Thylakoid attachment to the plasma membrane in Synechocystis sp. PCC 6803 requires the AncM protein

Thylakoids are the highly specialized internal membrane systems that harbor the photosynthetic electron transport machinery in cyanobacteria and in chloroplasts. In Synechocystis sp. PCC 6803, thylakoid membranes (TMs) are arranged in peripheral sheets that occasionally converge on the plasma membrane (PM) to form thylakoid convergence membranes (TCMs). TCMs connect several thylakoid sheets and form local contact sites called thylapses between the two membrane systems, at which the early steps of photosystem II (PSII) assembly occur. The protein CurT is one of the main drivers of TCM formation known so far. Here, we identify, by whole-genome sequencing of a curT- suppressor strain, the protein anchor of convergence membranes (AncM) as a factor required for the attachment of thylakoids to the PM at thylapses. An ancM- mutant is shown to have a photosynthetic phenotype characterized by reductions in oxygen-evolution rate, PSII accumulation, and PS assembly. Moreover, the ancM- strain exhibits an altered thylakoid ultrastructure with additional sheets and TCMs detached from the PM. By combining biochemical studies with fluorescence and correlative light-electron microscopy-based approaches, we show that AncM is an integral membrane protein located in biogenic TCMs that form thylapses. These data suggest an antagonistic function of AncM and CurT in shaping TM ultrastructure.