SecYEG assembles into a tetramer to form the active protein translocation channel

Translocase mediates preprotein translocation across the Escherichia coli i nner membrane, It consists of the SecYEG integral membrane protein complex and the peripheral ATPase SecA. Here we show by functional assays, negative -stain electron microscopy and mass measurements with the scanning transmis sion microscope that SecA recruits SecYEG complexes to form the active tran slocation channel, The active assembly of SecYEG has a side length of 10.5 nm and exhibits an similar to 5 nm central cavity, The mass and structure o f this SecYEG as well as the subunit stoichiometry of SecA and SecY in a so luble translocase-precursor complex reveal that translocase consists of the SecA homodimer and four SecYEG complexes.