AJP - Cell Physiology, Vol 265, Issue 3 C851-C856, Copyright © 1993 by American Physiological Society
Imaging of reconstituted biological channels at molecular resolution by atomic force microscopy
R. Lal, H. Kim, R. M. Garavito and M. F. Arnsdorf
Department of Medicine, University of Chicago, Illinois 60637.
Using atomic force microscopy (AFM), we obtained high-resolution surface
images of the bacterial outer membrane channels Escherichia coli OmpF porin
and Bordetella pertussis porin that were reconstituted in artificial
bilayer membranes as two-dimensional crystalline arrays. These porins were
chosen because they are among the most extensively studied proteins of this
type and are known for their well-defined crystalline nature in the native
membrane. Such reconstituted membrane proteins are ideal specimens to
assess the suitability and resolution of AFM for imaging biomembranes and
associated proteins. Although OmpF porin often showed a mixed pattern of
rectangular and hexagonal arrays with approximately 8.4 x 9.8- and
approximately 7.2-nm-spacings, respectively, B. pertussis porin showed
mostly a rectangular pattern with an approximately 7.9 x 13.8-nm spacing.
The packing patterns of the E. coli OmpF porin in the membrane are very
close to those found in electron-microscopic studies. When B. pertussis
porin was imaged in a buffer solution, its trimeric subunits were
apparently resolved, and the surface of each monomer revealed beadlike
structures. This is the first report of such a high-resolution structural
analysis of B. pertussis porin by any imaging method. We also imaged the
lipid bilayer itself as an internal control for imaging and to further
ascertain the resolution. Individual polar head groups of bilayer lipid
molecules were resolved, suggesting the intrinsic resolution of AFM for
bioimaging.
