Single-molecule visualization of sequence-specific RNA binding by a designer PPR protein

Pentatricopeptide repeat proteins (PPR) are a large family of modular RNA-binding proteins, whereby each module can be modified to bind to a specific ssRNA nucleobase. As such, there is interest in developing 'designer' PPRs (dPPRs) for a range of biotechnology applications, including diagnostics or in vivo localization of ssRNA species; however, the mechanistic details regarding how PPRs search for and bind to target sequences is unclear. To address this, we determined the structure of a dPPR bound to its target sequence and used two- and three-color single-molecule fluorescence resonance energy transfer to interrogate the mechanism of ssRNA binding to individual dPPRs in real time. We demonstrate that dPPRs are slower to bind longer ssRNA sequences (or could not bind at all) and that this is, in part, due to their propensity to form stable secondary structures that sequester the target sequence from dPPR. Importantly, dPPR binds only to its target sequence (i.e. it does not associate with non-target ssRNA sequences) and does not 'scan' longer ssRNA oligonucleotides for the target sequence. The kinetic constraints imposed by random 3D diffusion may explain the long-standing conundrum of why PPR proteins are abundant in organelles, but almost unknown outside them (i.e. in the cytosol and nucleus).