Colicin A and the Tol proteins involved in its translocation are preferentially located in the contact sites between the inner and outer membranes of Escherichia coli cells.

Colicin A is a bacterial toxin which forms channels in the cytoplasmic membrane of Escherichia coli. Its translocation through the envelope requires the participation of bacterial proteins encoded by the tolQ, -R, -A, and -B genes. Overproduction of the Tol proteins decreased the time needed for colicin A translocation and increased the number of channels formed in vivo. Cells overproducing radioactively labeled Tol proteins and containing or not colicin A were fractionated. The Tol proteins were mainly recovered in the inner membrane and in the contact sites between the two membranes. The presence of colicin A increased the specific radioactivity of the Tol proteins in the contact sites. Our data suggest that the Tol proteins form a complex of definite stoichiometry in the membranes and that colicin A is associated to this complex upon channel formation. We discuss the possibility that the channel activity determined in vivo is due to the colicin A-Tol proteins complex.