Final answer:
Proteins destined for mitochondria or chloroplasts are initially synthesized in the cytoplasm with an N-terminal signal peptide that helps target them to the correct organelle. Chaperone proteins like HSP70 unfold the protein for transport through membrane contact proteins. Once inside the organelle, the signal peptide is removed, and the protein is refolded into its functional form.
Step-by-step explanation:
When a protein is to be sent to the mitochondria or chloroplast, it is initially synthesized in the cytoplasm and must be targeted to the correct organelle. This process is different from the co-translational packaging of proteins by the Rough Endoplasmic Reticulum (RER). Mitochondrial proteins have an N-terminal signal peptide that is recognized by a receptor protein on the outer mitochondrial membrane. This receptor protein spans both the outer and cristae membranes of the mitochondria. The folded protein cannot cross the membrane on its own and requires the help of chaperone proteins, such as HSP70, which assists in unfolding the protein so that it can pass through the mitochondrial membranes via membrane contact proteins that act as channels.
Once inside, the signal peptide is removed, and another HSP70 molecule inside the mitochondrion helps refold the protein into its active conformation. Similarly, for proteins destined for chloroplasts in plant cells, a specific signal sequence at the amino terminus directs the protein to the chloroplast, where it is usually modified and the signal sequence removed upon arrival.