Anomalous proton selectivity in a large channel: colicin A.
Some of the bactericidal proteins known as colicins exert their toxic action by forming a large, nonselective channel in the inner membrane of target bacteria. The structure of this channel is unknown. It conducts large ions but has a much smaller conductance than would be expected for a channel of its deduced size. Here we report that the colicin channel, particularly the colicin A channel, is selective for protons over other cations (and anions) by many orders of magnitude. This was deduced from measurements of reversal potentials in pH gradients across planar lipid bilayers containing these channels. For example, in symmetric 0.1 M KCl with a pH 5/pH 8 gradient across the membrane, the reversal potential of colicin A is -21 mV, rather than 0. Such a result would be unremarkable for a narrow channel but is beyond explanation by current understanding of permeation for a channel of its diameter. For this reason, we re-examined the issue of the diameter of the channel lumen and confirmed that the lumen is indeed "too large" ( approximately 10 A) to select for protons by the amount that we measure. We are thus compelled to propose that an unorthodox mechanism is at work in this protein.
Slatin
Stephen L
SL
Department of Physiology and Biophysics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, USA. slatin@aecom.yu.edu
Finkelstein
Alan
A
Kienker
Paul K
PK
D003087:Colicins / Q000737:chemistry*; D011522:Protons
D003087:Colicins; D011522:Protons