The objectives of this effort will be to seek a greater understanding of how protein structure and dynamics correlates with detailed properties. The gramicidin channel formed by a dimer of a 15 amino acid polypeptide provides a unique opportunity to identify such correlations. A high resolution structure of the channel backbone has been achieved for the first time in a lipid environment (Ketchem, Hu & Cross, Science 261:1457- 1460). This structure has been achieved with a new structural approach - the use of orientational constraints by solid state NMR of aligned lipid bilayers. The structural resolution is very high - approximately plus/minus 3 degrees for each of the torsion angles. A very detailed dynamic description of both the sidechains and backbone has also been achieved. However, these descriptions have been accomplished in the absence of cations. To achieve functional correlations the dynamic and structural descriptions will be revisited in the presence of cations. While high resolution structural and dynamic characterizations are available for this channel there is an equally impressive array of functional properties that have been characterized. Single channel lifetimes, conductance rates, channel noise, binding constants and flickering rates have been characterized in detail with numerous cations ranging from alkali metal cations to organic cations. A great many modifications to the channel sequence have been made and the functional properties described. Now there is an opportunity to bring these two realms of knowledge together in an effort to understand in a very detailed way how this molecule functions, what interferes with its function and why it is so efficient at conducting cations. Why specific amino acid changes affect a wide range of functional properties. Through this effort not only a detailed understanding of the gramicidin channel will be achieved, but much of this understanding will have direct relevance to membrane proteins, lipid-protein interactions and ion-protein interactions. The structural stability of proteins in a membrane environment will assessed from an understanding of the monomer-dimer equilibrium. The roles for tryptophan - its electrostatic influence on function and its hydrogen bonding capacity to the membrane surface are general features for this amino acid in membrane proteins. The role for correlated motions in promoting function will be pursued and may have important implications for many enzymes and transport proteins.
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