Complexes between protein export chaperone SecB and SecA - Evidence for separate sites on SecA providing binding energy and regulatory interactions

During localization to the periplasmic space or to the outer membrane of Es cherichia coli some proteins are dependent on binding to the cytosolic chap erone SecB, which in turn is targeted to the membrane by specific interacti on with SecA, a peripheral component of the translocase. Five variant forms of SecB, previously demonstrated to be defective in mediating export in vi vo (Grannon, P, M., and Kumamoto, C, A. (1993) J. Biol, Chem. 288, 1590-159 5; Kimsey, H. K,, Dagarag, M, D,, and Kumamoto, C, A. (1995) J, Biol, Chem. 270, 22831-22835) were investigated with respect to their ability to bind SecA both in solution and at the membrane translocase. We present evidence that at least two regions of SecA are involved in the formation of active c omplexes with SecB, The variant forms of SecB were all capable of interacti ng with SecA in solution to form complexes with stability similar to that o f complexes between SecA and wild-type SecB, However, the variant forms wer e defective in interaction with a separate region of SecA, which was shown to trigger a change that was correlated to activation of the complex. The r egion of SecA involved in activation of the complexes was defined as the ex treme carboxyl-terminal 21 aminoacyl residues.