Fig. 2a shows the 3D map
for a single supercomplex with dimensions of 165 Å x 330 Å
x 110 Å overall width (x), length (y) and height (z),
respectively, and the volume enclosed by the surface is
approximately 3.5 x 106 Å cubed. The thickness of the
supercomplex is approximately 60 Å at its edges, as
expected for a membrane-spanning region, and the
protrusions extend maximally to 50 Å above the lumenal
surface. The protrusions marked A and A` have been shown
previously to be the 33 kDa manganese stabilizing protein
of the OEC based on earlier studies (7,8).
As previously suggested (4)
the protrusions labelled B and B` are in part due to the
23 kDa protein and, given that the isolated supercomplex
used for the present study contained, in addition, the 17
kDa protein, we conclude that this OEC subunit is also
located in this protrusion. Indeed, biochemical studies
have indicated that the 17 kDa protein binds to the 23 kDa
protein (9). The
structure of the B/B` protrusion, with a height of 50 Å,
is more extensive than that of the protrusion assigned to
the 33 kDa protein which has a height of about 40 Å, and
could therefore accommodate both the 23 kDa and 17 kDa
proteins. It has been suggested that there may be two
copies of the 33 kDa protein (10)
although others have argued for one (11).
From considerations of molecular mass and volume, our 3D
map indicates a single copy of this protein per reaction
centre. The centre-to-centre distances, as judged from Fig. 2e, between A and A` and
B and B` are 68 Å and 109 Å, respectively, and that
between A and B or A` and B` of each monomer is 65 Å. To
further aid the interpretation of the 3D map, three 10 Å
thick projections at different regions of the supercomplex
are presented in Fig. 2. On
the stromal side (Fig. 2c)
the distribution of features is relatively even throughout
the section but, in contrast, on the lumenal side (Fig.
2d) the region assigned to the surface of the
dimeric core, has markedly higher density as compared to
the peripheral regions containing LHCII, CP29 and CP26 (7, 8, 12). The
dimeric core contains the D1, D2, CP47 and CP43 proteins (13) that possess
substantial lumenal loops joining their transmembrane
helices. It is likely that these lumenal domains are
responsible for the high density observed in this region,
particularly the extensive loops belonging to the CP43 and
CP47 proteins (14).
Fig. 2e shows the 33 and
23/17 kDa proteins and clearly demonstrates their
tetrameric organisation on the surface of the dimer.
Next Section
|