FHUA, A TRANSPORTER OF THE ESCHERICHIA-COLI OUTER-MEMBRANE, IS CONVERTED INTO A CHANNEL UPON BINDING OF BACTERIOPHAGE-T5

The Escherichia coli outer membrane protein FhuA catalyzes the transpo rt of Fe3+-ferrichrome and is the receptor of phage T5 and Phi 80. The purified protein inserted into planar lipid bilayers showed no channe l activity. Binding of phage T5 to FhuA resulted in the appearance of high conductance ion channels. The electrophysiological characteristic s of the channels (conductance, kinetic behavior, substates, ion selec tivity including the effect of ferrichrome) showed similarities with t hose of the channel formed by a FhuA derivative from which the 'gating loop' (Delta 322-355) had been removed. Binding of phage T5 to FhuA i n E.coli cells conferred SDS sensitivity to the bacteria, suggesting t hat such channels also exist in vivo. These data suggest that binding of T5 to loop 322-355 of FhuA, which constitutes the T5 binding site, unmasks an inner channel in FhuA, Both T5 and ferrichrome bind to the closed state of the channel but only T5 can trigger its opening.