Molecular Basis for Semidominance of Missense Mutations in the XANTHA-H (42-kDa) Subunit of Magnesium Chelatase

Andreas Hansson, C Gamini Kannangara, Diter von Wettstein, Mats Hansson

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During biosynthesis of bacteriochlorophyll or chlorophyll, three protein subunits of 140, 70, and 42 kDa interact to insert Mg2+ into protoporphyrin IX. The semi-dominant Chlorina-125,-157, and -161 mutants in barley are deficient in this step and accumulate protoporphyrin IX after feeding on 5-aminolevulinate. Chlorina-125,-157, and -161 are allelic to the recessive xantha-h mutants and contain G559A, G806A, and C271T mutations, respectively. These mutations cause single amino acid substitutions in residues that are conserved in all known primary structures of the 42-kDa subunit. In vitro complementation and reconstitution of Mg-chelatase activity show that the 42-kDa subunits are defective in the semidominant Chlorina mutants. A mutated protein is maintained in the Chlorina plastids, unlike in the xantha-h plastids. Heterozygous Chlorina seedlings have 25-50% of the Mg-chelatase activity of wild-type seedlings. Codominant expression of active and inactive 42-kDa subunits in heterozygous Chlorina seedlings is likely to produce two types of heterodimers between the strongly interacting 42-kDa and 70-kDa subunits. Reduced Mg-chelatase activity is explained by the capacity of heterodimers consisting of mutated 42-kDa and wild-type 70-kDa protein to the 140-kDa subunit. The 42-kDa subunit is similar to chaperones that refold denatured polypeptides with respect to its ATP-to-ADP exchange activity and its ability to generate ATPase activity with the 70-kDa subunit. We hypothesize that the association of the 42-kDa subunit with the 70-kDa subunit allows them to form a specific complex with the 140-kDa subunit and that this complex inserts Mg2+ into protoporphyrin IX.
Original languageEnglish
Pages (from-to)1744-1749
JournalProceedings of the National Academy of Sciences
Issue number4
Publication statusPublished - 1999

Subject classification (UKÄ)

  • Biological Sciences


  • Plant Biology


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