Heat Shock Proteins in the Hot Mitochondrion: Identity and Putative Roles

The mitochondrion is known as the powerhouse of eukaryotic cells since it is the main site of adenosine 5 '-triphosphate (ATP) production. Using a temperature-sensitive fluorescent probe, it has recently been suggested that the stray free energy, not captured into ATP, is potentially sufficient to sustain mitochondrial temperatures higher than the cellular environment, possibly reaching up to 50 degrees C. By 50 degrees C, some DNA and mitochondrial proteins may reach their melting temperatures; how then do these biomolecules maintain their structure and function? Further, the production of reactive oxygen species (ROS) accelerates with temperature, implying higher oxidative stresses in the mitochondrion than generally appreciated. Herein, it is proposed that mitochondrial heat shock proteins (particularly Hsp70), in addition to their roles in protein transport and folding, protect mitochondrial proteins and DNA from thermal and ROS damage. Other thermoprotectant mechanisms are also discussed.