The role of the manganese exporter Mnx in manganese homeostasis of photoautotrophs

• Applicant: Privatdozentin Dr. Marion Eisenhut
• Subject Area: Plant Physiology
• Term: from 2014 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 255560821

Manganese (Mn) is an essential micronutrient to all organisms. It is involved in a wide range of physiological reactions by either activating enzymatic reactions or functioning as redoxactive catalyst of central enzymes. In oxygenic photosynthetic organisms Mn is essential for the splitting of water within photosystem II (PSII), enabling the use of water as electron source and evolution of oxygen. While in photoautotrophs sufficient provision of Mn to PSII and other Mn-dependent enzymes needs to be ensured, cytoplasmic Mn overload needs to be avoided to prevent mismetallation of metalloproteins. In the previous funding period we have identified and characterized proteins of the previously unknown protein family 0016 (UPF0016) as the long-sought Mn transporters being responsible for Mn transport at the chloroplast envelope and thylakoid membrane. The thylakoid protein Mnx is critical for maintenance of cellular Mn homeostasis in the cyanobacterium Synechocystis. The envelope transporter CMT1 and the thylakoid transporter PAM71 facilitate the sequential uptake of Mn into the chloroplast and delivery into the thylakoid lumen to enable Mn incorporation into PSII in the model plant Arabidopsis. In the renewal proposal we will address the role of UPF0016 proteins in Mn-dependent processes across a broad range of photosynthetic and non-photosynthetic organisms. In our own prework we identified two half-sized UPF0016 proteins in Synechocystis. We will test the hypothesis that these proteins, hemi manganese exporter 1 (Hmx1) and Hmx2, function as heteromers in Mn transport at the plasma membrane. Possibly, this transporter serves Mn provision to PSII in biogenesis centers and detoxification upon Mn excess. Furthermore, we will study whether Hmx1 and Hmx2 represent the ancestral form of full-length Mnx proteins. Another aim of this project is to evaluate the importance of plastid stroma Mn-loading for Mn-requiring enzymes. To this end, we will on the one hand investigate the MEP pathway enzyme 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) for Mn dependency, and on the other hand test the Mnx ortholog in the Apicomplexa parasite Toxoplasma gondii, TgMNX, for Mn transport activity. The proposed project will not only shed light on the evolution of Mnx proteins but will also determine the biological role of Mnx orthologs in a wide range of organisms.

• DFG Programme: Research Grants