Processing of exported proteins in Escherichia coli

The mechanism of export of protein in E. coli can be summarized in terms of the 'signal hypothesis'. The proteins are synthesized on membrane-bound polyribosomes in the form of precursors, which carry N-terminal extensions of amino acids, the 'signal'. The proteins are vectorially transferred through the membrane during synthesis and the signal sequence is removed to generate the mature protein. The basic principle is established and it is now important to elucidate the molecular mechanism of export. We have attempted to detail the proteolytic removal of the signal. We have shown that the precursors are processed post-translationally, and we have data suggesting that two cleavages may be involved. It appears that processing is not necessary to activate the mature proteins. Why then is the signal removed? Perhaps the answer will shed light on the other yet unanswered questions: what is the energy source for the translocation? how does the cell differentiate between the two classes of exported proteins, those in the periplasm and those in the outer membrane? The next few years should see the resolution of these questions.