The phycobilisome linker protein ApcG interacts with photosystem II and regulates energy transfer to photosystem I in Synechocystis sp. PCC 6803

Abstract

Photosynthetic organisms harvest light using pigment-protein super-complexes. In cyanobacteria, these are water-soluble antennae known as phycobilisomes (PBSs). The light absorbed by PBS is transferred to the photosystems in the thylakoid membrane to drive photosynthesis. The energy transfer between these super-complexes implies that protein-protein interactions allow the association of PBS with the photosystems. However, the specific proteins involved in the interaction of PBS with the photosystems are not fully characterized. Here, we show that the newly discovered PBS linker protein ApcG interacts specifically with photosystem II through its N-terminal region. Growth of cyanobacteria is impaired in apcG deletion strains under light-limiting conditions. Furthermore, complementation of these strains using a phospho-mimicking version of ApcG exhibit reduced growth under normal growth conditions. Interestingly, the interaction of ApcG with photosystem II is affected when a phospho-mimicking version of ApcG is used, targeting the positively charged residues interacting with thylakoid membrane suggesting a regulatory role mediated by phosphorylation of ApcG. Low temperature fluorescence measurements showed increased photosystem I fluorescence in apcG deletion and complementation strains. The photosystem I fluorescence was the highest in the phospho-mimicking complementation strain while pull-down experiment showed no interaction of ApcG with PSI under any tested condition. Our results highlight the importance of ApcG for selectively directing energy harvested by the PBS and implies that the phosphorylation status of ApcG plays a role in regulating energy transfer from PSII to PSI.