An Iron-induced Nitric Oxide Burst Precedes Ubiquitin-dependent Protein Degradation for Arabidopsis AtFer1 Ferritin Gene Expression

Abstract

Ferritins play an essential role in iron homeostasis by sequestering iron in a bioavailable and non-toxic form. In plants, ferritin mRNAs are highly and quickly accumulated in response to iron overload. Such accumulation leads to a subsequent ferritin protein synthesis and iron storage, thus avoiding oxidative stress to take place. By combining pharmacological and imaging approaches in an Arabidopsis cell culture system, we have identified several elements in the signal transduction pathway leading to the increase of AtFer1 transcript level after iron treatment. Nitric oxide quickly accumulates in the plastids after iron treatment. This compound acts downstream of iron and upstream of a PP2A-type phosphatase to promote an increase of AtFer1 mRNA level. The AtFer1 gene transcription has been previously shown to be repressed under low iron conditions with the involvement of the cis-acting element iron-dependent regulatory sequence identified within the AtFer1 promoter sequence. We show here that the repressor is unlikely a transcription factor directly bound to the iron-dependent regulatory sequence; such a repressor is ubiquitinated upon iron treatment and subsequently degraded through a 26 S proteasome-dependent pathway.As the major cofactor of proteins involved in essential processes like photosynthesis, respiration, DNA replication, or nitrogen fixation, iron is an essential element for life. Nonetheless, in the free ionic form, iron is toxic as it can catalyze the formation of reactive oxygen species through the Fenton reaction. These reactive oxygen species damage the cell membranes, DNA, and proteins (1, 2). Thus, iron homeostasis has to be tightly regulated, to avoid starvation that impairs the metabolism, and to avoid excess that may lead to cell death. Iron homeostasis is strongly dependent on ferritins, which are ironstorage proteins, found in bacteria, animals, and plants. Plant and animal ferritin structures are very similar, and are formed by 24 subunits arranged to form a hollow sphere able to sequester iron in a non-toxic and bioavailable form (3).In animals, ferritin synthesis is mainly regulated at the posttranscriptional level (3, 4). Ferritin mRNAs contain iron-responsive elements in their 5Ј-untranslated regions that function as binding sites for two related trans-acting factors, namely iron regulatory proteins IRP1 and IRP2. When bound to the iron-responsive element in the ferritin mRNA, the IRP inhibit translation of the transcript (4). IRP1 is a bifunctional protein that when iron is abundant possesses a 4Fe-4S cluster and acts as cytoplasmic aconitase. When iron levels are low, the 4Fe-4S cluster disassembles and the apoprotein acquires IRP 3 activity, thus repressing ferritin translation. High levels of iron lead to the 4Fe-4S cluster reconstitution and therefore the protein aconitase activity. In contrast to IRP1, IRP2 cannot assemble a iron-sulfur cluster and lacks aconitase activity. IRP2 shares about 60% amino acid sequence identity with IRP1, but differs only in having a 73-amino aci...

Ferritin is required for rapid remodeling of the photosynthetic apparatus and minimizes photo‐oxidative stress in response to iron availability in Chlamydomonas reinhardtii

“…In line with this, ferritin mRNAs are also strongly accumulated in response to iron overload in vascular plants (Arnaud et al, 2006;Lescure et al, 1991;Petit et al, 2001b).…”

Hunting for Plant Nitric Oxide Synthase Provides New Evidence of a Central Role for Plastids in Nitric Oxide Metabolism

“…Treatment with a specific NOS inhibitor reduced NO levels in elicited control plants but not in silenced lines, confirming an involvement of NOS1 in the observed NO burst (Asai et al, 2008). Other reports, however, found that NO accumulation in response to different hormones or oxidative stress was similar in nos1 and wild-type plants (Arnaud et al, 2006;Kolbert et al, 2008;Tun et al, 2008). Impaired NO generation in mutant plants in response to some treatments but not in response to others has been observed even under similar experimental conditions.…”

“…Nonenzymatic production of NO from nitrite involving the plastidial pigments carotenoids has also been reported (Cooney et al, 1994). Interestingly, NO synthesis in response to iron, elicitors, high temperatures, salinity, or osmotic stress is first detected in chloroplasts using NO-sensitive diaminofluorescein probes (Foissner et al, 2000;Gould et al, 2003;Arnaud et al, 2006). Although many pieces of the puzzle are still missing, and the search for the elusive plant NOS is not yet over, these results corroborate the hypothesis that plastids are key players for the control of NO levels in plant cells.…”

NMR reveals pathway for ferric mineral precursors to the central cavity of ferritin

“…A large cavity that occupies about 30% of the total protein volume at the center of the cage is the mineralization site of ferritin ferrihydrite. The importance of ferritin is illustrated by embryonic lethality of gene deletion in mammals (1) and resistance to oxidants in animals, plants, bacteria, and archea that reflects consumption of iron(II) and dioxygen or hydrogen peroxide (2)(3)(4)(5). Some classes of nanomaterials use ferritin nanocages as templates (6).…”