title: GUN4-porphyrin complexes bind the ChlH/GUN5 subunit of Mg-Chelatase and promote chlorophyll biosynthesis in Arabidopsis.
abstract: The GENOMES UNCOUPLED4 (GUN4) protein stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits protoporphyrin IX to chlorophyll biosynthesis. This stimulation depends on GUN4 binding the ChlH subunit of Mg-chelatase and the porphyrin substrate and product of Mg-chelatase. After binding porphyrins, GUN4 associates more stably with chloroplast membranes and was proposed to promote interactions between ChlH and chloroplast membranes-the site of Mg-chelatase activity. GUN4 was also proposed to attenuate the production of reactive oxygen species (ROS) by binding and shielding light-exposed porphyrins from collisions with O₂. To test these proposals, we first engineered Arabidopsis thaliana plants that express only porphyrin binding-deficient forms of GUN4. Using these transgenic plants and particular mutants, we found that the porphyrin binding activity of GUN4 and Mg-chelatase contribute to the accumulation of chlorophyll, GUN4, and Mg-chelatase subunits. Also, we found that the porphyrin binding activity of GUN4 and Mg-chelatase affect the associations of GUN4 and ChlH with chloroplast membranes and have various effects on the expression of ROS-inducible genes. Based on our findings, we conclude that ChlH and GUN4 use distinct mechanisms to associate with chloroplast membranes and that mutant alleles of GUN4 and Mg-chelatase genes cause sensitivity to intense light by a mechanism that is potentially complex.
authors.affiliation: Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: Neil D
authors.initials: ND
authors.lastname: Adhikari
authors.forename: John E
authors.initials: JE
authors.lastname: Froehlich
authors.forename: Deserah D
authors.initials: DD
authors.lastname: Strand
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Buck
authors.forename: David M
authors.initials: DM
authors.lastname: Kramer
authors.forename: Robert M
authors.initials: RM
authors.lastname: Larkin
mesh_terms: D000483:Alleles; D017360:Arabidopsis / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002734:Chlorophyll / Q000096:biosynthesis*; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D018506:Gene Expression Regulation, Plant / Q000187:drug effects / Q000528:radiation effects; D017343:Genes, Plant / Q000235:genetics; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D008027:Light; D008190:Lyases / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D017440:Photoperiod; D030821:Plants, Genetically Modified; D011166:Porphyrins / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects / Q000528:radiation effects; D021122:Protein Subunits / Q000378:metabolism*; D011524:Protoporphyrins / Q000494:pharmacology; D017382:Reactive Oxygen Species / Q000494:pharmacology; D013489:Suppression, Genetic / Q000187:drug effects / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; C521350:EXECUTER1 protein, Arabidopsis; C472017:GUN4 protein, Arabidopsis; D047908:Intracellular Signaling Peptides and Proteins; D011166:Porphyrins; D021122:Protein Subunits; D011524:Protoporphyrins; D017382:Reactive Oxygen Species; D002734:Chlorophyll; C028025:protoporphyrin IX; D008190:Lyases; C086788:magnesium chelatase; C561080:GUN5 protein, Arabidopsis