Structures of LPOR-Chlide complexes reveal the structural basis of membrane remodeling and photocatalysis.

Abstract

Light-dependent protochlorophyllide oxidoreductase (LPOR) is a light-driven enzyme in flowering plants. It is involved in chlorophyll biosynthesis while also reorganizing membrane lipids into the cubic membrane network that supports chloroplast development. However, the structural basis of these two activities and their relationship have remained unclear. Here, cryo-electron microscopy of chlorophyllide-bound LPOR oligomers reveals nine distinct assembly states, including helical filaments, stacked rings and segmented strings of dimers. We find that strings of LPOR dimers reshape lipid bilayers into a range of membrane architectures through combinations of three inter-string interfaces, providing a structural explanation for the flexibility of these assemblies. The highest-resolution map (2.55 Å), shows the pigment-binding region in sufficient detail to reveal a solvent-accessible channel near the pigment and a conformation of the propionate group may support hydride transfer from NADPH. Together, these findings establish a structural framework linking LPOR oligomerization, membrane remodeling and photocatalysis, and suggest that chlorophyllide-bound LPOR assemblies may have a regulatory function in mature leaves.