|
Although a Pcb–PSI supercomplex consisting of 18 subunits
of Chl a/b-binding Pcb protein surrounding a trimeric PSI reaction
center core has been found to occur in the prochlorophytes P.
marinus SS120 (17), we were unable to detect a
similar PSI supercomplex in Prochloron, despite adopting
the same solubilization and isolation procedures. It should be noted,
however, that Prochloron contains an isiA-like gene (10)
and perhaps under iron stress conditions or other environmental
variables, such a PSI supercomplex assembles, as in the case of
cyanobacteria (14–16). At present it is not possible to test
this hypothesis by using Prochloron because there is no reported
culturing procedure. However, we have identified a large supercomplex
that has Pcb proteins associated with PSII. This supercomplex in
Prochloron is the largest particle present anywhere in the
sucrose density gradient, and we interpret it to be a single PSII
core dimer with five Pcb proteins attached to each side. Consistent
with this conclusion is the fact that the particle is found in a
sucrose density fraction, F4, which is enriched in PSII relative
to PSI. Both room and low temperature fluorescence characteristics
of the fraction containing the Pcb–PSII supercomplex suggest
that energy is transferred efficiently from the Pcb proteins to
PSII, indicating it is a functional photosynthetic unit within the
thylakoid membrane.
Assuming the PSII core monomer binds 35 Chl and Pcb proteins are
like CP43 and bind 13 Chls (33), then the Pcb
antenna system of the supercomplex detected in F4 increases the
light-harvesting capacity of PSII by almost 200%. This is
a substantial increase in the absorption cross section of PSII and
presumably compensates for the lack of phycobiliproteins
in this organism. Thus the situation in Prochloron anticipates very
closely that found in green plastids of higher plants, where
the major light-harvesting Chl-binding proteins are attached to
PSII (11), as predicted from previous studies of Prochloron
(19, 34).
Interestingly, in no case was a complete ring of Pcb proteins observed
around a PSII core dimer, which contrasts with the
arrangement of Pcb proteins around PSI in Prochlorococcus
SS120 (17). The fact that some PSII core dimers had ~10 Pcb proteins
associated with them is probably the consequence of the detergent
solubilization procedure used, which, although very mild, did bring
about the release of some Pcb proteins, which were clearly detected
in the F1 fraction. This partial and in some cases complete stripping
down of the Pcb–PSII supercomplex gave rise to subpopulations,
particularly in the F2 and F3 fractions.
The work presented here represents the structural description of
the association of Pcb proteins with PSII core dimers and
potentially may be representative of Pcb–PSII complexes in
other related prochlorophytes. The model shown in Fig.
5, in
which the x-ray data of Zouni et al. (27) has been incorporated,
is a representation of the largest Pcb–PSII supercomplex that
we detected in Prochloron, but we cannot be certain that it is the
largest complex in vivo. However, this model shows that the Pcb
subunits closest to PSII are adjacent to CP47 and CP43. This arrangement
shows that the minimum Chl-Chl distances between the Pcb antenna
and CP47 and CP43 subunits are 10–15 and ~20 Å, respectively.
Chl molecules in the other Pcb subunits are distant from those within
the PSII core, suggesting that energy migration occurs between the
Pcb antenna proteins before being transferred to the PSII reaction
center. Note that the position of the Pcb proteins within the density
of the projection map in Fig. 5
has allowed for a detergent layer of ~10 Å, which is typical
for negatively stained membrane protein complexes (35).
Whether the lack of a complete Pcb ring around PSII is a general
feature required to facilitate quinone diffusion from the
QB site of PSII to the cytochrome b6f complex has yet to
be determined, but this idea has often been argued for the possible
lack of a complete light harvesting one antenna ring around the
purple bacterial reaction center (36). Interestingly the thylakoid
membranes of Prochloron are organized into stacked and unstacked
regions (37) reminiscent of the grana and stromal lamellae of higher
plant chloroplasts. It is possible that the Pcb–PSII supercomplex
is located in the stacked regions because there is some evidence
that there is lateral separation between PSI and PSII in the thylakoid
membranes of prochlorophytes equivalent to that found in higher
plant chloroplasts (38).
|
