Final answer:
Co-chaperones for HSP70, like J-domain proteins, and nucleotide exchange factors, assist HSP70 in refolding proteins, especially during heat stress. These proteins are involved in the HSP70 cycle and help maintain protein homeostasis.
Step-by-step explanation:
The HSP70 cycle involves co-chaperones that assist in its function of refolding misfolded proteins, especially during stress conditions such as heat shock. These co-chaperones include J-domain proteins (also known as Hsp40), nucleotide exchange factors (NEFs) like BAG1 or GrpE, and T-complex protein-1 ring complex (TRiC). HSP70 binds to an unfolded protein and stabilizes it in an ATP-dependent manner. J-domain proteins stimulate the ATPase activity of HSP70, leading to the tight binding of the unfolded protein. Subsequently, NEFs promote the exchange of ADP for ATP, which releases the refolded protein.
In the specific context of mitochondrial protein import, a cytosolic HSP70 first interacts with the unfolded protein, facilitating its translocation across the mitochondrial membrane. Following the removal of the signal peptide by mitochondrial signal peptidase, a mitochondrial-resident HSP70 aids in the proper refolding of the protein.
Heat shock proteins are crucial in maintaining protein homeostasis and are highly upregulated in response to stress, such as elevated temperatures, to prevent aggregation of denatured proteins and assist in their proper refolding.