The Arginine-Rich RNA-Binding Motif of HIV-1 Rev Is Intrinsically Disordered and Folds upon RRE Binding

Arginine-rich motifs (ARMs) capable of binding diverse RNA structures play critical roles in transcription, translation, RNA trafficking, and RNA packaging. The regulatory HIV-1 protein Rev is essential for viral replication and belongs to the ARM family of RNA-binding proteins. During the early stages of the HIV-1 life cycle, incompletely spliced and full-length viral mRNAs are very inefficiently recognized by the splicing machinery of the host cell and are subject to degradation in the cell nucleus. These transcripts harbor the Rev Response Element (RRE), which orchestrates the interaction with the Rev ARM and the successive Rev-dependent mRNA export pathway. Based on established criteria for predicting intrinsic disorder, such as hydropathy, combined with significant net charge, the very basic primary sequences of ARMs are expected to adopt coil-like structures. Thus, we initiated this study to investigate the conformational changes of the Rev ARM associated with RNA binding. We used multidimensional NMR and circular dichroism spectroscopy to monitor the observed structural transitions, and described the conformational landscapes using statistical ensemble and molecular-dynamics simulations. The combined spectroscopic and simulated results imply that the Rev ARM is intrinsically disordered not only as an isolated peptide but also when it is embedded into an oligomerization-deficient Rev mutant. RRE recognition triggers a crucial coil-to-helix transition employing an induced-fit mechanism.