Phycobilisome Linker Proteins Are Phosphorylated in Synechocystis sp. PCC 6803

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.

A low molecular weight protein tyrosine phosphatase from Synechocystis sp. strain PCC 6803: enzymatic characterization and identification of its potential substrates

“…Previously, phosphorylation of linker proteins (33 kDa, 35 kDa, 99 kDa and 27 kDa) on threonine in Synechocystis sp. PCC 6803 and the possible function of these linker proteins by phosphorylation/dephosphorylation as a signal for protein degradation during assembly/disassembly of the phycobilisome complex were reported by Piven et al (28). Future studies need to be performed to firmly establish the effect of tyrosine phosphorylation of this 9.5 kDa phycocyanin-associated rod linker protein, CpcD (Ssl3093), as a substrate for SynPTP and to identify the tyrosine phosphorylation sites on phycocyanin, PsaD and CpcD.…”

“…In Synechocystis sp. PCC 6803, a number of serine/threonine kinases (25,26), protein serine/threonine phosphatases (PP2A) (27,28), and serine/threonine phosphoproteins (29 32) are found. However, no specific PTKs, not any member of conventional or LMW PTPs, or any tyrosinephosphorylated proteins have been detected yet.…”

Proteomic analysis of post translational modifications in cyanobacteria

“…The nitrogen regulatory protein PII (GlnB), which plays a critical role in sensing changes in the internal N/C ratio, is the most intensively studied and best elucidated phosphoprotein in cyanobacteria (reviewed by Forchhammer, 2004Forchhammer, , 2008 [31,66]. Another example is the phosphorylation of phycobiliproteins and phycobilisome linker proteins [9,67]. KaiC, a central clock protein in cyanobacteria, was also reported to undergo circadian oscillations through autocatalytic phosphorylation/ de-phosphorylation cycle (reviewed by Williams, 2007& Axmann, 2014 [29,30].…”

“…These acetylated proteins are involved in diverse biological processes and render particular enrichment to photosynthesis and metabolic processes, which are closely linked to cellular energy status. Lysine acetylation is remarkably observed at the active sites of many components of the photosynthetic apparatus, including subunits of photosystem I (PsaA, PsaB, PsaD, and PsaF), photosystem II (PsbA, PsbB, PsabC, PsbO, PsbP, PsbV, and PsbU), phycocyanin (CpcA, CpcB, CpcC, and CpcG) and allophycocyanin (ApcA, ApcB, ApcD, ApcF, and ApcE), suggesting that lysine acetylation is another critical mechanism of photosynthetic functional regulation [9,10]. This study would be an important advance in understanding the physiological functions of lysine acetylation in Synechocystis and in cyanobacteria in general.…”

“…To adapt to changing environmental conditions and survive in extreme environments, cyanobacteria have developed a delicate signal transduction network to sense external signals and implement adaptive changes [9,10].…”