Biological membranes orchestrate cellular life by creating the cell shape, protecting the cell in the changing environment, and spatially separating metabolic processes into different cellular compartments. We know now that biological membranes organize the membrane-integral proteins into specialized domains so that the molecular partners can be concentrated and tightly associated together with each other to drive biological processes.
A paradigm of biological membranes is the specialized intracellular photosynthetic membranes in diverse phototrophs, known as the thylakoid membranes. In cyanobacteria, algae and plants, the thylakoid membranes carry out the light reactions of photosynthesis to convert energy from sunlight into chemical energy and produce oxygen necessary for life on Earth.
Cyanobacteria thylakoid membranes are unique compared with their counterparts in algae and plants, because they accommodate the membrane protein complexes not only for plant-like photosynthetic light reactions, including photosystem II (PSII), photosystem I (PSI), cytochrome b6f (Cyt b6f), and ATP synthase, but also for respiration (Liu, 2016). The longstanding questions are how these different membrane complexes are organized in the naturally designed thylakoid membranes and how they interact with each other to promote electron flux and adaption in the changing environments.
My research group (www.luningliu.org) is particularly interested in deciphering the structure and function of native photosynthetic membranes using a range of microscopic techniques, including fluorescence microscopy, electron microscopy, and atomic force microscopy (AFM) (Liu and Schuering, 2013; Liu and Zhang, 2019). A challenge is to overcome the limitations of microscopic imaging on the biological membranes that are very thin, mechanically soft and highly dynamic.
AFM is a powerful tool in studying protein organization and interactions in biological membranes at the near-native conditions. Unlike other microscopic techniques, AFM imaging does not require complicated sample preparation procedures; we conduct AFM imaging in buffers, at room temperature and under ambient so that the biological samples are still “alive”; as AFM can offer a high signal-to-noise ratio, there is no need for complicated image processing and averaging (Liu and Schuering, 2013).
The journey of AFM explorations started from my PhD study on the thylakoid membrane structure of red algae. I have revealed the native organization of phycobilisomes, the giant light-harvesting antenna on thylakoid membranes (Liu et al., 2008). I have also enjoyed probing the photosynthetic membranes of purple bacteria (Liu et al., 2011). AFM studies on cyanobacterial thylakoid membranes were initiated after I became a PI at the University of Liverpool. In 2017, we reported the medium-resolution AFM topographs of cyanobacterial thylakoid membranes and described how photosynthetic complexes are located and dynamically move in thylakoid membranes using both AFM and confocal imaging (Casella et al., 2017).
In this Nature Plants paper, we reported a high-resolution AFM study on the native architecture of thylakoid membranes from a model cyanobacterium (Zhao et al., 2020). This allowed us to observe how bioenergetic proteins are positioned in native thylakoid membranes at the nanometer scale and how cells change the thylakoid membrane organization to adapt to light variations.
Read the paper: https://www.nature.com/articles/s41477-020-0694-3.
We first grew cyanobacterial cells under different light intensities and then harvested thylakoid membranes for AFM imaging. We found that under high light, a chlorophyll-binding protein called IsiA was highly produced. The resulting IsiA proteins associate with PSI and form circular IsiA−PSI supramolecular complexes. The spectroscopic assays showed that the extra IsiA assemblies increase the absorption capacity of PSI. This is of physiological importance when the amount of PSI was reduced in cyanobacterial thylakoids under high light.
AFM topograph of high-light-adapted thylakoid membranes with densely packed PSI complexes and IsiA proteins.
However, IsiA−PSI complexes do not exhibit identical structures. Instead, there are different IsiA−PSI structures observed in the same membranes. These results indicated that the IsiA−PSI association is highly flexible.
We further illustrated that the thylakoid membranes create specific domains to allocate different photosynthetic complexes. For example, there are membrane regions that are enriched with PSI complexes; some membrane domains are occupied by parallel arrays of PSII complexes. The special membrane “islands” may facilitate directed and efficient electron flow between various protein complexes and the “repair” of damaged protein complexes.
We were also excited to see the close contacts between different membrane complexes in native membranes, such as the PSI−PSII−Cyt b6f, IsiA−PSI−ATP synthase, and PSI−NDH-1 assemblies. These results indicated experimentally the presence of “supercomplex assemblies” in nature, which can offer advantages to the photosynthetic machinery to fulfil and regulate photosynthetic activities.
By drawing the landscape of thylakoid membranes, we hope to gain a better understanding of how phototrophs produce and organize functional thylakoid membranes for photosynthesis and respiration. Lessons learned from nature will teach us how to tune the photosynthetic apparatus and build artificial photosynthetic systems to underpin the production of sustainable bioenergy.
Casella S, Huang F, Mason D, Zhao GY, John GN, Mullineaux CW, Liu LN. Dissecting the native architecture and dynamics of cyanobacterial photosynthetic machinery. Molecular Plant, 2017, 10(11): 1434–1448.
Liu LN, Aartsma TJ, Thomas JC, Lamers GEB, Zhou BC, Zhang YZ. Watching the native supramolecular architecture of photosynthetic membrane in red algae: Topography of phycobilisomes, and their crowding, diverse distribution patterns. J Biol Chem, 2008, 283 (50): 34946-34953.
Liu LN, Duquesne K, Oesterhelt F, Sturgis JN, Scheuring S. Forces guiding assembly of light-harvesting complex 2 in native membranes. Proc Natl Acad Sci USA, 2011, 108 (23): 9455-9459.
Liu LN, Scheuring S. Investigation of photosynthetic membrane structure using atomic force microscopy. Trends Plant Sci, 2013, 18(5): 277-286.
Liu LN, Zhang YZ. Cryo-electron microscopy delineates the in situ structure of the thylakoid network. Molecular Plant, 2019, 12(9): 1176-1178.
Liu LN. Distribution and dynamics of electron transport complexes in cyanobacterial thylakoid membranes. Biochim Biophys Acta - Bioenergetics, 2016, 1857(3): 256-265.
Zhao LS, Huokko T, Wilson S, Simpson DM, Wang Q, Ruban AV, Mullineaux CW, Zhang YZ, Lu-Ning Liu. Structural variability, coordination, and adaptation of a native photosynthetic machinery. Nature Plants, 2020, 6: 869-882. DOI: 10.1038/s41477-020-0694-3.