Homonuclear 1H NMR and circular dichroism study of the HIV-1 Tat Eli variant

Background: The HIV-1 Tat protein is a promising target to develop AIDS therapies, particularly vaccines, due to its extracellular role that protects HIV-1-infected cells from the immune system. Tat exists in two different lengths, 86 or 87 residues and 99 or 101 residues, with the long form being predominant in clinical isolates. We report here a structural study of the 99 residue Tat Eli variant using 2D liquid-state NMR, molecular modeling and circular dichroism. Results: Tat Eli was obtained from solid-phase peptide synthesis and the purified protein was proven biologically active in a trans-activation assay. Circular dichroism spectra at different temperatures up to 70 C showed that Tat Eli is not a random coil at 20 C. Homonuclear H-1 NMR spectra allowed us to identify 1639 NMR distance constraints out of which 264 were interresidual. Molecular modeling satisfying at least 1474 NMR constraints revealed the same folding for different model structures. The Tat Eli model has a core region composed of a part of the N-terminus including the highly conserved Trp 11. The extra residues in the Tat Eli C-terminus protrude from a groove between the basic region and the cysteine-rich region and are well exposed to the solvent. Conclusion: We show that active Tat variants share a similar folding pattern whatever their size, but mutations induce local structural changes.