Purification and Functional Characterization of Wild-Type and Mutant HIV-1 and HIV-2 Tat Proteins Expressed inEscherichia coli

Abstract

We describe a single method to purify milligram amounts of authentic wild-type and mutant HIV-1 and HIV-2 Tat proteins overexpressed inEscherichia coli.This method takes advantage of the highly basic, positively charged RNA binding domain present in both HIV-1 and HIV-2 Tat, which also facilitated purification of HIV-1 and HIV-2 mutant Tat proteins. In contrast to previously described methods, our method does not require use of denaturing or reducing agents, since Tat is present in the soluble fraction after bacterial lysis. The activity of purified wild-type and mutant HIV-1 and HIV-2 Tat proteins was determined using cell-based uptake,in vitrotranscription, and TAR binding assays. As expected, both HIV-1 and HIV-2 Tat efficiently transactivated the HIV-2 LTR, whereas HIV-2 Tat transactivated the HIV-1 LTR less efficiently than HIV-1 Tat. Purified HIV-2 Tat proteins in which the glutamic acid residue at position 77 was changed to either glycine or glutamine transactivated the HIV-1 LTR more efficiently than wild-type Tat-2, providing additional evidence that the net charge of this region may be responsible for nonreciprocal transactivation between Tat-1 and Tat-2. Our results demonstrate that this method can be used to rapidly purify authentic wild-type and mutant Tat proteins which are suitable for cell-based andin vitrofunctional studies.