EXBB ACTS AS A CHAPERONE-LIKE PROTEIN TO STABILIZE TONB IN THE CYTOPLASM

The TonB protein is required to transduce energy from the cytoplasmic membrane to outer membrane transport proteins of Gram-negative bacteri a. Two accessory proteins, ExbB and ExbD, are required for TonB functi on and it has been suggested that TonB and ExbBD form a complex in the membrane. In this paper we demonstrate that there are two spatially d istinct, functional interactions between ExbBD and TonB. First, there is an interaction between ExbBD and the N-terminal signal-like peptide of TonB, probably the formation of a stable complex in the membrane. Second, ExbB interacts with TonB in the cytoplasm. This interaction in volves the domain of TonB that is normally periplasmic. Thus, this is a transient interaction which occurs during the synthesis and/or local ization of TonB, implying a chaperone-like role for ExbB. The transmem brane topology of ExbB was shown to be consistent with this role.