Structure of chlorophyll synthase in complex with the LHC-like protein HliD

Chlorophyll is the central cofactor in oxygenic photosynthesis, responsible for both light capture in antenna complexes and light-driven charge separation in photosystems. The tetraprenyl tail of chlorophyll is attached to the chlorophyllide macrocycle by the transmembrane enzyme chlorophyll synthase (ChlG). In the cyanobacterium Synechocystis sp. PCC 6803, ChlG forms a complex with the LHC-like high-light-inducible protein HliD. To understand the substrate specificity and catalytic mechanism of ChlG, and how it is photoprotected by HliD, we determined the structure of a ChlG2HliD2 complex in both substrate-free and geranylgeranyl pyrophosphate (GGPP)-bound states using cryogenic electron microscopy (cryo-EM). A homodimer of HliD, which binds four chlorophylls and two zeaxanthin molecules in a quenched state, is flanked by two ChlG monomers. AlphaFold modelling placed chlorophyllide adjacent to the structurally resolved GGPP bound to the active site of ChlG. Cryo-EM data, site-directed mutagenesis and molecular dynamics were used to formulate a molecular mechanism for ChlG catalysis. The structure of the ChlG2HliD2 complex shows how Hlips bind to the synthase, reveals the arrangements of carotenoids and chlorophylls that mediate energy dissipation, and sheds light on the evolution of eukaryotic LHC antennae from their cyanobacterial ancestors.