Trax (Translin-associated factor X), a primarily cytoplasmic protein, inhibits the binding of TB-RBP (Translin) to RNA

Trax (Translin-associated factor X) has been shown to interact with TB-RBP/ Translin by its coimmunoprecipitation and in yeast two-hybrid assays. Here we demonstrate that Trax is widely expressed, does not bind to DNA or RNA, but forms heterodimers with TB-RBP under reducing conditions. The heterodim er of TB-RBP and Trax inhibits TB-RBP binding to RNA, but enhances TB-RBP b inding to specific single stranded DNA sequences. The in vitro interactions between TB-RBP and Trax are confirmed by similar interactions in the yeast two-hybrid system. Cell fractionation and confocal microscope studies reve al that Trax is predominantly cytoplasmic, In contrast, TB-RBP is present i n both the nuclei and cytoplasm of transfected cells and uses a highly cons erved nuclear export signal to exit nuclei. In addition to a leucine zipper , two basic domains in TB-RBP are essential for RNA binding, but only one o f these domains is needed for DNA binding. Trax restores DNA binding to TB- RBP containing an altered form of this domain, These data suggest that Trax -TB-RBP interactions modulate the DNA- and RNA-binding activity of TB-RBP.