Association of AUUUA-binding Protein with A + U-rich mRNA during nucleo-cytoplasmic transport

Abstract

Resealed nuclear envelope (NE) vesicles from rat liver containing entrapped exogenous RNA were used to study the effect of adenosine + uridine binding factor (AUBF), present in cytosolic cell extracts, on ATP-dependent transport of A + U-rich RNA (AU +RNA) and A + U-free RNA (AU −RNA) across the NE. This factor specifically binds to A + U-rich sequences present in the 3′ untranslated regions of lymphokine and cytokine mRNAs. containing overlapping AUUUA boxes (granulocyte—macrophage colony stimulating factor interleukin-3). Addition of AUBF to the extravesicular compartment markedly increased the efflux of the in vitro transcribed, capped and polyadenylated AU + RNAs. Export of entrapped AU − control RNA, such as β-globin RNA, was not affected by AUBF, in contrast to chimeric AU + β-globin RNA containing the A + U-rich sequence of human Interferon-α mRNA (6 reiterated AUUUA motifs). Competition experiments revealed that AUBF enhances the affinity of poly(A)-containing AU + RNAs to the NE poly(A)-binding component (poly(A)-recognizing mRNA carrier p106), and thereby accelerates nuclear export of these RNAs. We could demonstrate that AUBF added to the extravesicular space forms stable complexes with polyadenylated AU + RNA with relative molecular masses of about 45,000, 62,000 and 70,000 inside the vesicles or during ATP-dependent export. In addition we determined that AUBF may affect mRNA stability by protecting A + U-rich RNA against degradation by trans-acting, nuclear matrix-associated and A + U-specific endoribonuclease V.