TolC, an Escherichia coli outer membrane protein required for hemolysin secretion.

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Abstract
Secretion of Escherichia coli alpha-hemolysin into the medium does not require the cleavage of an N-terminal signal peptide. The specific secretion apparatus was shown to consist of two proteins, HlyB and HlyD, both located in the inner membrane and encoded by genes contiguous to the hemolysin structural gene (hlyA). It was proposed that these two proteins constitute a membrane-bound translocator for hemolysin [Mackman, N., Nicaud, J. M., Gray, L. & Holland, I. B. (1986) Curr. Top. Microbiol. Immunol. 125, 159-181]. We show here that an E. coli outer membrane protein, the TolC protein, encoded by a gene not located in the hly cluster, is specifically required for hemolysin secretion. This result suggests that an outer membrane protein might be a component of the secretion apparatus allowing a specific interaction between the inner and the outer membrane.