Proteinimport

Protein import refers to how cells move polypeptides from the cytosol into organelles and compartments. Most organellar proteins are synthesized as precursors on cytosolic ribosomes and must be delivered post-translationally or co-translationally. Successful import depends on targeting signals, chaperones, translocons, and energy sources.

Secretory and membrane proteins enter the endoplasmic reticulum co-translationally. A signal peptide emerges from the ribosome and is recognized by the signal recognition particle, docking the ribosome to the Sec61 translocon in the ER membrane. As translation proceeds, the polypeptide passes into the ER lumen or membrane, where disulfide bonds form, and signal peptidase cleaves the signal sequence.

Mitochondria and chloroplasts import use distinct outer and inner membrane complexes. Mitochondrial proteins use the TOM and TIM complexes; N-terminal presequences guide import and are cleaved by mitochondrial processing peptidase; import is powered by membrane potential and ATP-driven motors. Chloroplast proteins use the TOC/TIC system and transit peptides; processing occurs in the stroma by stromal processing peptidase.

Nuclear import delivers proteins through the nuclear pore complex using nuclear localization signals and transport receptors (importins) in a Ran-GTP–dependent cycle. Peroxisomal import uses PTS signals and Pex receptors. Failure in protein import can disrupt organelle function and lead to disease; import fidelity is ensured by chaperones and quality-control pathways.