Energy-coupled colicin transport through the outer membrane of Escherichia coli K-12: mutated TonB proteins alter receptor activities and colicin uptake.

The current model of TonB-dependent colicin transport through the outer membrane of Escherichia coli proposes initial binding to receptor proteins, vectorial release from the receptors and uptake into the periplasm from where the colicins, according to their action, insert into the cytoplasmic membrane or enter the cytoplasm. The uptake is energy-dependent and the TonB protein interacts with the receptors as well as with the colicins. In this paper we have studied the uptake of colicins B and Ia, both pore-forming colicins, into various tonB point mutants. Colicin Ia resistance of the tonB mutant (G186D, R204H) was consistent with a defective Cir receptor-TonB interaction while colicin Ia resistance of E. coli expressing TonB of Serratia marcescens, or TonB of E. coli carrying a C-terminal fragment of the S. marcescens TonB, seemed to be caused by an impaired colicin Ia-TonB interaction. In contrast, E. coli tonB (G174R, V178I) was sensitive to colicin Ia and resistant to colicin B unless TonB, ExbB and ExbD were overproduced which resulted in colicin B sensitivity. The differential effects of tonB mutations indicate differences in the interaction of TonB with receptors and colicins.