A live bovine endothelial cell stained to reveal different intracellular compartments. The lacelike membranes of the endoplasmic reticulum were stained with a green fluorescent dye, and the wormlike mitochondria were stained with an orange fluorescent dye. [Molecular Probes, Inc.]
Atypical mammalian cell contains up to 10,000 different kinds of proteins; a yeast cell, about 5000. The vast majority of these proteins are synthesized by cyto-solic ribosomes, and many remain within the cytosol. However, as many as half the different kinds of proteins produced in a typical cell are delivered to a particular cell membrane, an aqueous compartment other than the cytosol, or to the cell surface for secretion. For example, many hormone receptor proteins and transporter proteins must be delivered to the plasma membrane, some water-soluble enzymes such as RNA and DNA polymerases must be targeted to the nucleus, and components of the extracellular matrix as well as polypeptide signaling molecules must be directed to the cell surface for secretion from the cell. These and all the other proteins produced by a cell must reach their correct locations for the cell to function properly.
The delivery of newly synthesized proteins to their proper cellular destinations, usually referred to as protein targeting or protein sorting, encompasses two very different kinds of processes. The first general process involves targeting of a protein to the membrane of an intracellular organelle and can occur either during or soon after synthesis of the pro tein by translation at the ribosome. For membrane proteins, targeting leads to insertion of the protein into the lipid bi-layer of the membrane, whereas for water-soluble proteins, targeting leads to translocation of the entire protein across the membrane into the aqueous interior of the organelle.
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