Iron-binding haemerythrin RING ubiquitin ligases regulate plant iron responses and accumulation

Abstract

Iron is essential for most living organisms. Plants transcriptionally induce genes involved in iron acquisition under conditions of low iron availability, but the nature of the deficiency signal and its sensors are unknown. Here we report the identification of new iron regulators in rice, designated Oryza sativa Haemerythrin motif-containing Really Interesting New Gene (RING)- and Zinc-finger protein 1 (OsHRZ1) and OsHRZ2. OsHRZ1, OsHRZ2 and their Arabidopsis homologue BRUTUS bind iron and zinc, and possess ubiquitination activity. OsHRZ1 and OsHRZ2 are susceptible to degradation in roots irrespective of iron conditions. OsHRZ-knockdown plants exhibit substantial tolerance to iron deficiency, and accumulate more iron in their shoots and grains irrespective of soil iron conditions. The expression of iron deficiency-inducible genes involved in iron utilization is enhanced in OsHRZ-knockdown plants, mostly under iron-sufficient conditions. These results suggest that OsHRZ1 and OsHRZ2 are iron-binding sensors that negatively regulate iron acquisition under conditions of iron sufficiency.

Iron-Binding E3 Ligase Mediates Iron Response in Plants by Targeting Basic Helix-Loop-Helix Transcription Factors

“…6A). These results are consistent with the amount of iron associated with BTS rice ortholog OsHRZ1 and the OsHRZ1DH constructs described by Kobayashi et al (2013). Residual iron present in BTS DHHE protein samples might be due to Figure 6.…”

“…Notably, both BTS and the BTS rice orthologs OsHRZ1 and OsHRZ2 bind iron and zinc (Kobayashi et al, 2013), and deletion of the entire first two-thirds of the OsHRZ1 protein region containing the HHE domains decreases iron as well as zinc binding of the truncated OsHRZ1 and OsHRZ2 protein (Kobayashi et al, 2013). We hypothesized that iron binding at the HHE domains could cause a conformational change in BTS structure and activity similar to that exhibited by FBXL5.…”

“…Previously, BTS and the BTS rice orthologs OsHRZ1 and OsHRZ2 were shown to exhibit some level of ubiquitination capacity (Kobayashi et al, 2013). The E3 ligase activity of BTS protein and its derivatives (Supplemental Fig.…”

“…The presence of a RING domain near the BTS C terminus suggests that it may have E3 ligase capability. BTS and the BTS orthologs in rice, Oryza sativa Hemerythrin motif-containing RING and Zinc-Finger Protein1 (OsHRZ1) and OsHRZ2, were shown to bind iron at the HHE domains and have ubiquitination capacity (Kobayashi et al, 2013). However, no study has examined whether the iron-responsive PYEL proteins that BTS interacts with are targeted by this E3 ligase activity.…”

“…BTS encodes a unique plant protein consisting of several conserved domains, including three hemerythrin (HHE) cation-binding domains located near the N terminus, a CHY zinc-finger domain, and a Really Interesting New Gene (RING) domain near the C terminus (Long et al, 2010;Kobayashi et al, 2013). The conserved HHE domains are left-twisted 4-a-helical bundles that provide a hydrophobic pocket in which O 2 binds diiron.…”