fd coat protein structure in membrane environments: structural dynamics of the loop between the hydrophobic trans-membrane helix and the amphipathic in-plane helix.

By performing multidimensional solution NMR experiments on micelle samples it was possible to determine the structure of the membrane-bound form of fd coat protein based on short-range distance and dihedral angle constraints using distance geometry and simulated annealing calculations. Its dynamics were described by 15N relaxation measurements (T1, T2, heteronuclear nuclear Overhauser enhancement (NOE)) fitted with the Lipari-Szabo model-free formalism adapted for the transmembrane and in-plane helices of a membrane protein. The overall correlation time of the protein in micelles was found to be approximately 9 ns, and the local motion of each backbone N-H vector was described by an order parameter and an effective correlation time. The 50 residue protein has an amphipathic alpha-helix (residues 7 to 16) and a hydrophobic alpha-helix (residues 27 to 44), which were found to be approximately perpendicular on the basis of NOEs in the residues that connect the two helices. The residues connecting the helices are of particular interest in membrane proteins, and in this case the loop consists of two turns. The relaxation data show the presence of an extra motion in the amphipathic alpha-helix on the nanosecond timescale and additional flexibility of several residues in the loop connecting the two helices.