3D
map of the plant photosystem two supercomplex obtained
by cryoelectron microscopy and single particle
analysis
- adapted from Nield J.,
Orlova E.V., Morris E.P., Gowen B., van Heel M. and
Barber J. (2000) Nature Structural Biology7,
44-47
ABSTRACT:
Here we describe the first 3D structure of photosystem
II (PSII) supercomplex of higher plants constructed by
single particle analysis of images obtained by
cryoelectron microscopy. This large multisubunit
membrane protein complex functions to absorb light
energy and catalyse the oxidation of water and reduction
of plastoquinone. The resolution of the 3D structure is
24 Å and emphasises the dimeric nature of the
supercomplex. The extrinsic proteins of the
oxygen-evolving complex (OEC) are readily observed as a
tetrameric cluster bound to the lumenal surface. By
considering higher resolution data, obtained from
electron crystallography, it has been possible to relate
the binding sites of the OEC proteins with the
underlying intrinsic membrane subunits of the
photochemical reaction centre core. The model suggests
that the 33 kDa OEC protein is located towards the
CP47/D2 side of the reaction centre but is also
positioned over the C-terminal helices of the D1 protein
including its CD lumenal loop. In contrast, the model
predicts that the 23/17 kDa OEC proteins are positioned
at the N-terminus of the D1 protein incorporating the AB
lumenal loop of this protein and two other unidentified
transmembrane helices. Overall the 3D model represents a
significant step forward in revealing the structure of
the photosynthetic OEC whose activity is required to
sustain the aerobic atmosphere on our planet.