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.