Zn2+ binding to cysteine-rich domain of extracellular human immunodeficiency virus type 1 Tat protein is associated with Tat protein-induced apoptosis

The Tat protein has several functional domains, one of which is the cysteine-rich domain that is a highly conserved region in spite of the presence of many subtypes of human immunodeficiency virus type 1 (HIV-1). Although the cysteine-rich domain is a potential site for Zn2+ binding, it is controversial whether Zn2+ is substantially essential for the structure and activities of the Tat protein. To study the significance of Zn2+ in the cysteine-rich domain of the Tat protein particularly released to the extracellular space, we raised the monoclonal antibody (MAb) 5A4, which has an attractive property of recognizing the Zn2+-binding Tat(20-41) peptide but not the apo-Tat(20-41) peptide. MAb 5A4 inhibited the trans-activation of the HIV long terminal repeat (LTR) in HeLa-CD4-LTR/beta-gal cells induced by treatment with the recombinant Tat protein, indicating that MAb 5A4 can recognize the full-length Tat protein and inhibit its trans-activity. The antibody also inhibited the apoptosis of Jurkat cells induced by treatment with the released native-Tat-protein-containing supernatant from the culture of HIV-1(JRFL)-infected cells. These results suggest that Zn2+, whose structure is closely associated with not only the trans-activation of HIV-LTR but also the induction of apoptosis, binds to the extracellular native Tat protein. The Zn2+-binding cysteine-rich domain therefore can be a molecular target in the development of an anti-Tat vaccine and agents for the control of extracellular-Tat-protein-mediated pathogenesis leading to the progression of acquired immunodeficiency syndrome.