Phycobilisome Linker Proteins Are Phosphorylated in Synechocystis sp. PCC 6803

Irina Piven,Ghada Ajlani,Anna Sokolenko

doi:10.1074/jbc.m412967200

Abstract

The controversial issue of protein phosphorylation from the photosynthetic apparatus of Synechocystis sp. PCC 6803 has been reinvestigated using new detection tools that include various immunological and in vivo labeling approaches. The set of phosphoproteins detected with these methods includes ferredoxin-NADPH reductase and the linker proteins of the phycobilisome antenna. Using mutants that lack a specific set of linker proteins and are affected in phycobilisome assembly, we show that the phosphoproteins from the phycobilisomes correspond to the membrane, rod, and rod-core linkers. These proteins are in a phosphorylated state within the assembled phycobilisomes. Their dephosphorylation requires partial disassembly of the phycobilisomes and further contributes to their complete disassembly in vitro. In vivo we observed linker dephosphorylation upon long-term exposure to higher light intensities and under nitrogen limitation, two conditions that lead to remodeling and turnover of phycobilisomes. We conclude that this phosphorylation process is instrumental in the regulation of assembly/disassembly of phycobilisomes and should participate in signaling for their proteolytic cleavage and degradation.

Highlights

The controversial issue of protein phosphorylation from the photosynthetic apparatus of Synechocystis sp
Linker proteins can be divided into four groups: (i) rod-core linkers (LRC) that attach the peripheral rods to the PBS core; (ii) rod linkers (LR10, LR33, LR35) that associate PC substructures into rod segments; (iii) the small core linkers (LC8 ) that are
To check whether similar phosphoprotein patterns resulted from serological cross-reactivity of phosphothreonine and phosphoserine residues or from the presence of both phosphoresidues, inhibitors blocking phosphothreonine or phosphoserine antisera were applied during the immunological assay

Summary

Introduction

The controversial issue of protein phosphorylation from the photosynthetic apparatus of Synechocystis sp. In vivo we observed linker dephosphorylation upon long-term exposure to higher light intensities and under nitrogen limitation, two conditions that lead to remodeling and turnover of phycobilisomes We conclude that this phosphorylation process is instrumental in the regulation of assembly/disassembly of phycobilisomes and should participate in signaling for their proteolytic cleavage and degradation. Phycobilisomes (PBSs) are large multimeric protein structures that function as an extrinsic light-harvesting antenna in cyanobacteria and red algae They are located at the outer surface of thylakoid membranes where they transfer their excitation energy to the photosynthetic reaction centers that are embedded within the lipid bilayer. PBSs represent a major biosynthetic commitment of a cyanobacterial cell; apart from their light-harvesting function in photosynthesis, PBSs are recruited as a principal nutrient source under starvation conditions This dual function implies specific regulation for assembling, disassembling, and remodeling the PBS structure according to changes in metabolic and energy requirements. Its post-translational modification by phosphorylation at some threonine residues triggers changes in protein conformation and redistribution of the antenna between the two photosystems in a process known as state transition [25, 26]

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Conclusion