The Rex regulatory protein of human T-cell lymphotropic virus type I binds specifically to its target site within the viral RNA.

The Rex protein of human T-cell leukemia virus type I (HTLV-I) was expressed in bacteria and partially purified. Rex was shown to bind in vitro specifically to an RNA sequence located in the 3' long terminal repeat of HTLV-I, named Rex-responsive element (RXRE). Rex also bound in vitro to the human immunodeficiency virus type 1 (HIV-1) Rev-responsive element (RRE), while purified HIV-1 Rev protein did not bind to the RXRE. The binding results obtained in vitro are therefore in agreement with the nonreciprocal function of Rev and Rex in vivo. Rex binds specifically to both RRE and RXRE and activates expression in both HIV-1 and HTLV-I, while Rev binds to RRE and activates only HIV-1. Binding of Rex to RRE deletion mutants previously shown to lack either the Rev-responsive or the Rex-responsive portion suggested preferential binding of Rex to a distinct target within the RRE. These results demonstrated that Rex, like Rev, acts by binding to a specific RNA target. IMAGES:

人类T细胞白血病病毒I型（Human T-cell Leukemia Virus Type I, HTLV-I）的Rex蛋白在细菌中表达并经部分纯化。实验证实，Rex可在体外特异性结合HTLV-I 3'长末端重复序列中的一段RNA序列，该序列被命名为Rex应答元件（Rex-responsive element, RXRE）。Rex在体外还可结合人类免疫缺陷病毒1型（Human Immunodeficiency Virus Type 1, HIV-1）的Rev应答元件（Rev-responsive element, RRE），而纯化的HIV-1 Rev蛋白则无法结合RXRE。因此，体外获得的结合实验结果与Rev和Rex在体内的不对称功能特性相符。Rex可特异性结合RRE与RXRE，并同时激活HIV-1与HTLV-I的基因表达；而Rev仅能结合RRE，且仅激活HIV-1的基因表达。针对此前被证实缺失Rev应答区域或Rex应答区域的RRE缺失突变体进行的结合实验结果显示，Rex更倾向于结合RRE内一个独特的靶位点。上述结果证实，与Rev类似，Rex通过结合特异性RNA靶标发挥功能。IMAGES: