The N- and C-terminal domains of the OCP are joined by a
long (∼25 amino acids) peptide chain. Even though this is the
only region of the protein that is not strongly conserved, we
found that K167 and K170 in this linker are cross-linked to
ApcB K58 and ApcE K4, respectively (Table S3 of the
Supporting Information). It seems that this flexible linker could
pass conformational information from the C-terminal domain
to the buried N-terminal domains of the protein or vice versa
and thereby allow the energy coupling of 3′-hECN and
phycocyanobilins (PCBs). According to our model, PCB
pigments from APC660 are located spatially closer to the
pigment of the OCP than PCB is to ApcE. Because spatial
distance dictates the excitation energy transfer efficiency, we
hypothesize that the OCPr will preferentially quench the
excitation energy from APC660. Similar conclusions about
quenching sites were recently proposed on the basis of
spectroscopic methods and mutation studies.26,27,40 We think
that our results constitute direct structural evidence that
supports this hypothesis. There are reports showing that ApcE
(LCM) could also be the binding partner of the OCP and that
ApcE is directly involved in the OCP-mediated NPQ.23,25 Our
data do not exclude this hypothesis; however, if this were the
case, the quenching efficiency would be very low in vivo,
because of the increased spatial distance between 3′-hECN and
the terminal emitter on ApcE (Figure 3). Additionally,
nonphysiological conditions (2 M urea at pH 2.5 or 60 mM
formic acid at pH 3.0) used in the preparation of ApcE and in
the reconstitution of the ApcE−OCP complex in these two
reports could lead to unexpected results.4