Question

The translocon of the mitochondrial outer membrane is functionally equivalent to ER translocon.  However, why is Hsc70 needed for mitochondrial import but not for import into the ER?

Answer 1

The great majority of mitochondrial proteins are synthesized by cytosolic ribosomes and then imported into the organelle post - translationally. The translocase of the outer membrane (TOM) is a proteinaceous machinery that contains surface receptors for preprotein recognition and also serves as the main entry gateway into mitochondria . Mitochondrial targeting requires various cytosolic factors , in particular the molecular chaperones Hsc70/Hsp70 and Hsp90. The chaperone activity of Hsc70/Hsp70 and Hsp90 occurs in coordinated cycles of ATP hydrolysis and substrate binding , and is regulated by a number of co-chaperone proteins . The import receptor Tom 70 is a member of the tetratricopeptide repeat co-chaperone family and contains a conserved TPR clamp domain for interaction with Hsc70 and Hsp90. Such interaction is essential for the initiation of the import process . This review will discuss the role of Hsc70 and Hsp90 in mitochondrial import and summarize recent progress in understanding these pathways .