A carboxy-terminal fragment of colicin Ia forms ion channels |
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Authors: | Partho Ghosh Stephanie F Mel Robert M Stroud |
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Institution: | (1) Department of Biochemistry and Biophysics, University of California, 94143-0448 San Francisco, California;(2) Department of Experimental Pathology, University of California, 94143-0506 San Francisco, California;(3) Present address: Department of Biochemistry and Molecular Biology, Harvard University, 7 Divinity Avenue, 02138 Cambridge, Massachusetts;(4) Present address: Department of Microbiology and Molecular Genetics, Harvard Medical School, 200 Longwood Avenue, 02115 Boston, Massachusetts |
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Abstract: | A carboxy-terminal, 18 kD fragment of colicin Ia, a bacterial toxin, forms ion channels in artificial phospholipid bilayers. This fragment, which comprises a quarter of the intact 70 kD molecule, is resistant to extensive protease digestion and probably constitutes a structural domain of the protein. The ion channels formed by the 18 kD fragment are functionally heterogeneous, having conductances that range from 15 to 30 pS at positive voltages and from 70 to 250 pS at negative voltages, and open lifetimes that range from at least 25 msec to 5 sec. In contrast, ion channels formed by whole colicin Ia open only at negative voltages, at which their conductances range from 6 to 30 pS, and their open lifetimes range from 1 sec to 3 min. Additionally, the open state of the 18 kD fragment channel is characterized by noisy fluctuations in current, while the open state of the whole molecule ion channel is often marked by numerous, stable subconductance states. Since the properties of the fragment channel differ substantially from those of the whole molecule channel, we suggest that portions of the molecule outside of the 18 kD fragment are involved in forming the whole molecule ion channel. |
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Keywords: | Colicin Ia fragment Artificial lipid bilayers Ion channels Proteolysis |
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