Abstract 1938 The RNA-binding protein SERBP1 regulates mTOR expression by interacting with G-quadruplexes

SERBP1 is an RNA binding protein that regulates translation and plays a role in neurogenesis. Recently we uncovered that high level of SERBP1 expression in glioblastomas is a prognostic marker for poor patient outcomes. The protein is comprised of two RG/RGG boxes yet lacks other known nucleic acid binding motifs. Previously, we used NMR and biophysical techniques to characterize the structural properties of SERBP1 revealing its intrinsically disordered character. Further, our data revealed SERBP1 preferentially binds GC-rich sequences mediated primarily through the C-terminal RGG box and successive residues. Bioinformatic approaches by us and others identified SERBP1 target transcripts that contain GC-rich sequences in their 5' untranslated region (UTR) and demonstrated these GC-rich motifs fold as G-quadruplexes (G4), nucleic acid structures known to be involved in the regulation of biological events. Here, we show that SERBP1 is a key regulator of mTOR expression levels by interacting with RNA-quadruplexes within mTOR mRNA. We then used a combination of NMR spectroscopy and biophysical approaches to decipher the binding mechanisms of SERBP1 to G4s. We show that the protein uses its second RGG domain and C-terminal region to bind different targets but is selective for G4 structures. This discrimination between different sequences based on three-dimension shape likely plays a key role in mTOR translation regulation. Finally, phosphomimic mutations showed reduced binding, indicating that this regulation mechanism is affected by phosphorylation. Altogether, our results provide a better understanding of the mechanisms of SERBP1 interaction with G4 targets, as well as insights into the mechanism of target selection by intrinsically disordered RNA-binding proteins and is a first step toward developing novel interventions for the treatment of glioblastoma. This work was supported by a Voelker Fund Young Investigator Grant (DSL) and Cancer Prevention and Research Institute of Texas (RP200595 to LP). AB is the recipient of a Center for Biomedical Neuroscience Science Postdoctoral Fellowship.