Protein secretion by hybrid bacterial ABC‐transporters: specific functions of the membrane ATPase and the membrane fusion protein.

The Erwinia chrysanthemi metalloprotease C and the Serratia marcescens haem acquisition protein HasA are both secreted from Gram‐negative bacteria by a signal peptide‐independent pathway which requires a C‐terminal secretion signal and a specific ABC‐transporter made up of three proteins: a membrane ATPase (the ABC‐protein), a second inner membrane component belonging to the membrane fusion protein family and an outer membrane polypeptide. HasA and protease C transporters are homologous although the secreted polypeptides share no sequence homology. Whereas protease C can use both translocators, HasA is secreted only by its specific transporter. Functional analysis of protease C and HasA secretion through hybrid transporters obtained by combining components from each system demonstrates that the ABC‐protein is responsible for the substrate specificity and that inhibition of protease C secretion in the presence of HasA results from a defective interaction between HasA and the ABC‐protein. We also show that the outer membrane protein, TolC, can combine with the membrane fusion protein HasE in the presence of either ABC‐protein to form a functional transporter but not with the membrane fusion protein, PrtE. This indicates a specific interaction between the outer membrane component and the membrane fusion protein.