HIGH-RESOLUTION ATOMIC FORCE MICROSCOPY OF NATIVE MEMBRANES - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 26

2.2.2 Outer Membrane

The OM of gram-negative bacteria protects the cell against bactericidal
substances. Passage of nutrients and waste is assured by porins, the β -barrel
transmembrane channels in OMs.

(a) (b)

(c)

(d) (e)

(f)

Figure 2.4. AFM images of the native extracellular and the periplasmic OM surfaces of
Roseobacter denitriicans . 22 (a) Topograph of the top layer exposing the extracellular
surface to the AFM tip. The arrow marks a border strip, which exposes the underlying
layer showing the periplasmic surface. Encircled are the trimers representing two
different conformations of the extracellular domains. (b and c) The two conformations
of the extracellular protrusions correlate with their location within the membrane.
The “relaxed” conformation (top) was found inside the membrane patch (dashed
outlines). The three-fold symmetrized average discloses a well-preserved, central
indentation. The “contracted” conformation (bottom) displayed by porin located
close to the membrane's border (dashed outlines). (d) High-resolution analysis of the
periplasmic surface of the OM. (e) High-resolution average revealing substructure
comparable to (f ), the X-ray structure of a homologous porin (PDB 1PRN 39 ).

The irst high-resolution AFM view of a bacterial OM revealed that
porins cover 70% of the membrane surface and form locally regular lattices