HIGH-RESOLUTION ATOMIC FORCE MICROSCOPY OF NATIVE MEMBRANES - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 28

AFM imaging of native photosynthetic membranes has provided detailed
information about the architecture of RC-LH1 core complexes and the LH2
complexes. Indeed, the core-complex architecture varied considerably
between species: core complexes in a native membrane of Rps. viridis ,

24

Rsp.

photometricum 21,42,43 and Phsp. molischianum 44 had LH1 16 -RC L,M,H architecture,

topped by a non-membranous tetraheme cytochrome (4Hcyt), that was not
visible in Rb. blasticus 45 the core

complexes had (PufX 2 -LH1 13 -RC L,M,H ) 2 architecture probably like the

Phsp. molischianum . In native membranes of

Rba.

sphaeroides core, 46 but the precise Rhodobacter core-complex architecture

is still a matter of debate. Rps. palustris

membranes had W-LH1 15 -RC L,M,H architecture. 49 Similarly, high-resolution
AFM allowed studying LH2 complex architecture at the single-molecule
level and depicting molecular heterogeneity

45-48

Finally, core complexes in

. The structure of LH2
complexes in native membranes was found to be rather variable. In the study
of LH2 in

in situ

membranes, in addition to normal nonameric
LH2 complexes, several different types of complexes were observed:

Rsp. photometricum

About
10% of the complexes were octameric and another 10% were decameric.
This size heterogeneity was attributed to the known spectral heterogeneity
of LH2. In addition, some of them presented small rings containing six or
seven subunits, open C-shaped complexes, or large complexes containing
up to 14 subunits, maybe LH2/LH1 chimeric rings. 50 Such heterogeneous
stoichiometry appears to be an inherent feature of LH2, as it has also been
observed in

50

. 49
Beyond the structure analysis of the individual components, the AFM
gives the exciting possibility of studying supramolecular non-ordered
protein assemblies in the native membrane. Analysis of the distribution
of photosynthetic complexes showed clustering of LH2 and RC-LH1 core
complexes in native membranes ( Fig. 2.5 ) . Clustering of complexes is
a functional necessity, as each light-harvesting component must pass
its harvested energy to a neighbouring complex and eventually to the
RC. Clustering of bacterial photosynthetic complexes has been seen in
membranes from all different species studied so far.

Phsp. molischianum 44 and Rps. palustris

41

With the exception of

Rb. sphaeroides , 51 no regular structural assembly of LH2 and core complexes

were observed, and these photosynthetic complexes were not in an
ordered arrangement in the native membranes. Core complexes completely
surrounded by several LH2 and core complexes making multiple core-core
contacts were visualized ( Fig. 2.5a ) 21,42,43 . However, their organization is
far from random. Pair correlation function analysis has shown that there is
a most favourable assembly within these membranes, which is core-LH2-