Porphyrins promote the association of GENOMES UNCOUPLED 4 and a Mg-chelatase subunit with chloroplast membranes

J Biol Chem. 2009 Sep 11;284(37):24783-96. doi: 10.1074/jbc.M109.025205. Epub 2009 Jul 15.


In plants, chlorophylls and other tetrapyrroles are synthesized from a branched pathway that is located within chloroplasts. GUN4 (GENOMES UNCOUPLED 4) stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits porphyrins to the chlorophyll branch. GUN4 stimulates Mg-chelatase by a mechanism that involves binding the ChlH subunit of Mg-chelatase, as well as a substrate (protoporphyrin IX) and product (Mg-protoporphyrin IX) of Mg-chelatase. We chose to test whether GUN4 might also affect interactions between Mg-chelatase and chloroplast membranes, the site of chlorophyll biosynthesis. To test this idea, we induced chlorophyll precursor levels in purified pea chloroplasts by feeding these chloroplasts with 5-aminolevulinic acid, determined the relative levels of GUN4 and Mg-chelatase subunits in soluble and membrane-containing fractions derived from these chloroplasts, and quantitated Mg-chelatase activity in membranes isolated from these chloroplasts. We also monitored GUN4 levels in the soluble and membrane-containing fractions derived from chloroplasts fed with various porphyrins. Our results indicate that 5-aminolevulinic acid feeding stimulates Mg-chelatase activity in chloroplast membranes and that the porphyrin-bound forms of GUN4 and possibly ChlH associate most stably with chloroplast membranes. These findings are consistent with GUN4 stimulating chlorophyll biosynthesis not only by activating Mg-chelatase but also by promoting interactions between ChlH and chloroplast membranes.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / physiology
  • Buffers
  • Cell Membrane / enzymology
  • Chlorophyll / chemistry
  • Chloroplasts / enzymology*
  • Intracellular Membranes / enzymology*
  • Intracellular Signaling Peptides and Proteins / genetics
  • Intracellular Signaling Peptides and Proteins / physiology
  • Kinetics
  • Lyases / chemistry*
  • Models, Biological
  • Peas / enzymology
  • Plasmids / metabolism
  • Porphyrins / chemistry
  • Porphyrins / metabolism*
  • Protoporphyrins / chemistry
  • Thylakoids / enzymology


  • Arabidopsis Proteins
  • Buffers
  • GUN4 protein, Arabidopsis
  • Intracellular Signaling Peptides and Proteins
  • Porphyrins
  • Protoporphyrins
  • Chlorophyll
  • protoporphyrin IX
  • Lyases
  • magnesium chelatase