In vitro translocation of bacterial secretory proteins and energy requirements.

The recent establishment of in vitro assay systems has made biochemical studies on the process of membrane translocation of secretory proteins possible. This review summarizes what we have learned, using these in vitro systems, concerning the biochemical process of protein translocation, with special reference to energy requirements. Both ATP and the protonmotive force participate in the translocation reaction. The requirement of ATP is obligatory, whereas that of the protonmotive force differs, in terms of its level, with the secretory protein species. The possible roles of ATP and the protonmotive force in protein translocation are discussed with special reference to the function of SecA, an essential component of the secretory machinery. The effect of positive charges, which precede or follow the hydrophobic domain of signal peptides, on translocation is also discussed.