A ribosomal kinetic checkpoint governs selective mRNA recruitment

Abstract

Translation initiation begins with recruitment of mRNA to the ribosome, yet how mRNA engagement is converted into productive initiation remains unclear. Using real-time fluorescence assays with purified components, we show that mRNA recruitment proceeds through a branched kinetic pathway on the 40S subunit. Following rapid sampling, mRNAs partition into either a productive accommodated state or an arrested state that stabilizes ribosome binding before accommodation. mRNA structure, eIF3, and eIF3j bias recruitment toward arrest, whereas eIF4F promotes accommodation in an ATP-dependent manner coupled to displacement of eIF3j from the mRNA entry channel. Unstructured mRNAs accommodate independently of eIF4E, whereas structured mRNAs require an upstream eIF4E-dependent step, enabling selective recruitment under limiting eIF4E. Arrested complexes can convert directly into the accommodated state without dissociation, revealing a reversible standby intermediate poised for activation. Together, our findings establish mRNA accommodation as a ribosome-intrinsic checkpoint governing initiation and provide a framework for selective translation.