4.2 Redistribution of excitation energy, qT
In response to light quality and intensity, the balance of excitation
energy between the two photosystems is dynamically adjusted to avoid
photodamage. This is achieved through serine/threonine-protein kinase
(STN7)
(Bellafioreet al. 2005). STN7 is a transmembrane protein having its
catalytic domain extending into the stroma. STN7 also has two conserved
cysteines located on the lumen side of the thylakoid
(Lemeilleet al. 2009; Bergner et al. 2015).
The catalytic domain of STN7 is responsible for the phosphorylation of
light-harvesting complex II (LHCII)
(Puthiyaveetil
2011). Phosphorylation of LHCII changes its structure, which causes it
to detach from PSII and migrate to PSI. This process is important for
balancing the amount of light energy that is captured by each
photosystem under dynamic environmental changes
(Bellafiore
et al. 2005).
The proposed model for STN7 functions is that plastoquinol
(PQH2) binds to the Qo site of
Cytb6f and activates STN7 by forming
intermolecular disulfide bridges between the conserved cysteines in the
lumenal domain of STN7. However, the dimer formed by these disulfide
bridges is very transitory and can be easily converted back to a monomer
(Lemeilleet al. 2009; Wunder et al. 2013; Bergner et al.2015; Shapiguzov et al. 2016).
Wuet al. (2021) showed that the lumenal cysteines of STN7 are
maintained in the oxidized state by the Trx-like domain of LTO1, which
then transfers reducing power to the VKOR domain
(Wuet al. 2021). Under conditions where the PQ pool is oxidized,
e.g. when PSI is preferentially excited by light, STN7 becomes
inactivated, LTO1 no longer oxidizes STN7, and the antenna adopt the
state 1 configuration (Fig. 5). However, it is noteworthy that state
transitions are strongly decreased in the lto1 mutant, but they
are not completely abolished, unlike in the stn7 mutant
(Wuet al. 2021). This result suggests that other factors, in
addition to LTO1, are likely involved in keeping STN7 oxidized and
active in the acidic lumenal environments. In addition, it has shown
that STN7 is inactivated by Trx-m, in coordination with HCF164 and CcdA
(AncĂnet al. 2019) (Fig. 5). In this scenario, Trx-m breaks the
intermolecular disulfide bridges in STN7, leading to the formation of an
intramolecular disulfide bond, which inactivates STN7 by monomerizing
it. So once again, the function of STN7 is heavily impacted by its redox
regulation However, the exact oxidative mechanism by which STN7
functions in the acidified lumen, where the redox midpoint potentials of
the regulatory thiols increase, still needs further investigation.