A structure-based mechanism for recognition of oskar mRNA by Tropomyosin 1

Abstract

During Drosophila development, the transport of oskar (osk) mRNA to the oocyte posterior pole is driven by dimerization of its cis-acting Oocyte Entry Signal (OES) and interactions with the tropomyosin 1 I/C isoform (Tm1-I/C), which links osk to the motor protein kinesin. The structure of this critical signal, however, and how it interacts with Tm1-I/C are largely unknown. My work presented here shows that the OES is peppered with structural motifs, including an A-rich helical junction, an AUU triloop, an arginine sandwich motif, and an A-minor motif, all poised to act as protein binding sites. Importantly, as predicted, the OES homodimerizes as a kissing complex via its GC rich loop, albeit with additional intermolecular tandem AC base pair interaction, which provides a unique asymmetric recognition interface for the binding of the N-terminal domain of Tm1-I/C (N- Tm1-I/C) and induces its dimerization. N-Tm1-I/C is largely intrinsically disordered and remains so after OES recognition, with local structures mediating dimerization and specific interactions with the various RNA motifs. Overall, our study assigns the asymmetric OES structure as a trigger for stepwise tropomyosin recognition; assigns the RNA-driven dimerization function to the N-terminal domain of Tm1-I/C; and adds to our understanding of how local structures in intrinsically disordered proteins can drive critical functions such as transport.