What mechanism targets proteins to mitochondria?

The correct choice is that mitochondrial signal sequences are essential for targeting proteins to mitochondria. These sequences are specific amino acid motifs that are typically located at the N-terminus of mitochondrial proteins. When a protein is synthesized in the cytosol, if it contains a mitochondrial signal sequence, it is recognized by receptor proteins on the mitochondrial surface.

Once the signal sequence is identified, the protein is unfolded and transported across the mitochondrial membranes through translocases. The mitochondrial signal sequence not only aids in directing the protein to the mitochondria but also plays a crucial role in the proper localization of the protein within the mitochondria, ensuring that it reaches its intended destination inside the organelle.

Other mechanisms highlighted in the options do not specifically govern the targeting of proteins to mitochondria. Signal transduction pathways involve the transmission of signals from outside the cell to elicit a cellular response, while active transport mechanisms generally refer to the movement of molecules across membranes against their concentration gradient, not specifically involving mitochondrial targeting. Vesicle-mediated transport typically pertains to the transport of proteins and materials to and from the endoplasmic reticulum, Golgi apparatus, and plasma membrane, rather than directly to mitochondria.