has been accomplished, progress can be made in several areas: (1) use of heteronuclear-edited NOESY data to generate initial solution structures; (2) assignment of side-chain resonances (necessary for structure refinement); and (3) investigation of protein dynamics using 13C and 15N relaxation and heteronuclear NOE measurements.
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