ABSTRACT
Both in prokaryotes and eukaryotes, newly synthesized proteins must be delivered to a specific subcellular location or exported from the cell for correct activity. This phenomenon is called protein targeting. Proteins destined to be secreted from the eukaryotic cell are synthesized by ribosomes bound to the rough endoplasmic reticulum (RER). A typical secretory protein differs from a cytosolic protein by having a sequence about 13–35 amino acids long at its N-terminal end called asignal sequence or signal peptide. Integral plasma membrane proteins are also synthesized by ribosomes on the RER, but become inserted in the RER membrane rather than transported into the lumen. Transfer of the plasma membrane protein across the ER membrane occurs during synthesis by a mechanism similar to that for secretory proteins. Lysosomal enzymes and lysosomal membrane proteins are synthesized on the RER and transported to the cis compartment of the Golgi complex. Proteins are targeted to the mitochondrial matrix by an N-terminal matrix-targeting sequence.
