UV and X‐ray structural studies of a 101‐residue long Tat protein from a HIV‐1 primary isolate and of its mutated,detoxified, vaccine candidate

The 101‐residue long Tat protein of primary isolate 133 of the human immunodeficiency virus type 1 (HIV‐1), wt‐Tat₁₃₃ displays a high transactivation activity in vitro, whereas the mutant thereof, STLA‐Tat₁₃₃, a vaccine candidate for HIV‐1, has none. These two proteins were chemically synthesized and their biological activity was validated. Their structural properties were characterized using circular dichroism (CD), fluorescence emission, gel filtration, dynamic light scattering, and small angle X‐ray scattering (SAXS) techniques. SAXS studies revealed that both proteins were extended and belong to the family of intrinsically unstructured proteins. CD measurements showed that wt‐Tat₁₃₃ or STLA‐Tat₁₃₃ underwent limited structural rearrangements when complexed with specific fragments of antibodies. Crystallization trials have been performed on the two forms, assuming that the Tat₁₃₃ proteins might have a better propensity to fold in supersaturated conditions, and small crystals have been obtained. These results suggest that biologically active Tat protein is natively unfolded and requires only a limited gain of structure for its function. Proteins 2010. © 2009 Wiley‐Liss, Inc