AQUEOUS-SOLUTION STRUCTURE OF A HYBRID LENTIVIRAL TAT PEPTIDE AND A MODEL OF ITS INTERACTION WITH HIV-1 TAR RNA

Human immunodeficiency virus, type 1, (HIV-1) encodes a transactivatin g regulatory protein, called Tat, which is required for efficient tran scription of the viral genome. Tat acts by binding to a specific RNA s tem-loop element, called TAR, on nascent viral transcripts. The specif icity of binding is principally determined by residues in a short, hig hly basic domain of Tar. The structure in aqueous solution of a biolog ically active peptide, comprised of the ten-amino acid HIV-1 Tat basic domain linked to a 15-amino acid segment of the core regulatory domai n of another lentiviral Tat, i.e., that from equine infectious anemia virus (EIAV), has been determined. The restraint data set includes int erproton distance bounds determined from two-dimensional nuclear Overh auser effect (2D NOE) spectra via a complete relaxation matrix analysi s. Thirty structures consistent with the experimental data were genera ted via the distance geometry program DIANA. Subsequent restrained mol ecular mechanics calculations were used to define the conformational s pace subtended by the peptide. A large fraction of the 25-mer peptide assumes a structure in aqueous solution with the lysine- and arginine- rich HIV-1 basic domain being alpha-helical and the EIAV core domain b eing separated from the basic domain by a turn and characterized by a nascent helix as well. The Tat peptide/TAR complex could be modeled wi th the basic alpha-helix lying in the major groove of TAR such that im portant interactions of a putative specificity-endowing arginine are m aintained and very slight widening of the major groove is entailed.