Protein targeting to the bacterial cytoplasmic membrane

Proteins that perform their activity within the cytoplasmic membrane or out side this cell boundary must be targeted to the translocation site prior to their insertion and/or translocation. In bacteria, several targeting route s are known; the SecB- and the signal recognition particle-dependent pathwa ys are the best characterized. Recently, evidence for the existence of a th ird major route, the twin-Arg pathway, was gathered Proteins that use eithe r one of these three different pathways possess special features that enabl e their specific interaction with the components of the targeting routes. S uch targeting information is often contained in an N-terminal extension, th e signal sequence, but can also be found within the mature domain of the ta rgeted protein. Once the nascent chain starts to emerge from the ribosome, competition for the protein between different targeting factors begins. Aft er recognition and binding, the targeting factor delivers the protein to th e translocation sires at the cytoplasmic membrane. Only by means of a speci fic interaction between the targeting component and its receptor is the car go released for further processing and translocation. This mechanism ensure s the high-fidelity targeting of premembrane and membrane proteins to the t ranslocation site.