Escherichia coli translocase: the unravelling of a molecular machine

Protein translocation across the bacterial cytoplasmic membrane has been st udied extensively in Escherichia coli. The identification of the components involved and subsequent reconstitution of the purified translocation react ion have defined the minimal constituents that allowed extensive biochemica l characterization of the so-called translocase. This functional enzyme com plex consists of the SecYEG integral membrane protein complex and a periphe rally bound ATPase, SecA. Under translocation conditions, four SecYEG heter otrimers assemble into one large protein complex, forming a putative protei n-conducting channel. This tetrameric arrangement of SecYEG complexes and t he highly dynamic SecA dimer together form a proton-motive force- and ATP-d riven molecular machine that drives the stepwise translocation of targeted polypeptides across the cytoplasmic membrane. Recent findings concerning th e translocase structure and mechanism of protein translocation are discusse d and shine new light on controversies in the field.