Mechanisms of protein import into the endoplasmic reticulum and protein export in Escherichia coli.

Every polypeptide has a specific function as well as a unique functional location, i.e. an intraor extra-cellular location where it fulfils its function. There are two facts which turned the latter into a central problem in our understanding of gene expression in both the prokaryotic and the eukaryotic cell: (i) there is one site of protein synthesis, the cytoplasm, but there are many different potential functional locations, including the endoplasmic reticulum in the eukaryotic cell or the plasma membrane in the prokaryotic cell and the extracellular space in both systems; and (ii) the site of synthesis is separated from these locations by biological membranes. Therefore, there must exist mechanisms which guarantee the specific transport of proteins across membranes and the assembly of proteins into membranes [ 11. The mechanisms of transport of proteins across bacterial plasma membranes and membranes of the endoplasmic reticulum show a number of homologies. In both systems there is a need for a signal/leader peptide on the respective precursor protein and for a signallleader-peptide receptor on the cis side of the target membrane as well as for a signal/ leader peptidase on the trans side of the target membrane. Furthermore, there generally is no mechanistic coupling between translation and the actual membrane transport, but the folding of the precursor protein into a stable tertiary structure has to be prevented or reversed to allow transport. In addition, in both systems thkre appear to exist two classes of precursor proteins with respect to their molecular requirements for membrane transport. There is, however, one striking difference between the two systems: protein export in Escherichia coli depends on a membrane potential, but protein import into microsomes does not show a membrane potential effect.