Bilin-dependent regulation of chlorophyll biosynthesis by GUN4

Abstract

Biosyntheses of chlorophyll and heme in oxygenic phototrophs share a common trunk pathway that diverges with insertion of magnesium or iron into the last common intermediate, protoporphyrin IX. Since both tetrapyrroles are pro-oxidants, it is essential that their metabolism is tightly regulated. Here, we establish that heme-derived linear tetrapyrroles (bilins) function to stimulate the enzymatic activity of magnesium chelatase (MgCh) via their interaction with GENOMES UNCOUPLED 4 (GUN4) in the model green alga Chlamydomonas reinhardtii. A key tetrapyrrole-binding component of MgCh found in all oxygenic photosynthetic species, CrGUN4, also stabilizes the bilin-dependent accumulation of protoporphyrin IX-binding CrCHLH1 subunit of MgCh in light-grown C. reinhardtii cells by preventing its photooxidative inactivation. Exogenous application of biliverdin IXα reverses the loss of CrCHLH1 in the bilin-deficient heme oxygenase (hmox1) mutant, but not in the gun4 mutant. We propose that these dual regulatory roles of GUN4:bilin complexes are responsible for the retention of bilin biosynthesis in all photosynthetic eukaryotes, which sustains chlorophyll biosynthesis in an illuminated oxic environment.

Transcriptional regulation of photoprotection in dark-to-light transition - more than just a matter of excess light energy

“…Furthermore, CRYs are also required for phytochrome signaling functions associated with the circadian clock in plants (Devlin & Kay, 2000). Although currently there are no identified phytochromes in Chlamydomonas, there are red light stimulated processes in this alga and it has the capacity to synthesize bilins (linear tetrapyrroles that often absorb red light), which may function as signaling molecules and control the biosynthesis of chlorophyll (Duanmu et al, 2013;Zhang et al, 2021). The Chlamydomonas genome also encodes proteins with potential bilin binding domains, like those of phytochromes.…”

Impact of Porphyrin Binding to GENOMES UNCOUPLED 4 on Tetrapyrrole Biosynthesis in planta

“…Previously, it was hypothesized that GUN4 might function as a buffer for excessive porphyrins, and efficient shielding of porphyrins from the interaction with molecular oxygen could diminish the phototoxic effects arising from unbound porphyrin accumulation ( Peter and Grimm, 2009 ; Adhikari et al, 2011 ; Li et al, 2021 ). This hypothesis is further supported by GUN4’s ability to bind various TPS pathway intermediates ( Adhikari et al, 2009 ; Hu et al, 2021 ; Zhang et al, 2021 ). The modification or degradation of the GUN4:porphyrin complex was proposed as an efficient way to catabolize a surplus of TPS intermediates.…”

“…Because the GUN4-1 mutant protein with increased affinity for porphyrins ( Davison et al, 2005 ) was similarly degraded in HL treated gun4-1 mutants ( Adhikari et al, 2011 ), GUN4 degradation upon HL shift is most likely not connected to the formation of GUN4:porphyrin complexes. More recently, a stabilizing effect of GUN4 on the MgCh H1 subunit in the presence of PIX, biliverdin (BV) and other linear tetrapyrroles was reported in Chlamydomonas reinhardtii ( Hu et al, 2021 ; Zhang et al, 2021 ). Also, for Synechocystis , a connection between porphyrin binding to GUN4 and accumulation of CHLH after recovery from nitrogen depletion has been reported ( Kopecna et al, 2015 ).…”

“…Biochemical and structural analysis revealed that GUN4 from cyanobacteria, green algae and plants binds the substrate and product of the MgCh reaction but also other TPS intermediates ( Davison et al, 2005 ; Verdecia et al, 2005 ; Adhikari et al, 2009 ; Chen et al, 2015b ; Tarahi Tabrizi et al, 2015 ; Hu et al, 2021 ; Zhang et al, 2021 ). Tetrapyrrole binding is stabilized by hydrophobic and electrostatic interactions between the substrate(s) and conserved amino acids in helix α2 and a flexible loop between helices α6 and α7 of GUN4 (see also Figure 1 ).…”

“…Porphyrin-dependent association of pea and Arabidopsis GUN4 to thylakoid membranes and the MgCh H subunit was shown ( Adhikari et al, 2009 , 2011 ), but this seems to be not the case for Synechocystis GUN4 ( Kopecna et al, 2015 ). Despite its impact on MgCh, the binding of porphyrins to GUN4 influences the abundance of GUN4 in high light-treated Arabidopsis plants ( Adhikari et al, 2011 ), the stability of MgCH H1 subunit ( Zhang et al, 2021 ) and 1 O 2 formation and retrograde signaling in Chlamydomonas ( Brzezowski et al, 2014 ; Tarahi Tabrizi et al, 2016 ).…”