Structure, biogenesis, and evolution of thylakoid membranes

A small genome in chloroplasts encodes polypeptide subunits of the photosynthetic electron transport complexes in the membranes of thylakoid vesicles in the chloroplast stroma. Trans-membrane subunits of these complexes undergo co-translational membrane insertion during their synthesis by ribosomes of the bacterial-like genetic system of this semiautonomous organelle. While thylakoid membranes are sites of translation, evidence in the unicellular alga Chlamydomonas reinhardtii supports translation also on non-canonical membranes in a discrete translation-zone in the chloroplast. To characterize the membranous platforms for translation and the biogenesis of thylakoid membrane, we profiled membranes during chloroplast development, using the yellow-in-the-dark 1 mutant, and carried out proteomic analyses on membranes of interest. The results support roles of two membrane types in preliminary and ongoing stages of translation: a low-density membrane and a denser chloroplast translation membrane, respectively. These roles are based on correlations of the relative levels of each membrane type and the translational status of the chloroplast before, during and after chloroplast differentiation and results of proteomic analyses. Our results support a model of photosynthesis complex biogenesis in a spatiotemporal assembly line involving LDM and CTM as sequential stages leading to photosynthetic thylakoid membranes.

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Molecular architecture of thylakoid membranes within intact spinach chloroplasts

Wietrzynski,

Lamm,

Wood

et al. 2024

Preprint

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Thylakoid membranes coordinate the light reactions of photosynthesis across multiple scales, coupling the architecture of an elaborate membrane network to the spatial organization of individual protein complexes embedded within this network. Previously, we used in situ cryo-electron tomography (cryo-ET) to reveal the native thylakoid architecture of the green alga Chlamydomonas reinhardtii [1] and then map the molecular organization of these thylakoids with single-molecule precision [2]. However, it remains to be shown how generalizable this green algal blueprint is to the thylakoids of vascular plants, which possess distinct membrane architecture subdivided into grana stacks interconnected by non-stacked stromal lamellae. Here, we continue our cryo-ET investigation to reveal the molecular architecture of thylakoids within intact chloroplasts isolated from spinach (Spinacia oleracea). We visualize the fine ultrastructural details of grana membranes, as well as interactions between thylakoids and plastoglobules. We apply and further develop AI-based computational approaches for automated membrane segmentation and membrane protein picking [3], enabling us to quantify the organization of photosynthetic complexes within the plane of the thylakoid membrane and across adjacent stacked membranes. Our analysis reveals that, despite different 3D architecture, the molecular organization of thylakoid membranes in vascular plants and green algae is strikingly similar. In contrast to isolated plant thylakoids, where semi-crystalline arrays of photosystem II (PSII) appear to hold some membranes together, we find in intact chloroplasts that PSII is non-crystalline and has uniform concentration both within the membrane plane and across stacked grana membranes. Similar to C. reinhardtii, we observe strict lateral heterogeneity of PSII and PSI at the boundary between appressed and non-appressed thylakoid domains, with no evidence for a distinct grana margin region where these complexes have been proposed to intermix. Based on these measurements, we support a simple two-domain model for the molecular organization of thylakoid membranes in both green algae and plants.

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Structure, biogenesis, and evolution of thylakoid membranes

Cited by 3 publications

References 235 publications

Focus on photosynthesis

Focus on photosynthesis

Membranous translation platforms in the chloroplast ofChlamydomonas reinhardtii

Molecular architecture of thylakoid membranes within intact spinach chloroplasts

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