title: Acetolactate synthase regulatory subunits play divergent and overlapping roles in branched-chain amino acid synthesis and Arabidopsis development.
abstract: Branched-chain amino acids (BCAAs) are synthesized by plants, fungi, bacteria, and archaea with plants being the major source of these amino acids in animal diets. Acetolactate synthase (ALS) is the first enzyme in the BCAA synthesis pathway. Although the functional contribution of ALS to BCAA biosynthesis has been extensively characterized, a comprehensive understanding of the regulation of this pathway at the molecular level is still lacking.
  
  
  
  To characterize the regulatory processes governing ALS activity we utilized several complementary approaches. Using the ALS catalytic protein subunit as bait we performed a yeast two-hybrid (Y2H) screen which resulted in the identification of a set of interacting proteins, two of which (denoted as ALS-INTERACTING PROTEIN1 and 3 [AIP1 and AIP3, respectively]) were found to be evolutionarily conserved orthologues of bacterial feedback-regulatory proteins and therefore implicated in the regulation of ALS activity. To investigate the molecular role AIPs might play in BCAA synthesis in Arabidopsis thaliana, we examined the functional contribution of aip1 and aip3 knockout alleles to plant patterning and development and BCAA synthesis under various growth conditions. Loss-of-function genetic backgrounds involving these two genes exhibited differential aberrant growth responses in valine-, isoleucine-, and sodium chloride-supplemented media. While BCAA synthesis is believed to be localized to the chloroplast, both AIP1 and AIP3 were found to localize to the peroxisome in addition to the chloroplast. Analysis of free amino acid pools in the mutant backgrounds revealed that they differ in the absolute amount of individual BCAAs accumulated and exhibit elevated levels of BCAAs in leaf tissues. Despite the phenotypic differences observed in aip1 and aip3 backgrounds, functional redundancy between these loci was suggested by the finding that aip1/aip3 double knockout mutants are severely developmentally compromised.
  
  
  
  Taken together the data suggests that the two regulatory proteins, in conjunction with ALS, have overlapping but distinct functions in BCAA synthesis, and also play a role in pathways unrelated to BCAA synthesis such as sodium-ion homeostasis, extending to broader aspects of patterning and development.
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada.|Present address: Department of Genetics, Harvard Medical School, Boston, MA, 02115, USA.
authors.forename: Mohammad H
authors.initials: MH
authors.lastname: Dezfulian
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada.
authors.forename: Curtis
authors.initials: C
authors.lastname: Foreman
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada.
authors.forename: Espanta
authors.initials: E
authors.lastname: Jalili
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada.
authors.forename: Mrinal
authors.initials: M
authors.lastname: Pal
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada.
authors.forename: Rajdeep K
authors.initials: RK
authors.lastname: Dhaliwal
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada.
authors.forename: Don Karl A
authors.initials: DK
authors.lastname: Roberto
authors.affiliation: Department of Biochemistry and Molecular Biology, Michigan State University, Lansing, MI, USA.
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Imre
authors.affiliation: Department of Biology, University of Western Ontario, London, ON, Canada.
authors.forename: Susanne E
authors.initials: SE
authors.lastname: Kohalmi
authors.affiliation: Department of Biological Sciences, University of Windsor, Windsor, ON, Canada. bcrosby@uwindsor.ca.
authors.forename: William L
authors.initials: WL
authors.lastname: Crosby
mesh_terms: D000095:Acetolactate Synthase / Q000235:genetics / Q000378:metabolism*; D000597:Amino Acids, Branched-Chain / Q000096:biosynthesis* / Q000378:metabolism; D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D007532:Isoleucine / Q000096:biosynthesis / Q000378:metabolism; D007930:Leucine / Q000096:biosynthesis / Q000378:metabolism
chemical_list: D000597:Amino Acids, Branched-Chain; D029681:Arabidopsis Proteins; D007532:Isoleucine; D000095:Acetolactate Synthase; D007930:Leucine
keywords: AHAS; ALS; Acetolactate synthase; Branched-chain amino acids; Isoleucine; Leucine; Valine