Voltage and pH-induced channel closure of porin OmpF visualized by atomic force microscopy

doi:10.1006/jmbi.1998.2359

Journal of Molecular Biology

Volume 285, Issue 4, 29 January 1999, Pages 1347-1351

https://doi.org/10.1006/jmbi.1998.2359 Get rights and content

Abstract

Gram-negative bacteria are protected by an outer membrane in which trimeric channels, the porins, facilitate the passage of small solutes. The pores are formed by membrane-spanning antiparallel β-strands, which are connected by short turns on the periplasmic side and long loops on the extracellular side. Voltage and pH-dependent conformational changes of these extracellular loops have now been visualized by atomic force microscopy of two-dimensional crystals of Escherichia coli porin OmpF. The observed conformational changes accompany the closure of the channel entrance, and suggest that this is a mechanism that the cells have evolved to protect themselves from drastic changes of the environment.

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Acknowledgements

We thank A. Hoenger and M. Cyrklaff for providing porin OmpF 2D crystals, and H.-J. Butt, H. Gaub, S. A. Müller, R. Dutzler, J. Rosenbusch, T. Schirmer and M. Winterhalter for helpful discussions. This work was supported by the Swiss National Foundation for Scientific Research and the Maurice Müller Foundation of Switzerland.

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To gain a mechanistic insight in the functioning of the OmpF-like porin from Yersinia pseudotuberculosis (YOmpF), we compared the effect of pH variation on the ion channel activity of the protein in planar lipid bilayers and its binding to lipid membranes. The behavior of YOmpF channels upon acidification was similar to that previously described for Escherichia coli OmpF. In particular, a decrease in pH of the bathing solution resulted in a substantial reduction of YOmpF single channel conductance, accompanied by the emergence of subconductance states. Similar subconductance substates were elicited by the addition of lysophosphatidylcholine. This observation, made with porin channels for the first time, pointed to the relevance of lipid-protein interactions, in particular, the lipid curvature stress, to the appearance of subconductance states at acidic pH. Binding of YOmpF to membranes displayed rather modest dependence on pH, whereas the channel-forming potency of the protein tremendously decreased upon acidification.

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Journal of Molecular Biology

CommunicationVoltage and pH-induced channel closure of porin OmpF visualized by atomic force microscopy1

Abstract

Section snippets

Acknowledgements

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J. Struct. Biol.

J. Mol. Biol.

FEBS Letters

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J. Struct. Biol.

J. Struct. Biol.

Curr. Opin. Cell Biol.

J. Biol. Chem.

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Communication
Voltage and pH-induced channel closure of porin OmpF visualized by atomic force microscopy¹