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

Abstract

Iron uptake and metabolism are tightly regulated in both plants and animals. In Arabidopsis (Arabidopsis thaliana), BRUTUS (BTS), which contains three hemerythrin (HHE) domains and a Really Interesting New Gene (RING) domain, interacts with basic helix-loop-helix transcription factors that are capable of forming heterodimers with POPEYE (PYE), a positive regulator of the iron deficiency response. BTS has been shown to have E3 ligase capacity and to play a role in root growth, rhizosphere acidification, and iron reductase activity in response to iron deprivation. To further characterize the function of this protein, we examined the expression pattern of recombinant ProBTS::b-GLUCURONIDASE and found that it is expressed in developing embryos and other reproductive tissues, corresponding with its apparent role in reproductive growth and development. Our findings also indicate that the interactions between BTS and PYE-like (PYEL) basic helix-loop-helix transcription factors occur within the nucleus and are dependent on the presence of the RING domain. We provide evidence that BTS facilitates 26S proteasome-mediated degradation of PYEL proteins in the absence of iron. We also determined that, upon binding iron at the HHE domains, BTS is destabilized and that this destabilization relies on specific residues within the HHE domains. This study reveals an important and unique mechanism for plant iron homeostasis whereby an E3 ubiquitin ligase may posttranslationally control components of the transcriptional regulatory network involved in the iron deficiency response.

A Program for Iron Economy during Deficiency Targets Specific Fe Proteins

“…Four other BHLH transcription factors, BHLH38, BHLH39, BHLH100, and BHLH101, are Fe responsive (Wang et al, 2007). BHLH100 and BHLH101 act separately from PYE and from BHLH38, BHLH39, and FIT, which interact to mediate regulation of root Fe uptake (Yuan et al, 2008) but all are implied in the regulation of Fe uptake or distribution between tissues (Long et al, 2010, Sivitz et al, 2012, Selote et al, 2015. The bHLH family of transcription factors thus plays an important role in Fe homeostasis.…”

“…FIT and PYE are major regulators of root Fe uptake (for review, see Brumbarova et al, 2015). PYE associates with similar BHLH transcription factors (Selote et al, 2015). Four other BHLH transcription factors, BHLH38, BHLH39, BHLH100, and BHLH101, are Fe responsive (Wang et al, 2007).…”

The bHLH Transcription Factor bHLH104 Interacts with IAA-LEUCINE RESISTANT3 and Modulates Iron Homeostasis in Arabidopsis

“…Hence, BTS might control Fe contents in plants by regulating the abundance of bHLH104 and ILR3. Recently, a study by Selote et al (2015) demonstrated that BTS could target ILR3 and bHLH115 but not bHLH104 for the 26S proteasomemediated degradation, using a cell-free degradation assay. However, the mutation of bHLH104 could partially complement the phenotype of BTS lesion (Figure 9), which suggested that the loss of bHLH104 might disrupt the formation of heterodimers with ILR3 and bHLH115 that function to activate the Fe deficiency responses in bts-2 plants.…”

A WRKY Transcription Factor Regulates Fe Translocation under Fe Deficiency

“…The bHLH protein-dependent transcriptional regulation (FIT1-and PYE-dependent) plays a key role in the control of Fe uptake and homeostasis in plants (Colangelo and Guerinot, 2004;Long et al, 2010;Selote et al, 2015). Here, we show another kind of transcription factor that is from WRKY family participating in Fe deficiency response in Arabidopsis.…”

“…Furthermore, some other bHLH proteins, e.g. POPEYE (PYE) and its homologs, are capable of forming heterodimers and play an important role in the regulation of Fe homeostasis probably independent of FIT1-mediated signaling pathway (Long et al, 2010;Selote et al, 2015). This evidence reveals that the bHLH transcription factors have key roles in regulating Fe uptake and homeostasis in plants.…”