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fluorescence emission spectra of Synechocystis cells lacking both photo-
systems under excitation at 532 nm in a wide range of laser photon flux
density. Deconvolution of the fluorescence spectra into three Gaussians
assigned to PC, APC 660 and APC 680 fluorescence and analysis of the non-
linear dependence of fluorescence and the laser photon flux density allowed
them to conclude that both APC 660 and APC 680 are quenched ( Kuzminov,
Karapetyan, et al., 2012 ). Thus, the possibility that the OCP binds an APC 680
trimer in some phycobilisomes or that OCP binding allows the simultane-
ous quenching of 660 and 680 nm emission remains an open question.
Nevertheless, based on our knowledge of the stringent specificity of OCP
binding and of the interaction between the OCP r and the phycobilisome,
we think that this is not likely.
4.3. What Part of the OCP Interacts with the Phycobilisome?
Several recent experiments have provided the first insights into the structural
basis of the interaction between the OCP and phycobilisome to mediate
quenching. To dissect out the role of specific amino acids, it is first necessary
to consider the structural anatomy of the OCP. As noted above, the OCP
is composed of two domains joined by a flexible linker ( Fig. 1.1 ). Given
that the OCP-mediated photoprotective mechanism requires the OCP o to
OCP r conversion, and is known from FTIR measurements to involve pro-
tein motion, and to be sensitive to viscosity, the assumption is that there are
protein conformation changes that are requisite for photoprotection; per-
haps these are important for exposing and orienting the carotenoid.
The two domains interact through two regions ( Fig. 1.1 ). The first 19
amino acids of the OCP extend from the N-terminal domain and form
part of the C-terminal domain in full-length OCP. This N-terminal inter-
domain interface buries 947 and 775 Å 2 of the N- and C-terminal domains,
respectively, and is composed of conserved residues from both domains
( Wilson, Kinney, et al., 2010 ). The second interface, across which the carot-
enoid spans the protein, buries 628 and 723 Å 2 of the N- and C-terminal
domains, respectively. It contains a salt bridge between R155 and E244.
This salt bridge stabilizes the interaction between the two domains ( Fig. 1.1 ).
Interestingly, the OCP mutants R155L and R155E are photoactive but
unable to induce fluorescence quenching in Synechocystis cells ( Wilson,
Gwizdala, et al., 2012 ; Wilson, Kinney, et al., 2010 ). In vitro , accumula-
tion of the red form of R155L-OCP and R155E-OCP is faster than that
of WT-OCP and their OCP r forms are more stable than the WT-OCP r
( Wilson, Gwizdala, et al., 2012 ). The E244L-OCP mutant shows similar
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