The GluTR-binding protein is the heme-binding factor for feedback control of glutamyl-tRNA reductase.

Andreas S Richter,Claudia Banse,Bernhard Grimm

doi:10.7554/elife.46300

Andreas S Richter, Claudia Banse + Show 1 more

Open Access

https://doi.org/10.7554/elife.46300

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Journal: eLife	Publication Date: Jun 13, 2019
Citations: 42	License type: CC BY 4.0

Affiliation: Humboldt-Universität zu Berlin

Abstract

Synthesis of 5-aminolevulinic acid (ALA) is the rate-limiting step in tetrapyrrole biosynthesis in land plants. In photosynthetic eukaryotes and many bacteria, glutamyl-tRNA reductase (GluTR) is the most tightly controlled enzyme upstream of ALA. Higher plants possess two GluTR isoforms: GluTR1 is predominantly expressed in green tissue, and GluTR2 is constitutively expressed in all organs. Although proposed long time ago, the molecular mechanism of heme-dependent inhibition of GluTR in planta has remained elusive. Here, we report that accumulation of heme, induced by feeding with ALA, stimulates Clp-protease-dependent degradation of Arabidopsis GluTR1. We demonstrate that binding of heme to the GluTR-binding protein (GBP) inhibits interaction of GBP with the N-terminal regulatory domain of GluTR1, thus making it accessible to the Clp protease. The results presented uncover a functional link between heme content and the post-translational control of GluTR stability, which helps to ensure adequate availability of chlorophyll and heme.

Highlights

Tetrapyrrole biosynthesis (TBS) is one of the vital biochemical pathways found in organisms
We found that a group of proteins including GluTR2, GSA aminotransferase (GSAT), MgCh subunit I (CHLI), GENOMES UNCOUPLED 4 (GUN4), the CHL27 subunit of the MgPMME oxidative cyclase (CHL27, Pontier et al, 2007), the product of hypothetical open reading frame 54 [YCF54/LCAA, a subunit of Mg-protoporphyrin IX monomethylester (MgPMME) oxidative cyclase], ferrochelatase (FC) 2, caseinolytic protease (Clp) protease C subunit, and subunits of the photosynthetic protein complexes, such as LHCA1/B1, PetB and PsaL, showed little or no variation in the course of a 24 hr dark incubation (Figure 1A)
In comparison to the light-grown seedlings at t(0), GluTR1 amounts were slightly reduced after 4 hr and reduced by 40% after 24 hr of darkness without aminolevulinic acid (ALA) supply

Summary

Introduction

Tetrapyrrole biosynthesis (TBS) is one of the vital biochemical pathways found in organisms. Besides providing heme and its breakdown products, which are essential constituents of almost all organisms, the pathway supplies the chlorophyll (Chl) crucial for photosynthesis in cyanobacteria, algae, and plants (Tanaka and Tanaka, 2007). ALA is the universal precursor of all tetrapyrroles, and its synthesis is considered to be rate-limiting for TBS as a whole. Eight molecules of ALA are converted into the macrocyclic protoporphyrin IX (ProtoIX) in subsequent reactions. Thereafter, ProtoIX is directed into either the iron or magnesium branch for the synthesis of heme or Chl, respectively (Tanaka and Tanaka, 2007). Synthesis of Chl is initiated by the insertion of Mg2+ into ProtoIX, catalyzed by the Mg-chelatase (MgCh) complex. Chlide is converted to Chl a and b by enzymes of the Chl cycle (Tanaka and Tanaka, 2007)

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