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characteristics, implying that the salt bridge between Arg 155 and Glu
244 stabilizes the OCP
o
and strongly suggests that in the red form. This
region of interaction between the two domains is weakened or nonex-
istent (
Wilson, Gwizdala, et al., 2012
). In addition, R155L- and R155E-
OCPs are substantially impaired in phycobilisome binding. The change to
a neutral amino acid in R155L decreases the strength of OCP
r
binding
and the change to a negative charge in R155E OCP almost completely
abolishes it. The importance of the positive charge of Arg 155 for binding
to the phycobilisome is underscored by the relatively small effect on fluo-
rescence quenching in an R155K mutant (
Wilson, Gwizdala, et al., 2012
).
In contrast, binding of E244L-OCP to phycobilisomes is similar to that of
WT-OCP, indicating that this amino acid is not directly involved in the
interaction. These results demonstrated that the surface of the N-terminal
domain containing the Arg 155 is directly involved in the binding of OCP
r
to the core of phycobilisome. This implies that in the orange form, the
presence of the R155-E244 salt bridge stabilizes a 'closed' conformation
precluding the OCP
o
binding to the phycobilisomes. Accordingly, a model
is emerging for the OCP interaction with the phycobilisome: strong blue-
green light, by inducing carotenoid (and concomitant protein) confor-
mational changes, causes the breakage of the R155-E244 salt bridge; the
resulting domain motion exposes the surface of the N-terminal domain
containing Arg155 for interaction with the phycobilisome core, possibly
with the negative charges in one of the APC trimers of the phycobilisome
core, close to one of the bilin chromophores. It seems that there is only one
very specific site of OCP binding and the interaction between Arg155 and
APC permits a closer interaction between hECN and one APC chromo-
phore but probably less with two bilins.
5. DISCOVERY AND CHARACTERIZATION OF THE
FLUORESCENCE RECOVERY PROTEIN
In vitro
, the isolated OCP
r
spontaneously reverts to OCP
o
in dark-
ness (
Wilson, Punginelli, et al., 2008
). In
Synechocystis
, however, attachment
of OCP
r
to the phycobilisomes stabilizes the OCP
r
and another protein
is needed to induce OCP
r
to OCP
o
conversion and detachment from the
phycobilisome (
Boulay, Wilson, et al., 2010
;
Gwizdala, Wilson, et al., 2011
).
In the
Synechocystis
genome, the
slr1963
gene encoding the OCP is fol-
lowed by a gene (
slr1964
) encoding a conserved protein; the co-occurrence
of this gene adjacent to that for the OCP in most available cyanobacterial
