title: Porphyrins promote the association of GENOMES UNCOUPLED 4 and a Mg-chelatase subunit with chloroplast membranes.
abstract: 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.
authors: Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824, USA.
authors: Neil D
authors: ND
authors: Adhikari
authors: Robert
authors: R
authors: Orler
authors: Joanne
authors: J
authors: Chory
authors: John E
authors: JE
authors: Froehlich
authors: Robert M
authors: RM
authors: Larkin
mesh_terms: D029681:Arabidopsis Proteins / Q000235:genetics / Q000502:physiology; D002021:Buffers; D002462:Cell Membrane / Q000201:enzymology; D002734:Chlorophyll / Q000737:chemistry; D002736:Chloroplasts / Q000201:enzymology*; D007425:Intracellular Membranes / Q000201:enzymology*; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000502:physiology; D007700:Kinetics; D008190:Lyases / Q000737:chemistry*; D008954:Models, Biological; D018532:Pisum sativum / Q000201:enzymology; D010957:Plasmids / Q000378:metabolism; D011166:Porphyrins / Q000737:chemistry / Q000378:metabolism*; D011524:Protoporphyrins / Q000737:chemistry; D020524:Thylakoids / Q000201:enzymology
chemical_list: D029681:Arabidopsis Proteins; D002021:Buffers; C472017:GUN4 protein, Arabidopsis; D047908:Intracellular Signaling Peptides and Proteins; D011166:Porphyrins; D011524:Protoporphyrins; D002734:Chlorophyll; C028025:protoporphyrin IX; D008190:Lyases; C086788:magnesium chelatase