title: Structure of the cyanobacterial Magnesium Chelatase H subunit determined by single particle reconstruction and small-angle X-ray scattering.
abstract: The biosynthesis of chlorophyll, an essential cofactor for photosynthesis, requires the ATP-dependent insertion of Mg(2+) into protoporphyrin IX catalyzed by the multisubunit enzyme magnesium chelatase. This enzyme complex consists of the I subunit, an ATPase that forms a complex with the D subunit, and an H subunit that binds both the protoporphyrin substrate and the magnesium protoporphyrin product. In this study we used electron microscopy and small-angle x-ray scattering to investigate the structure of the magnesium chelatase H subunit, ChlH, from the thermophilic cyanobacterium Thermosynechococcus elongatus. Single particle reconstruction of negatively stained apo-ChlH and Chl-porphyrin proteins was used to reconstitute three-dimensional structures to a resolution of ∼30 Å. ChlH is a large, 148-kDa protein of 1326 residues, forming a cage-like assembly comprising the majority of the structure, attached to a globular N-terminal domain of ∼16 kDa by a narrow linker region. This N-terminal domain is adjacent to a 5 nm-diameter opening in the structure that allows access to a cavity. Small-angle x-ray scattering analysis of ChlH, performed on soluble, catalytically active ChlH, verifies the presence of two domains and their relative sizes. Our results provide a basis for the multiple regulatory and catalytic functions of ChlH of oxygenic photosynthetic organisms and for a chaperoning function that sequesters the enzyme-bound magnesium protoporphyrin product prior to its delivery to the next enzyme in the chlorophyll biosynthetic pathway, magnesium protoporphyrin methyltransferase.
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom.
authors.forename: Pu
authors.initials: P
authors.lastname: Qian
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Marklew
authors.forename: Joanne
authors.initials: J
authors.lastname: Viney
authors.forename: Paul A
authors.initials: PA
authors.lastname: Davison
authors.forename: Amanda A
authors.initials: AA
authors.lastname: Brindley
authors.forename: Christopher
authors.initials: C
authors.lastname: Söderberg
authors.forename: Salam
authors.initials: S
authors.lastname: Al-Karadaghi
authors.forename: Per A
authors.initials: PA
authors.lastname: Bullough
authors.forename: J Günter
authors.initials: JG
authors.lastname: Grossmann
authors.forename: C Neil
authors.initials: CN
authors.lastname: Hunter
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D000458:Cyanobacteria / Q000201:enzymology*; D008190:Lyases / Q000737:chemistry*; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction
chemical_list: D001426:Bacterial Proteins; D021122:Protein Subunits; D008190:Lyases; C086788:magnesium chelatase