[Intracellular protein transport--the cell as a mail carrier]

The eukaryotic cell has a highly developed machinery for routing proteins to their correct intra- or extra-cellular locations. Short stretches of amino acids ("signal peptides") serve as address labels that are recognized by receptors on the surface of the appropriate organelle, where-upon translocation across one or more membranes ensues. Premature folding of the nascent protein is prevented by cytoplasmic "chaperones", i.e. proteins that bind to unfolded or partially folded nascent chains. After import into the organelle, other chaperones are required for catalyzing the final folding of the protein, and for promoting its correct assembly into oligomeric protein complexes. Transport along the secretory pathway (endoplasmic reticulum to Golgi to trans-Golgi to plasma membrane) is mediated by vesicles that bud from a donor compartment and fuse with an acceptor compartment.