Multiprotein bridging factor 1 is required for robust activation of the integrated stress response on collided ribosomes

In yeast, multiprotein bridging factor 1 (Mbf1) has been proposed to function in the integrated stress response (ISR) as a transcriptional coactivator by mediating a direct interaction between general transcription machinery and the process's key effector, Gcn4. However, mounting evidence has demonstrated that Mbf1 (and its human homolog EDF1) is recruited to collided ribosomes, a known activator of the ISR. In this study, we connect these otherwise seemingly disparate functions of Mbf1. Our biochemical and structural analyses reveal that Mbf1 functions as a core ISR factor by interacting with collided ribosomes to mediate Gcn2 activation. We further show that Mbf1 serves no role as a transcriptional coactivator of Gcn4. Instead, Mbf1 is required for optimal stress-induced eukaryotic initiation factor 2α (eIF2α) phosphorylation and downstream de-repression of GCN4 translation. Collectively, our data establish that Mbf1 functions in ISR signaling by acting as a direct sensor of stress-induced ribosome collisions.