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The Orange Carotenoid Protein (OCP) is responsible for photoprotection in many cyanobacteria. Absorption of blue light drives the conversion of the orange, inactive form (OCP) to the red, active form (OCP). Concomitantly, the N-terminal domain (NTD) and the C-terminal domain (CTD) of OCP separate, which ultimately leads to the formation of a quenched OCP-PBS complex. The details of the photoactivation of OCP have been intensely researched. Binding site(s) of OCP on the PBS core have also been proposed. However, the post-binding events of the OCP-PBS complex remain unclear. Here, we demonstrate that PBS-bound OCP is not sufficient as a PBS excitation energy quencher. Using site-directed mutagenesis, we generated a suite of single point mutations at OCP Leucine 51 (L51) of Synechocystis 6803. Steady-state and time-resolved fluorescence analyses demonstrated that all mutant proteins are unable to quench the PBS fluorescence, owing to either failed OCP binding to PBS, or, if bound, an OCP-PBS quenching state failed to form. The SDS-PAGE and Western blot analysis support that the L51A (Alanine) mutant binds to the PBS and therefore belongs to the second category. We hypothesize that upon binding to PBS, OCP likely reorganizes and adopts a new conformational state (OCP) different than either OCP or OCP to allow energy quenching, depending on the cross-talk between OCP and its PBS core-binding counterpart.
This article was published in the following journal.
Name: Biochimica et biophysica acta. Bioenergetics
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