To actively transport many of its proteins to extracytoplasmic compartments such as the periplasm and outer membrane, E.coli employs a collection of Sec(secretion) proteins that catalyze the translocation of various polypeptides through the inner membrane. The targeting of periplamic proteins after the cleavage of signal peptide is commonly regarded as a default process, while the final destination of outer-membrane proteins needs the help of other factors and is well accepted as a periplasmic intermediate process.