SOLUTION STRUCTURE AND BACKBONE DYNAMICS OF THE PHOTOACTIVE YELLOW PROTEIN

The solution structure of photoactive yellow protein (PYP), a photosen sory protein from Ectothiorhodospira halophila, has been determined by multidimensional NMR spectroscopy. The structure consists of an open, twisted, 6-stranded, antiparallel beta-sheet, which is flanked by fou r alpha-helices on both sides. The final set of 26 selected structures is well-defined for the regions spanning residues Phe(6)-Ala(16), Asp (24)-Ala(112), and Tyr(118)-Val(125) and displays a root-mean-square d eviation, versus the average, of 0.45 Angstrom for the backbone and 0. 88 Angstrom for all heavy atoms. Comparison of the solution structure with an earlier published 1.4 Angstrom crystal structure (Borgstahl, G . E. O., Williams, D. R., and Getzoff, E. D. (1995) Biochemistry, 34, 6278-6287) reveals a similarity with a root-mean-square deviation of 1 .77 Angstrom for the backbone for the well-defined regions. The most d istinct difference in the backbone with the crystal structure is found near the N-terminus, for residues Asp(19)-Leu(23), which corresponds to an alpha-helix in the crystal structure and to one of the poorest d efined regions in the solution structure. To characterize the dynamic behavior of PYP in solution, we undertook a N-15 relaxation study and measurements of hydrogen/deuterium exchange. Determination of order pa rameters through the model-free Lipari-Szabo approach enabled the iden tification of several regions of enhanced dynamics. The comparison of atomic displacements in the backbone traces of the ensemble structures , with mobility measurements from NMR, show that the poorly defined re gions feature fast internal motions in the nanosecond to picosecond ti me scale.