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

Abstract

Chlorophyll synthase (ChlG) is a transmembrane enzyme that attaches the tetraprenyl tail to chlorophyllide to produce chlorophyll a, the key pigment of oxygenic photosynthesis. In the cyanobacterium Synechocystis sp. PCC 6803, ChlG forms a complex with the light-harvesting complex (LHC)-like high-light-inducible protein (Hlip) HliD. Here, to understand the catalytic mechanism of ChlG and the role of HliD, we used cryogenic electron microscopy to determine the structure of a ChlG2HliD2 complex in substrate-free and geranylgeranyl-pyrophosphate-bound states and AlphaFold to model binding of chlorophyllide at the active site of ChlG. In combination with site-directed mutagenesis and molecular dynamics simulations, our structural data allow us to propose the mechanisms of substrate specificity and catalysis by ChlG. Our structures also reveal how a central HliD homodimer binds two ChlG monomers, as well as the arrangement of the four chlorophyll and two carotenoid pigments bound by the Hlips, and shed light on the evolution of eukaryotic LHC antennae.

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