Transport of Vitamin B12 in Escherichia coli: Common Receptor Sites for Vitamin B12 and the E Colicins on the Outer Membrane of the Cell Envelope

AUTOR(ES)
RESUMO

The first step in the transport of cyanocobalamin (CN-B12) by cells of Escherichia coli was shown previously to consist of binding of the B12 to specific receptor sites located on the outer membrane of the cell envelope. In this paper, evidence is presented that these B12 receptor sites also function as the receptors for the E colicins, and that there is competition between B12 and the E colicins for occupancy of these sites. The cell strains used were E. coli KBT001, a methionine/B12 auxotroph, and B12 transport mutants derived from strain KBT001. Colicins E1 and E3 inhibited binding of B12 to the outer membrane B12 receptor sites, and CN-B12 protected cells against these colicins. Half-maximal protection was given by CN-B12 concentrations in the range of 1 to 6 nM, depending upon the colicin concentration used. Colicin E1 competitively inhibited the binding of 57Co-labeled CN-B12 to isolated outer membrane particles. Functional colicin E receptor sites were found in cell envelopes from cells of only those strains that possessed intact B12 receptors. Colicin K did not inhibit the binding of B12 to the outer membrane receptor sites, and no evidence was found for any identity between the B12 and colicin K receptors. However, both colicin K and colicin E1 inhibited the secondary phase of B12 transport, which is believed to consist of the energy-coupled movement of B12 across the inner membrane.

ACESSO AO ARTIGO

Documentos Relacionados