title: Recessiveness and dominance in barley mutants deficient in Mg-chelatase subunit D, an AAA protein involved in chlorophyll biosynthesis.
abstract: Mg-chelatase catalyzes the insertion of Mg2+ into protoporphyrin IX at the first committed step of the chlorophyll biosynthetic pathway. It consists of three subunits: I, D, and H. The I subunit belongs to the AAA protein superfamily (ATPases associated with various cellular activities) that is known to form hexameric ring structures in an ATP-dependant fashion. Dominant mutations in the I subunit revealed that it functions in a cooperative manner. We demonstrated that the D subunit forms ATP-independent oligomeric structures and should also be classified as an AAA protein. Furthermore, we addressed the question of cooperativity of the D subunit with barley (Hordeum vulgare) mutant analyses. The recessive behavior in vivo was explained by the absence of mutant proteins in the barley cell. Analogous mutations in Rhodobacter capsulatus and the resulting D proteins were studied in vitro. Mixtures of wild-type and mutant R. capsulatus D subunits showed a lower activity compared with wild-type subunits alone. Thus, the mutant D subunits displayed dominant behavior in vitro, revealing cooperativity between the D subunits in the oligomeric state. We propose a model where the D oligomer forms a platform for the stepwise assembly of the I subunits. The cooperative behavior suggests that the D oligomer takes an active part in the conformational dynamics between the subunits of the enzyme.
authors.affiliation: Department of Biochemistry, Lund University, SE-221 00 Lund, Sweden.
authors.forename: Eva
authors.initials: E
authors.lastname: Axelsson
authors.forename: Joakim
authors.initials: J
authors.lastname: Lundqvist
authors.forename: Artur
authors.initials: A
authors.lastname: Sawicki
authors.forename: Sara
authors.initials: S
authors.lastname: Nilsson
authors.forename: Ingrid
authors.initials: I
authors.lastname: Schröder
authors.forename: Salam
authors.initials: S
authors.lastname: Al-Karadaghi
authors.forename: Robert D
authors.initials: RD
authors.lastname: Willows
authors.forename: Mats
authors.initials: M
authors.lastname: Hansson
mesh_terms: D000595:Amino Acid Sequence; D002384:Catalysis; D002734:Chlorophyll / Q000096:biosynthesis*; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D005799:Genes, Dominant; D005808:Genes, Recessive; D001467:Hordeum / Q000201:enzymology* / Q000235:genetics*; D008190:Lyases / Q000737:chemistry / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism; D008666:Metalloendopeptidases / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D018515:Plant Leaves / Q000201:enzymology; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000737:chemistry / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016354:Rhodobacter capsulatus / Q000648:ultrastructure; D036226:Seedlings / Q000201:enzymology
chemical_list: D010940:Plant Proteins; D021122:Protein Subunits; D012333:RNA, Messenger; D002734:Chlorophyll; D008666:Metalloendopeptidases; C114583:m-AAA proteases; D008190:Lyases; C086788:magnesium chelatase