A direct role for a mitochondrial targeting sequence in triggering a stress response

Mitochondrial protein import is required for maintaining organellar function. Perturbations in this process are associated with various physiological and disease conditions. Several stress responses, including the mitoCPR, combat damage caused by mitochondrial protein import stress. However, it remains unknown how this defect is sensed. Here, we reveal that the conserved mitochondrial Hsp70 co-chaperone, Mge1, acts as a stress messenger in budding yeast. During mitochondrial stress, unimported Mge1 entered the nucleus and triggered the transcription of mitoCPR target genes. This was mediated by Mge1's interaction with the transcription factor Pdr3 on DNA regulatory elements. Mge1's mitochondrial targeting sequence was both sufficient and essential for mitoCPR induction, demonstrating that in addition to their roles in mitochondrial protein import, targeting sequences can also function as signaling molecules. Overall design: Experiments was performed using three biological replicates for each of the following six conditions: (1) PSD1 overexpressors, (2) PSD1 overexpressors harboring a PDR3 deletion, (3) overexpressors of the N-terminal 44 amino acids of MGE1, (4) overexpressors of the N-terminal 44 amino acids of MGE1 with a PDR3 deletion, (5) overexpressors of the N-terminal 53 amino acids of MSS116.