4.1 Energy-dependent quenching, qE
Energy-dependent quenching (qE) is the most rapidly-responding form of
NPQ. It is triggered by the acidification of the lumen (proton gradient
(∆pH) component of proton motive force (pmf ) ) which results in
the protonation of PsbS as well as the activation of violaxanthin
de-epoxidase (VDE), which catalyzes the conversion of violaxanthin (Vx)
to zeaxanthin (Zx). Zx subsequently dissipates excessive light energy
and induces qE along with PsbS
(Niyogiet al. 1998).
Recent studies have shown that both the proton gradient (∆pH) and also
the redox status of the thiol-disulfides within in VDE play a role in
controlling its function
(Arnoux,
Morosinotto, Saga, Bassi & Pignol 2009; Yu et al. 2014; Hallinet al. 2015;
Arnouxet al. 2009; Simionato et al. 2015). Specifically, the
proton gradient (∆pH) facilitates a structural change in VDE causing it
to change from a monomer to a homo-dimer complex upon lumen
acidification
(Arnouxet al. 2009). In addition, to VDE undergoing a structural
conformational change, VDE activity is also regulated in a
thiol-disulfide-dependent manner
(Hallet al. 2010; Kieselbach 2013; Yu et al. 2014; Simionatoet al. 2015; Hallin et al. 2015). This elaborate
regulation is achieved due to the unique structure of VDE. The VDE
protein consists of three domains: a Cys-rich N-terminal domain, a
lipocalin-like domain (predicted to bind violaxanthin)
(Sagaet al. 2010) and a glutamate-rich C-terminal domain. VDE has a
total of 13 conserved Cys residues, 12 of which are at the N-terminal
and these Cys residues play a major role in protein function.
Consequently, VDE is active only when fully oxidized, with six disulfide
bonds, is VDE active
(Simionatoet al. 2015), and this corresponds to a more compact, rigid,
thermostable form of the protein
(Hallinet al. 2015).
It has been proposed that the electron donor for VDE is Trx-m through
CcdA and HCF164
(Motohashi
& Hisabori 2006, 2010) and the oxidase is
LTO1(Luet al. 2013, 2015; Yu et al. 2014; Wu et al. 2021)
(Fig. 4). Coincidently, an lto1-2 mutant line showed a lower
de-epoxidation state of xanthophyll cycle pigment de-epoxidase index
(DEI), suggesting that de-epoxidation of Vx to Zx was suppressed in thelto1-2 mutant line(Yu
et al. 2014; Lu et al. 2015).
Another component that can affect the kinetics of qE is
K+ efflux antiporter3 (KEA3), a potassium/proton
antiporter. KEA3 exchanges protons (out of the lumen) and potassium ions
(into the lumen)
(Armbrusteret al. 2014). Consequently, when light transitions from high to
low, the KEA3 antiporter function, helps relax qE. Furthermore, it has
been recently reported that KEA3 has five Cys residues, but only one Cys
residue is located at the N-terminal which extends into the lumen
(Wanget al. 2017a). The location of this Cys residue suggests that
KEA3 may be regulated by a redox mechanism, acting as adimer. However,
the possible electron donors and oxidases involved in regulating KEA3
need further investigation.