Substrate Proteins†

ABSTRACT: The molecular chaperone SecB binds to hydrophobic sections of unfolded secretory proteins and thereby prevents their premature folding prior to secretion by the translocase of Escherichia coli. Here, we have investigated the effect of the single-residuemutation of leucine 42 to arginine (L42R) centrally positioned in the polypeptide binding pocket of SecB on its chaperonin function. The mutant retains its tetrameric structure and SecA targeting function but is defective in its holdase activity. Isothermal titration calorimetry and single-molecule optical tweezer studies suggest that the SecB(L42R) mutant exhibits a reduced polypeptide binding affinity allowing for partial folding of the bound polypeptide chain rendering it translocation-incompetent. The translocation of proteins across membranes is an essential process in life. In bacteria, a large number of proteins have to cross the cytoplasmic membrane to fulfill their function at the outer surface of the cell or in the extracellular medium. Many of these proteins are synthesized as precursors with an N-terminal signal sequence that is used for correct targeting to the Sec translocase at the membrane. The Sec translocase is a complex o