Escherichia coli translocase: the unravelling of a molecular machine.

Protein translocation across the bacterial cytoplasmic membrane has been studied extensively in Escherichia coli. The identification of the components involved and subsequent reconstitution of the purified translocation reaction have defined the minimal constituents that allowed extensive biochemical characterization of the so-called translocase. This functional enzyme complex consists of the SecYEG integral membrane protein complex and a peripherally bound ATPase, SecA. Under translocation conditions, four SecYEG heterotrimers assemble into one large protein complex, forming a putative protein-conducting channel. This tetrameric arrangement of SecYEG complexes and the highly dynamic SecA dimer together form a proton-motive force- and ATP-driven molecular machine that drives the stepwise translocation of targeted polypeptides across the cytoplasmic membrane. Recent findings concerning the translocase structure and mechanism of protein translocation are discussed and shine new light on controversies in the field.