High aggregation sensitivity of mammalian mitochondrial elongation factor Tu (Tufm) as a sensor for organellar stress

As proteins in mammalian cells exhibit optimal stability at natural temperatures, small temperature variations may cause unfolding and subsequent non-specific aggregation. As this process leads to a loss of function of the affected polypeptides as well as to further cytotoxic stress, aggregate formation has been recognized as a major pathogenic factor in human diseases. In this study we determined the impact of physiological heat stress on mammalian mitochondria on a proteomic level. The overall solubility of endogenous mitochondrial proteins was only marginally affected by a treatment at elevated temperatures. However, we identified a small subset of polypeptides that exhibited an exceptionally high sensitivity to heat stress. The mitochondrial translation elongation factor Tu (Tufm), a protein essential for organellar protein biosynthesis, was highly aggregation-prone and lost its solubility already under mild heat stress conditions. In parallel, mitochondrial translation as well as the import of cytosolic proteins was defective in heat stressed mitochondria. We propose that a shutdown of endogenous protein synthesis concomitant with a reduced preprotein import has a protective function by attenuating the proteotoxic stress caused by an accumulation of nascent polypeptides with a high tendency to misfold.