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The antibiotic protein colicin E1 forms ion channels in planar lipid bilayers that are capable of conducting monovalent organic cations having mean diameters of at least 9 Å. Polyvalent organic cations appear to be completely impermeant, regardless of size. All permeant ions, whether large or small, positively or negatively charged, are conducted by this channel at very slow rates. We have examined the permeability of colicin E1 channels to anionic probes having a variety of sizes, shapes, and charge distributions. In contrast to the behavior of cations, polyvalent as well as monovalent organic anions were found to permeate the colicin E1 channel. Inorganic sulfate was able to permeate the channel only when the pH was 4 or less, conditions under which the colicin E1 protein is predominantly in an anion-preferring conformational state. The less selective state(s) of the colicin E1 channel, observed when the pH was 5 or greater, was not permeable to inorganic sulfate. The sulfate salt of the impermeant cation Bis-T6 (N,N,N′,N′-tetramethyl-1,6-hexanediamine) had no effect on the single channel conductance of colicin E1 channels exposed to solutions containing 1 m NaCl at pH 5. The complete lack of blocking activity by either of these two impermeant ions indicates that both are excluded from the channel lumen. These results are consistent with our hypothesis that there is but a single location in the lumen of the colicin E1 channel where positively charged groups can be effectively hydrated. This site may coincide with the location of the energetic barrier which impedes the movement of anions. [ABSTRACT FROM AUTHOR]

The antibiotic protein colicin E1 forms ion channels in planar lipid bilayers that are capable of conducting monovalent organic cations having mean diameters of at least 9 A. Polyvalent organic cations appear to be completely impermeant, regardless of size. All permeant ions, whether large or small, positively or negatively charged, are conducted by this channel at very slow rates. We have examined the permeability of colicin E1 channels to anionic probes having a variety of sizes, shapes, and charge distributions. In contrast to the behavior of cations, polyvalent as well as monovalent organic anions were found to permeate the colicin E1 channel. Inorganic sulfate was able to permeate the channel only when the pH was 4 or less, conditions under which the colicin E1 protein is predominantly in an anion-preferring conformational state. The less selective state(s) of the colicin E1 channel, observed when the pH was 5 or greater, was not permeable to inorganic sulfate. The sulfate salt of the impermeant cation Bis-T6 (N,N,N',N'-tetramethyl-1,6-hexanediamine) had no effect on the single channel conductance of colicin E1 channels exposed to solutions containing 1 M NaCl at pH 5. The complete lack of blocking activity by either of these two impermeant ions indicates that both are excluded from the channel lumen. These results are consistent with our hypothesis that there is but a single location in the lumen of the colicin E1 channel where positively charged groups can be effectively hydrated. This site may coincide with the location of the energetic barrier which impedes the movement of anions. [ABSTRACT FROM AUTHOR]

Channels formed by colicin E1 in planar lipid bilayers have large diameters and conduct both cations and anions. The rates at which ions are transported, however, are relatively slow, and the relative anion-to-cation selectivity is modulated over a wide range by the pH of the bathing solutions. We have examined the permeability of these channels to cationic probes having a variety of sizes, shapes, and charge distributions. All of the monovalent probes were found to be permeant, establishing a minimum diameter at the narrowest part of the pore of approximately 9 Å. In contrast to this behavior, all of the polyvalent organic cations were shown to be impermeant. This simple exclusionary rule is interpreted as evidence that, when steric restrictions require partial dehydration of an ion, the structure of the channel is able to provide a substitute electrostatic environment for only one charged group at time. [ABSTRACT FROM AUTHOR]

Channels formed by colicin E1 in planar lipid bilayers have large diameters and conduct both cations and anions. The rates at which ions are transported, however, are relatively slow, and the relative anion-to-cation selectivity is modulated over a wide range by the pH of the bathing solutions. We have examined the permeability of these channels to cationic probes having a variety of sizes, shapes, and charge distributions. All of the monovalent probes were found to be permeant, establishing a minimum diameter at the narrowest part of the pore of approximately 9 A. In contrast to this behavior, all of the polyvalent organic cations were shown to be impermeant. This simple exclusionary rule is interpreted as evidence that, when steric restrictions require partial dehydration of an ion, the structure of the channel is able to provide a substitute electrostatic environment for only one charged group at time. [ABSTRACT FROM AUTHOR]