HIGH-RESOLUTION ATOMIC FORCE MICROSCOPY OF NATIVE MEMBRANES - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 27

in . 22 After nano-manipulation to

remove peptidoglycan remnants or stacked layers, both extracellular and
periplasmic surfaces were visualized. The extracellular surface of porins
exists in two distinct conformations: one with separated protrusions and
another with protrusions contracted at the three-fold axis ( Fig. 2.4a ). Their
occurrence was correlated with their position within the membrane: trimers
which were positioned in the centre of a membrane displayed the “relaxed”
conformation ( Fig. 2.4b ) , whereas the trimers at the membrane edges
displayed the “contracted” conformation ( Fig. 2.4c ), a phenomenon attributed
to trapped ions under the central membrane region. The periplasmic surface
of the porins exhibited oval-shaped cavities separated by walls crowned by
three major protrusions with three-fold axis at the pore brims ( Fig. 2.4d ) .
The unit cell in ordered regions is

Roseobacter denitriicans ( R. denitriicans )

= 60°. High-resolution
topography averages could be compared to molecular surface representations
of the X-ray structure 39 in great detail ( Fig. 2.4e,f ) .

a = b = 81 ± 2.5 Å, γ

2.3 PHOTOSYNTHETIC MEMBRANE

Owing to its high SNR ratio, AFM has emerged as an indispensable tool that
allows for the structural identiication of individual membrane proteins
not only with regular protein arrangements but also with non-ordered
assemblies. A striking breakthrough is the high-resolution AFM imaging of
bacterial photosynthetic membranes in contact mode, which has provided
the irst surface views of the organization of multi-component biological
membranes at submolecular resolution. 40,41
In photosynthesis, light capture and energy transfer are eficiently
accomplished by the strong cooperativeness among different photosynthetic
components, i.e., light-harvesting complexes (LH1 and LH2), reaction centre
(RC), cytochrome (cyt) bc 1 complex and ATPase, which are embedded in

the photosynthetic membranes. The shape of bacterial photosynthetic
membranes is highly species dependent. They may be arranged in regular
parallel layers ( Rhodopseudomonas [ Rps. ] viridis , Rhodopseudomonas [ Rps. ]

palustris , Rhodospirillum [ Rsp. ] photometricum , Phaeospirillum [ Phsp. ]

molischianum) or form vesicles ( Rhodobactor [ Rb. ] spheroides , Rhodobactor

[ Rb. ] blasticus ), or small regular stacks of tubular lamellae ( Rhodospirillum

[ ). The information concerning protein domain

formation, complex assembly and protein heterogeneity, derived from high-
resolution AFM imaging, provided important insights into the physiological
roles of the photosynthetic machinery.

Rsp. ] fulvum , Thiocapsa spp.