Structure-Based Mechanism for Retroviral Primer Annealing

Abstract

In order to prime reverse transcription, retroviruses require annealing of a tRNA molecule to the U5-primer binding site (U5-PBS) region of the viral genome. The residues essential for primer annealing are initially locked in intramolecular interactions, and hence, annealing requires the chaperone activity of the retroviral nucleocapsid (NC) protein to facilitate structural rearrangements. Understanding the mechanism of primer annealing has been a challenging problem, both due to the relatively low probability of these domains crystallizing and the complexity of studying them by nuclear magnetic resonance (NMR). In my talk, I will detail the NMR experiments that led to the discovery of the mechanism used by the Moloney murine leukemia virus (MLV) NC protein. I will show that unlike classical chaperones, the MLV-NC uses a unique mechanism, in which it specifically targets multiple structured regions in both the U5-PBS and tRNAPro primer that otherwise sequester residues necessary for annealing. This high-specificity and high-affinity binding by NC consequently liberates these sequestered residues_which are exactly complementary_for intermolecular interactions. Furthermore, I will show that NC utilizes a step-wise, entropy-driven mechanism to trigger both residue-specific destabilization and residue-specific release.