Thylakoid Membrane Architecture in Synechocystis Depends on CurT, a Homolog of the Granal CURVATURE THYLAKOID1 Proteins

Abstract

Photosynthesis occurs in thylakoids, a highly specialized membrane system. In the cyanobacterium sp PCC 6803 (hereafter 6803), the thylakoids are arranged parallel to the plasma membrane and occasionally converge toward it to form biogenesis centers. The initial steps in PSII assembly are thought to take place in these regions, which contain a membrane subcompartment harboring the early assembly factor PratA and are referred to as PratA-defined membranes (PDMs). Loss of CurT, the 6803 homolog of grana-shaping proteins of the CURVATURE THYLAKOID1 family, results in disrupted thylakoid organization and the absence of biogenesis centers. As a consequence, PSII is less efficiently assembled and accumulates to only 50% of wild-type levels. CurT induces membrane curvature in vitro and is distributed all over the thylakoids, with local concentrations at biogenesis centers. There it forms a sophisticated tubular network at the cell periphery, as revealed by live-cell imaging. CurT is part of several high molecular mass complexes, and Blue Native/SDS-PAGE and isoelectric focusing demonstrated that different isoforms associate with PDMs and thylakoids. Moreover, CurT deficiency enhances sensitivity to osmotic stress, adding a level of complexity to CurT function. We propose that CurT is crucial for the differentiation of membrane architecture, including the formation of PSII-related biogenesis centers, in 6803.

Hydrocarbons Are Essential for Optimal Cell Size, Division, and Growth of Cyanobacteria

“…Therefore, it is possible that, if hydrocarbons do alter membrane curvature, then this is augmented and orientated by CurT. In DcurT, the thylakoid membranes were still highly curved, indicating that other factors are involved in inducing membrane curvature (Heinz et al, 2016). Homologs of CurT are present in the majority of sequenced cyanobacterial strains (Supplemental Table S1).…”

“…A homologous protein in Synechocystis, CurT, has recently been shown to have a similar role in thylakoid membranes (Heinz et al, 2016). Deletion of CurT resulted in a reduction in growth and extreme differences in thylakoid membrane organization, with the thylakoids appearing to cross the cytoplasm and not converging on the "thylakoid centers."…”

What is Vipp1 good for?

“…Vipp1/IM30‐enriched regions were found to co‐localize with CURT1, a protein supposably inducing TM curvature (Armbruster et al ., ; Heinz et al ., ). Noteworthy, no evidence of direct IM30‐CURT1 interaction has been found yet (Bryan et al ., ; Heinz et al ., ), implying that co‐localization is caused by the membrane curvature. However, the lipid bilayer of curvature‐stressed membranes might be drastically destabilized, which exhibits the risk of proton leakage (Attard et al ., ).…”

“…In agreement with this observation, in the now presented study, IM30 puncta were preferentially identified at curvature‐stressed regions of the TM (Gutu et al ., ). Vipp1/IM30‐enriched regions were found to co‐localize with CURT1, a protein supposably inducing TM curvature (Armbruster et al ., ; Heinz et al ., ). Noteworthy, no evidence of direct IM30‐CURT1 interaction has been found yet (Bryan et al ., ; Heinz et al ., ), implying that co‐localization is caused by the membrane curvature.…”

“…In fact, the highly curved TM margin regions were suggested to be of special importance for PS II assembly/accumulation in cyanobacteria, albeit deletion of the curt1 gene in the cyanobacterium Synechocystis sp. PCC 6803 did not dramatically affect TM and PS biogenesis and/or cell viability (Heinz et al ., ).…”