Photocatalytic plant LPOR forms helical lattices that shape membranes for chlorophyll synthesis

Abstract

Chlorophyll (Chl) biosynthesis, crucial to life on Earth, is tightly regulated because its precursors are phototoxic. In flowering plants, the enzyme Light-dependent Protochlorophyllide OxidoReductase (LPOR) captures photons to catalyze the penultimate reaction: the reduction of a double-bond within protochlorophyllide (Pchlide) to generate chlorophyllide (Chlide). In darkness, LPOR oligomerizes to facilitate photon energy transfer and catalysis. However, the complete 3D structure of LPOR, the higher-order architecture of LPOR oligomers, and the implications of these self-assembled states for catalysis, including how LPOR positions Pchlide and the cofactor NADPH, remain unknown. Here we report the atomic structure of LPOR assemblies by electron cryo-microscopy (cryoEM). LPOR polymerizes with its substrates into helical filaments around constricted lipid bilayer tubes. Portions of LPOR and Pchlide insert into the outer membrane leaflet, targeting the product, Chlide, to the membrane for the final reaction site of chlorophyll biosynthesis. In addition to its crucial photocatalytic role, we show that in darkness LPOR filaments directly shape membranes into high-curvature tubules with the spectral properties of the prolammelar body, whose light-triggered disassembly provides lipids for thylakoid assembly. Our structure of the catalytic site, moreover, challenges previously proposed reaction mechanisms. Together, our results reveal a new and unexpected synergy between photosynthetic membrane biogenesis and chlorophyll synthesis in plants orchestrated by LPOR.

Photocatalysis as the ‘master switch’ of photomorphogenesis in early plant development

“…The size, shape and architecture of PLBs is known to be determined by the structural properties of the LPOR protein. These include its ability to form oligomers, 62,[65][66][67][68][69][70][71] the association of LPOR with the membrane 71,72 and the interaction of the enzyme with lipids. 44,59,71 The propensity of LPOR to form large, substrate-bound, PLB aggregates in the etioplastand potentially during the greening processwas first observed over 30 years ago 65 and has generated much interest in understanding this process in vitro.…”

“…These include its ability to form oligomers, 62,[65][66][67][68][69][70][71] the association of LPOR with the membrane 71,72 and the interaction of the enzyme with lipids. 44,59,71 The propensity of LPOR to form large, substrate-bound, PLB aggregates in the etioplastand potentially during the greening processwas first observed over 30 years ago 65 and has generated much interest in understanding this process in vitro. 62,[66][67][68][69][70][71] LPORs from different organisms can adopt a variety of soluble oligomeric forms in the absence of substrates, ranging from monomers in cyanobacterial LPORs 66,67,70 through to higher order oligomers in plant enzymes.…”

“…44,59,71 The propensity of LPOR to form large, substrate-bound, PLB aggregates in the etioplastand potentially during the greening processwas first observed over 30 years ago 65 and has generated much interest in understanding this process in vitro. 62,[66][67][68][69][70][71] LPORs from different organisms can adopt a variety of soluble oligomeric forms in the absence of substrates, ranging from monomers in cyanobacterial LPORs 66,67,70 through to higher order oligomers in plant enzymes. 62,68,69 In all cases, formation of a LPOR-Pchlide-NADPH ternary complex leads to formation of larger multimeric complexes (Figure 3).…”

“…62,68,69 In all cases, formation of a LPOR-Pchlide-NADPH ternary complex leads to formation of larger multimeric complexes (Figure 3). [66][67][68][69][70][71] In plant LPORs, binding of Pchlide is surmised to alter the oligomeric structure. This triggers further coupling of oligomers to form higher molecular weight aggregates.…”

“…This triggers further coupling of oligomers to form higher molecular weight aggregates. 62,68,69,71 Recent cryoelectron microscopy studies show that these LPOR oligomers form helical filaments with the lipid bilayer, allowing efficient exchange of the Pchlide substrate and Chlide product from the membrane. 71 Such LPOR filaments also directly shape the membranes into the characteristic curved tubules found in the PLB and may provide a source of lipids upon light-driven dissociation for the subsequent assembly of the thylakoids during biogenesis of the photosynthetic apparatus.…”