The Biogenesis Structure and Function of Biological Membranes

Nearly all life on Earth gets the food and oxygen it needs from plants, or from simpler bacterial photosynthetic organisms that live in oceans, lakes and ponds, thus underpinning all global food chains. Because the planet Earth is largely aquatic the quantity and activity of these photosynthetic bacteria is stupendous; billions of tonnes of photosynthetic bacteria grow in the oceans every year. This unseen microbial army inhabits every sea, even growing 100 metres below the surface. Although to us these are inky depths, photosynthetic bacteria can grow and thrive because they make so much chlorophyll that they can grab hold of every photon of light that comes their way. The humble photosynthetic bacteria are the start of the major food chains that make other life in the sea possible and so feed many of us too. There is so much chlorophyll on Earth that it weighs more than all of humankind, yet despite its omnipresence and its importance to all life, astonishingly, nobody understands fully how chlorophyll is made. What is more there are millions of chlorophyll molecules inside each photosynthetic cell, which have to be attached to proteins before they collect and use energy from the sun, but again despite its crucial importance, nobody understands anything about this attachment process. We want to find out how the chlorophylls and proteins inside cells are made, and how they are put together to capture light and convert it into ATP, which powers the thousands of chemical reactions that enable the cells to grow and divide. This knowledge is important to us all, not just because capturing and using solar energy fuels life, but it also holds the secret of designing and making devices that one day could give us clean, unlimited energy from sunlight. How can we gain this knowledge? We use photosynthetic bacteria, quick and easy to grow in illuminated bottles on a laboratory benchtop, and we then open up the bacteria, take out the chlorophyll-proteins and see how they work. The chlorophyll-proteins that capture solar energy are called light-harvesting complexes (LHCs). In much the same way as a satellite dish concentrates the weak TV signal onto the receiver, thousands of LHCs are grouped side-by-side to collect solar energy and deliver it to a small number of reaction centres (RC). The RC protein converts the energy harvested by the LHCs to electrical energy, in the form of positive and negative charges on either side of a membrane, like charging up a biological battery; this drives the production of ATP, the chemical fuels for all cells. We want to know how the cell makes these membranes, so amazingly efficient that 99% of the energy that falls on them is delivered to the RC. To get such highly efficient energy collection we know that LHCs must be packed in close contact with one another in the membrane but we do not know how the cell manages to do this. To understand how such a photosynthetic membrane works we must follow the sequence of events that leads to the functional light harvesting network inside the cells. To do this we will use an atomic force microscope to literally 'feel' the shapes of each LHC and RC as the cell makes networks of them and turn this information into a 'photograph' of where everything is in the membrane. By taking repeated pictures of membranes at different stages in their development we can see how nature achieves this feat of bio-engineering. How will we use this knowledge? We all need electricity and we want to make a start on learning lessons from nature by assembling our own artificial light harvesting system and following how the energy is captured by LHCs then channeled to a RC. Can we channel this energy efficiently? Can we 'plug' our artificial light harvesting system directly into a photovoltaic cell to make electricity? By bringing together a team of scientists from Biology, Physics and Chemistry we will explore these exciting possibilities in our research.

This multidisciplinary programme of research aims for a complete understanding of a biological membrane, including the biosynthesis of the protein and cofactor components, their assembly into membrane-cofactor complexes, and supramolecular organisation to form functional arrays in the native membrane. The model systems chosen for this work are the purple bacterium Rhodobacter sphaeroides and the cyanobacterium Synechocystis. An integrated study of chlorophyll biosynthesis will include the quantitation, structural and functional analysis of the pathway enzymes, with the eventual aim of using scanning probe and single molecule approaches to quantify the intermolecular forces that drive the formation and function of multisubunit complexes. Studies of the membrane bound terminal enzyme, chlorophyll synthase, will be used to elucidate the handover mechanism of newly synthesised chlorophylls and the role of assembly factors in the initiation of membrane biogenesis. The composition, cellular location, number and morphology of sites of initiation of membrane invagination, and of mature membranes, will be quantified by mass spectrometry and mapped using tomographic, optical and AFM approaches with the eventual aim of using near-field optics to provide simultaneous temporal and nanometer scale spatial imaging of ultrafast energy flow through photosynthetic architectures. In silico modelling will construct a full-atom membrane model to investigate the dynamics and of complexes, and the influence of membrane protein packing density on migration of quinones and quinols between complexes. Novel chemically defined surface binding strategies for controlled surface attachment and patterning of functional arrays of native and heterologously produced membrane proteins and enzymes will not only allow us to examine molecular interactions at the single molecule level but will act as prototypes for downstream 'lab on a chip' projects and hybrid photo-voltaic systems