Identification of specific molecular structures of human immunodeficiency virus type 1 Tat relevant for its biological effects on vascular endothelial cells

Human immunodeficiency virus type 1 (HIV-1) Tat transactivates viral genes and is released by infected cells, acting as a soluble mediator. In endothe lial cells (EC), it activates a proangiogenic program by activating vascula r endothelial growth factor receptor type 2 (VEGFR-2) and integrins. A stru cture-activity relationship study was performed by functional analysis of T at substitution and deletion variants to define the Tar determinants necess ary for EC activation, Variants were made (i) in the basic and (ii) in the cysteine-rich domains and (iii) in the C-terminaI region containing the RGD sequence required for integrin recognition. Our results led to the followi ng conclusions. (i) Besides a high-affinity binding site corresponding to V EGFR-2, EC express low-affinity binding sites. (ii) The basic and the cyste ine-rich variants bind only to the low-affinity binding sites and do not pr omote tyrosine phosphorylation of VEGFR-2. Furthermore, they have a reduced ability to activate EC in vitro, and they lack angiogenic activity. (iii) Mutants with mutations in the C-terminal region are partially defective for in vitro biological activities and in vivo angiogenesis, but they activate VEGFR-2 as Tat wild type. In conclusion, regions encoded by the first exon of tat are necessary and sufficient for activation of VEGFR-2. However, th e C-terminaI region, most probably through RGD-mediated integrin engagement , is indispensable for full activation of an in vitro and in vivo angiogeni c program.