Adaptive recognition in RNA complexes with peptides and protein modules

Recently, progress has been made towards the structural characterization of the novel folds of RNA-bound arginine-rich peptides and the architecture of their peptide-binding RNA pockets in viral and phage systems. These studies are based on an approach whereby the peptide and RNA components are minimalist modular domains that undergo adaptive structural transitions upon complex formation. Such complexes are characterized by recognition alignments in which the tertiary fold of the RNA generates binding pockets with the potential to envelop minimal elements of protein secondary structure. Strikingly, the peptides fold as isolated alpha-helical or beta-hairpin folds within their RNA major-groove targets, without the necessity of additional appendages for anchorage within the binding pocket. The RNA peptide-binding pocket architectures are sculptured through precisely positioned mismatches, triples and looped-out bases, which accommodate amino acid sidechains through hydrophobic, hydrogen bonding and ionic intermolecular contacts. By contrast, protein modules associated with the HIV-1 nucleocapsid and MS2 phage coat target their RNA binding sites through the insertion of specificity-determining RNA base residues within conserved hydrophobic pockets and crevices on the protein surface, with the bases anchored through hydrogen bonding interactions. These alternative strategies of RNA recognition at the peptide and protein module level provide novel insights into the principles, patterns and diversity of the adaptive transitions associated with the recognition process.