Chloroplasts rely on a tiny molecular machine to build the protein gates in their outer membranes, and researchers have now developed a way to watch that machine in action. This matters because the same type of protein-assembly machinery operates in bacteria and mitochondria, but scientists do not yet understand the common rules that govern how these machines recognise, fold, and release their protein cargo. Without that understanding, it is impossible to predict how to fix a broken assembly line or block a harmful one. If this project succeeds, it will produce the first detailed structural and mechanistic model of the chloroplast’s assembly machinery. That knowledge is fundamental science with no immediate practical application. However, a unified model across bacteria, mitochondria, and chloroplasts could eventually guide efforts to develop antibiotics that disable the bacterial version without harming human cells, or to engineer crops with more efficient photosynthesis by improving chloroplast protein transport. Past work on similar membrane protein machines has already led to unexpected drug targets and synthetic biology tools.
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A fundamental question in cell biology and biochemistry is how beta-barrel membrane proteins assemble into the outer membrane of Gram-negative bacteria and endosymbiotic organelles (mitochondria and chloroplasts). This is of profound importance because the function of many of these organelles depends upon the proper assembly of essential beta-barrel proteins involved in transporting ions, metabolites and proteins. Thus, a complete mechanistic understanding of this process may enable us to modify or control beta-barrel biogenesis to aid the development of specific therapies targeting resistant bacteria or mitochondrial diseases. Moreover, this understanding will contribute to the development of crops with enhanced yields (since chloroplasts are responsible for photosynthesis), thereby addressing population needs and pressure from climate change. Our current knowledge of bacterial and mitochondrial beta-barrel assembly machineries makes it difficult to discern common underlying principles, generalities, and disparities among three model systems. Furthermore, many critical mechanistic steps remain unclear, including how the assembly machineries recognise, fold, and release substrates. This is primarily due to our inability to capture the transient interactions involved within this process. Based on my recent discovery that native mass spectrometry (MS) can efficiently capture these transient interactions and the information obtained that can guide structural analysis, I propose to decipher the molecular architecture and mechanism of the chloroplast beta-barrel assembly machinery and thereby contribute to a greater understanding of a unified mechanistic model. This ambitious project integrates several recent breakthrough discoveries in structural biology, such as native MS, top-down MS, mass photometry, and cryo-EM. Over the last year, I have laid the foundation and developed a pipeline to facilitate these studies and have demonstrated the project's overall feasibility.
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