Protein import mechanisms vary depending on the destination organelle:
Nuclear Import: Proteins destined for the nucleus contain nuclear localization signals (NLS) that are recognized by importin proteins. The importins facilitate the transport of these proteins through the nuclear pore complex. Mitochondrial Import: Mitochondrial proteins have mitochondrial targeting sequences (MTS) that are recognized by receptors on the mitochondrial surface. They are then translocated across the mitochondrial membranes via the TOM and TIM complexes. Endoplasmic Reticulum (ER) Import: Proteins that enter the ER often have a signal peptide that directs them to the ER membrane. The signal recognition particle (SRP) binds to the signal peptide and directs the ribosome to the ER membrane, where the protein is translocated into the ER lumen. Peroxisomal Import: Peroxisomal proteins contain peroxisomal targeting signals (PTS) that are recognized by specific receptors. These proteins are then imported through the peroxisomal membrane using PEX proteins.
