ION SELECTIVITY OF COLICIN E1 .3. ANION PERMEABILITY

The antibiotic protein colicin E1 forms ion channels in planar lipid b ilayers that are capable of conducting monovalent organic cations havi ng mean diameters of at least 9 Angstrom. Polyvalent organic cations a ppear to be completely impermeant, regardless of size. All permeant io ns, whether large or small, positively or negatively charged, are cond ucted by this channel at very slow rates. We have examined the permeab ility of colicin E1 channels to anionic probes having a variety of siz es, shapes, and charge distributions. In contrast to the behavior of c ations, polyvalent as well as monovalent organic anions were found to permeate the colicin E1 channel. Inorganic sulfate was able to permeat e the channel only when the pH was 4 or less, conditions under which t he colicin E1 protein is predominantly in an anion-preferring conforma tional state. The less selective state(s) of the colicin E1 channel, o bserved when the pH was 5 or greater, was not permeable to inorganic s ulfate. The sulfate salt of the impermeant cation Bis-T6 (N,N,N',N'-te tramethyl-1,6-hexanediamine) had no effect on the single channel condu ctance of colicin E1 channels exposed to solutions containing 1 M NaCl at pH 5. The complete lack of blocking activity by either of these tw o impermeant ions indicates that both are excluded from the channel lu men. These results are consistent with our hypothesis that there is bu t a single location in the lumen of the colicin E1 channel where posit ively charged groups can be effectively hydrated. This site may coinci de with the location of the energetic barrier which impedes the moveme nt of anions.