Understanding the role of carotenoids in bacterial light-harvesting proteins

This project will quantify how "light-harvesting" (LH) proteins can protect photosynthetic organisms from exposure to intense sunlight, in a process called "photo-protection". To do this, we will use genetically-modified versions of the LH proteins and artificial biomembranes that were recently developed by our team. These LH proteins are naturally localised within specialised biomembranes found in photosynthetic bacteria or plant chloroplasts. Bright sunlight can cause damage to plants and photosynthetic bacteria, so, to prevent this they activate photo-protective processes which safely remove the excess energy. This process is essential for fitness of plants and phototrophic bacteria but there are gaps in our knowledge of it. We know that carotenoid pigments are involved and also that the clustering of LH proteins increases energy dissipation, but the molecular details are uncertain. This project will determine how these two factors (carotenoids and protein clustering) can cause photo-protection by comparing LH proteins from photosynthetic bacteria to LH proteins from plants. Samples will be analysed with a world-class suite of microscopy and spectroscopy instruments. Firstly, we will assess how energy dissipation in the LH proteins is related to their carotenoid content for the simplest situation of isolated proteins. Secondly, we will assess how energy dissipation relates to the dual effects of protein clustering and carotenoids using artificial biomembranes which offer us greater control over the system. Thirdly, we will assess natural biomembranes and compare these to the artificial biomembranes to show whether the more complicated native system has other factors which alter the behaviour of LH proteins. Revealing the shared molecular mechanisms for how plants and bacteria control the distribution of energy within LH membranes will help us to understand the critical stages of solar energy capture which are fundamental to the global process of photosynthesis. This could provide new information that helps other researchers develop improved crops that have higher yields.

This project will delineate the photophysical mechanisms for important protective processes that occur within light-harvesting (LH) protein complexes, using mutant strains of bacteria and model membranes that were recently developed by our team. Photosynthetic organisms can be damaged by exposure to high light intensity, therefore they activate "photoprotective" energy dissipation processes in their LH proteins. This is crucial for maintaining the efficiency of plant photosynthesis but the influence of two key factors is not well understood. First, carotenoids are known to be involved in energy dissipation but the underlying photophysical pathways are highly debated. Second, the clustering of LH proteins within natural membranes appears to promote their energy-dissipative states, however the molecular basis is unknown. This project will quantify how protein clustering modulates the energy dissipation pathways related to carotenoids, by performing a detailed photophysical and structural comparison of four variants of the bacterial LH2 complex that have different carotenoids and contrasting these to the plant LHCII complex. Work Package (WP) 1 will quantify how the photophysics of LH complexes relates to their carotenoid content by assessing proteins that are isolated from each other. WP2 will quantify how the photophysics of LH complexes relates to the dual effects of carotenoid variation together with protein clustering, by assessing model lipid membranes where we can exert control over the proteins' aggregation state. Finally, WP3 will assess natural membrane extracts and compare these to the model membranes, allowing us to determine whether the photophysical trends are inherent to the LH complexes or depend upon external factors. A combination of spectroscopy and microscopy techniques (TA, FLIM, AFM) will provide definitive mechanistic insights. The outcome will be a step-change in our understanding of the molecular basis for photoprotection for diverse organisms.