The structural basis of protein targeting and translocation in bacteria

In Gram-negative bacteria, two distinct targeting routes assist in the prop er localization of secreted and membrane proteins. Signal recognition parti cle (SRP) mainly targets ribosome-bound nascent membrane proteins, whereas SecB facilitates the targeting of periplasmic and outer membrane proteins. These routes converge at the translocase, a protein-conducting pore in the membrane that consists of the SecYEG complex associated with the peripheral ATPase, SecA. Recent structural studies of the targeting and the transloca ting components provide insights into how substrates are recognized and sug gest a mechanism by which proteins are transported through an aqueous pore in the cytoplasmic membrane.