The ChlD subunit links the motor and porphyrin binding subunits of magnesium chelatase

Abstract

Magnesium chelatase initiates chlorophyll biosynthesis, catalysing the MgATP^2--dependent insertion of a Mg²⁺ ion into protoporphyrin IX. The catalytic core of this large enzyme complex consists of three subunits: Bch/ChlI, Bch/ChlD and Bch/ChlH (in bacteriochlorophyll and chlorophyll producing species, respectively). The D and I subunits are members of the AAA⁺ (ATPases associated with various cellular activities) superfamily of enzymes, and they form a complex that binds to H, the site of metal ion insertion. In order to investigate the physical coupling between ChlID and ChlH <i>in vivo</i> and <i>in vitro</i>, ChlD was FLAG-tagged in the cyanobacterium <i>Synechocystis</i> sp. PCC 6803 and co-immunoprecipitation experiments showed interactions with both ChlI and ChlH. Co-production of recombinant ChlD and ChlH in <i>Escherichia coli</i> yielded a ChlDH complex. Quantitative analysis using microscale thermophoresis showed magnesium-dependent binding (<i>K</i>_d 331 ± 58 nM) between ChlD and H. The physical basis for a ChlD-H interaction was investigated using chemical cross-linking coupled with mass spectrometry (XL-MS), together with modifications that either truncate ChlD or modify single residues. We found that the C-terminal integrin I domain of ChlD governs association with ChlH, the Mg²⁺ dependence of which also mediates the cooperative response of the <i>Synechocystis</i> chelatase to magnesium. The interaction site between the AAA⁺ motor and the chelatase domain of magnesium chelatase will be essential for understanding how free energy from the hydrolysis of ATP on the AAA⁺ ChlI subunit is transmitted via the bridging subunit ChlD to the active site on ChlH.