TRIFLUOROETHANOL STABILIZES A HELIX-TURN-HELIX MOTIF IN EQUINE INFECTIOUS-ANEMIA-VIRUS TRANSACTIVATOR PROTEIN

The solution structure of the 75-amino-acid trans-activator (Tat) prot ein of the equine infectious-anemia virus in trifluoroethanol-containi ng solution was determined by two-dimensional and three-dimensional nu clear magnetic resonance spectroscopy, resulting in a total of 838 nuc lear-Over-hauser-enhancement distance restraints, and restrained molec ular-dynamics simulations. In contrast to the recently determined stru cture of this protein in trifluoroethanol-free pH 6.3 solution, the hy drophobic core and the adjacent basic RNA-binding region of the protei n showed well-defined alpha-helical secondary structure in trifluoroet hanol-containing solution. The helical regions comprise those parts of the molecule whose helix-forming tendencies were noted earlier in tri fluoroethanol-free solution. Two helices (Gln38-Arg43 and Asp48-Ala64) are connected by a tight type-II turn centered at the strictly conser ved Gly46 leading to a helix-turn-helix motif in the core and basic re gion of the protein. A third helix (Thr9-Asn13) is located in the less well defined N-terminal part of the protein. These observations may s upport the notion that the protein adopts a helical structure in the R NA-binding region on complex formation. Although the secondary-structu re elements become better defined in trifluoroethanol-containing solut ion, the opposite is true for the hydrophobically stabilized tertiary structure. This adds a caveat to studies of protein structures in trif luoroethanol-containing solution in general.