Transport of Vitamin B 12 in Escherichia coli : Common Receptor Sites for Vitamin B 12 and the E Colicins on the Outer Membrane of the Cell Envelope

Abstract

The first step in the transport of cyanocobalamin (CN-B 12 ) by cells of Escherichia coli was shown previously to consist of binding of the B 12 to specific receptor sites located on the outer membrane of the cell envelope. In this paper, evidence is presented that these B 12 receptor sites also function as the receptors for the E colicins, and that there is competition between B 12 and the E colicins for occupancy of these sites. The cell strains used were E. coli KBT001, a methionine/B 12 auxotroph, and B 12 transport mutants derived from strain KBT001. Colicins E1 and E3 inhibited binding of B 12 to the outer membrane B 12 receptor sites, and CN-B 12 protected cells against these colicins. Half-maximal protection was given by CN-B 12 concentrations in the range of 1 to 6 nM, depending upon the colicin concentration used. Colicin E1 competitively inhibited the binding of 57 Co-labeled CN-B 12 to isolated outer membrane particles. Functional colicin E receptor sites were found in cell envelopes from cells of only those strains that possessed intact B 12 receptors. Colicin K did not inhibit the binding of B 12 to the outer membrane receptor sites, and no evidence was found for any identity between the B 12 and colicin K receptors. However, both colicin K and colicin E1 inhibited the secondary phase of B 12 transport, which is believed to consist of the energy-coupled movement of B 12 across the inner membrane.