Subunit interactions in the Escherichia coli protein translocase: SecE and SecG associate independently with SecY

We used hexahistidine-tagged SecE and SecY to study how the core subunits (SecY, SecE and SecG) of Escherichia coli protein translocase interact with each other. Detergent extracts were prepared from the plasma membranes and fractionated by Ni2+-NTA agarose affinity binding. Although His6-SecE, expressed in wild-type cells, brought down both SecY and SecG, neither of them was brought down when the same protein was expressed in the secY24 mutant cells. His6-SecY brought down both SecE and SecG, as expected. Interestingly, His6-SecY24 was able to bring down SecG but not SecE. These results confirm our previous conclusion that the secY24 alteration impairs the SecY-SecE interaction, and demonstrate that SecY and SecG can form a complex that does not contain SecE. Likewise, SecY-SecE complex could be isolated from the secG-deleted strain. The trimeric complex, in detergent extracts, dissociated at a critical temperature between 23 and 26 degrees C, whereas the SecY-SecE complex without SecG dissociated at a slightly lower temperature (20-23 degrees C). We conclude that each of SecE and SecG independently binds to SecY, the central subunit of protein translocase, although the trimeric complex is more stable than the binary complexes.