 Organelle protein import

Organelle protein import brings externally synthesized proteins into an organelle. Many plastid proteins made from nuclear genes carry targeting information recognized by machinery that transports them across the envelope membranes. This delivery enables nuclear genes to supply functions inside a plastid, but the presence of targeting information is not itself evidence that a gene was acquired through endosymbiotic gene transfer. ^[ref]

For the endoplasmic reticulum, cotranslational protein translocation illustrates how organelle protein import can begin before a polypeptide chain is complete. Import into the ER lumen and insertion as a transmembrane protein are different outcomes; neither alone establishes subsequent secretion from the biological cell.