3D
map of the plant photosystem two supercomplex obtained by cryoelectron
microscopy and single particle analysis
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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.