Under positive regulator PYE (Long et al., 2010;

Under Fe deprivation, PYE is co-expressed with BRUTUS (BTS), a protein
with RING E3 ligase -function that contains a haemerythrin / HHE cation-binding
motif for metal binding such as Fe and Zn (Long et al., 2010; Kobayashi et al., 2013; Selote et
al., 2015). bts-1 knock-down mutants have longer
roots, increased acidification capacity and contain more Fe. Hence, they are
more robust to Fe deficiency which suggests an opposite function of BTS
compared to the positive regulator PYE (Long et al., 2010; Zhang et al., 2015). Likewise, the two redundant
Arabidopsis BTS paralogs, BTS LIKE1 (BTSL1) and BTS LIKE2 (BTSL2), as well as
the rice homologs OzHRZ1 and OzHRZ2, negatively affect the Fe deficiency
response (Kobayashi et al., 2013; Hindt et al., 2017). Besides its
expression in the vasculature, BTS is broadly expressed in other tissues, which
matches its additional function in developing processes (Selote et al., 2015). Interestingly, PYELs, but not PYE, interact
with BTS (Long et al., 2010; Selote et al., 2015; Liang et al.,
2017). The interaction
inhibits the activity of PYELs and it is suggested that this is conferred by
their ubiquitination and subsequent degradation (Selote et al., 2015). The interaction and degradation is likely mediated
via the BTS RING E3 domain, since BTS?E3 deletion mutants cannot
form heterodimers (Selote et al., 2015). The RING E3 domain is additionally important
for BTS activity (Matthiadis and Long, 2016). The HHE domain renders BTS less stable as seen
by the accumulation of BTS-GFP in BTSDHHE mutants (Selote et al., 2015; Matthiadis and Long, 2016). In vitro expressed BTS was shown to be
degraded in a dose-dependent manner following an increasing concentration of
Fe. Similar results were obtained upon transient expression of BTS-EGFP in
tobacco leaves, in which excess Fe promoted the degradation of the fusion
protein. Chelation of Fe3+, however, arrested the degradation (Selote et al., 2015). This suggests, that BTS is less stable upon Fe
binding, which is controversially discussed in the literature (Selote et al., 2015; Hindt et al., 2017). It is not very beneficial
for the plant to degrade a negative regulator of Fe deficiency responses upon the
presence of Fe, leading to enhanced expression of subgroup Ib transcription
factors and hence even more Fe accumulation. The degradation of BTS was, however,
solely shown in an in vitro
situation, where problems during wheat germ-based protein translation cannot be
ruled out, particularly since MG132 treatment could not prevent BTS degradation.
On the other hand, BTS might display a modulator to prevent Fe
over-accumulation in a nutrient deprived situation. This could be conferred by
constantly degrading PYELs in order to finely adjust initiation of subgroup Ib
bHLH translation. This in turn depicts also an effective mechanism to quickly
respond to changing Fe conditions (Selote et al., 2015; Hindt et al., 2017). A similar model was
suggested for FIT regulation (Lingam et al., 2011; Meiser et al., 2011; Sivitz et
al., 2011). Hence, Hindt et al.,
2017 suggest a model, where BTS is stabilized upon metal binding as seen for
human Fe sensor FBXL5 (Salahudeen et al., 2009) and rice OsHRZ (Kobayashi et al., 2013) and therefore blocking subgroup Ib-mediated
upregulation of Fe deficiency genes