Project Title: Dissecting the role of MCE proteins in Cyanobacteria
Summary: Cyanobacteria are ancient oxygenic prokaryotes capable of harvesting solar energy for photosynthesis. They have a specialised internal membrane environment of thylakoids to host the photosynthetic machinery. The photosynthesis of the thylakoid membrane is affected by the type and integrity of lipids on the membrane.
MCE proteins are conserved lipid-binding proteins in diderm bacteria and eukaryotic chloroplasts. Studies in various organisms have shown that it plays an essential role in lipid transport and cell membrane homeostasis. For instance, structures of E. coli Mce/MlaD proteins (MlaD, PqiB, LetB) have been characterised using X-ray Crystallography & Cryo-EM. Interestingly, E. coli Mce/MlaD protein forms a hexameric ring associated with an ABC transporter complex in the inner membrane. It has been shown to bind to lipids. E.coli LetB protein consists of a stack of seven modular rings, creating a long hydrophobic tunnel through the centre of the complex. Lipids bind inside the tunnel, suggesting that it functions as a pathway for lipid transport. Besides, the plant's MCE family protein TGD2 can transport PA (phosphatidic acid) from the endoplasmic reticulum to the chloroplasts for the synthesis of galactolipids.
At present, there are two MCE proteins, SII1002 and SII0751, and two possible Mce transport complexes with permease SII1001 and SII1041, respectively, which have been described in Synechocystis sp. PCC 6803. Specifically, SII0751 and SII1041 are involved in acid stress tolerance. Nevertheless, we do not know the localisation of these proteins or the architecture and specific function and mechanism of action for the MCE transport assembly.
This project will explore which membrane these proteins reside in and the structure of MCE transport assembly in Synechocystis sp. PCC 6803 with X-ray Crystallography & Cryo-EM. The help of biochemistry and molecular biology methods will further verify and explore whether and how these proteins participate in lipid transport. All of these will help elucidate the role of these proteins and the molecular mechanism underlying stress tolerance.