STUDY OF SUGAR BINDING TO THE SUCROSE-SPECIFIC SCRY CHANNEL OF ENTERIC BACTERIA USING CURRENT NOISE-ANALYSIS

ScrY, an outer membrane channel of enteric Gram-negative bacteria, whi ch confers to the bacteria the rapid uptake of sucrose through the out er membrane was reconstituted into lipid bilayer membranes and the cur rent noise was investigated in the open and in the carbohydrate-induce d closed state of the channel. The open state of the channel exhibited up to about 200 Hz 1/f-noise with a rather small spectral density. Up on addition of carbohydrates to the aqueous phase the current through the ScrY channels decreased in a dose-dependent manner. Simultaneously , the spectral density of the current noise increased drastically, whi ch indicated interaction of the carbohydrates with the binding site in side the channel and its reversible block. The frequency dependence of the spectral density was of the Lorentzian type but very often two Lo rentzians were observed, from which the slow one may not be related to carbohydrate binding. Analysis of the power density spectra of the se cond Lorentzian using a previously proposed simple model of carbohydra te binding allowed the evaluation of the on- and the off-rate constant s for the carbohydrate association with the binding site inside the Sc rY channel and of a mutant (ScrY Delta 3-72), in which 70 amino acids at the N-terminus are deleted. The binding of carbohydrates to ScrY wa s compared to those of the closely related maltoporin channels of Esch erichia coli and Salmonella typhimurium by assuming that only the time constant and spectral density of the high frequency Lorentzian is rel ated to carbohydrate transport.