3. Connecting thiol regulation across the thylakoid membrane
Recent work has shown that the stromal redox pool communicates with redox-regulated proteins in the lumen, and there is strong evidence that several lumenal enzymes or proteins are modulated by thiol redox state (Ströher & Dietz 2008; Hall et al. 2010; Kieselbach 2013; Yu, Lu, Du, Peng & Wang 2014; Simionato et al. 2015; Hallin, Guo & Åkerlund 2015; Wang et al. 2017a; Wu et al. 2021). The special properties of the chloroplast compartments impose distinct requirements for the stromal and lumenal compartments. As with the stromal system, three functionalities are required to operate an effective redox regulatory network: 1) controlled injection of reducing equivalents (electrons, hydrogen); 2) balanced removal of electrons; and 3) adjustment of redox potentials and kinetics to match the specific regulatory requirements.
Upon illumination, the electron and proton transport reaction deposits protons into the lumen, generating a proton motive force (pmf ), which is composed of two components, the electric field (Δψ) and the proton gradient (∆pH). Both pmf components drive the synthesis of ATP at the chloroplast ATP synthase (Kramer et al. 2003). The initial form of pmf is exclusively stored as the electric field component due to the lower capacity of electric capacitance and higher buffering capacity of ∆pH in the lumen (Kanazawa & Kramer 2002; Cruz et al. 2005; Takizawa, Kanazawa & Kramer 2008). As counterions move into the lumen, the proton gradient builds up, resulting in acidification of the lumen to values as low as about 5.5 (Kramer et al. 1999; Takizawa et al. 2007). By contrast, stromal pH tends to increase in the light to about 7.5-8.0 (Werdan and Heldt 1972; Heldt et al. 1973; Werdan et al. 1975; Reardon-Robinson 1985; Wu and Berkowitz 1992). In addition, lumen acidification is known to have crucial photoprotective regulatory roles, such as inducing PSII photoprotection mechanisms (section 4) and “photosynthetic control” of electron flow by slowing down the electron transfer rate at the Cytb6f complex, thus preventing PSI photodamage.
However, each of these functions is strongly impacted by the specific properties of the lumen and its role in photosynthetic energy transduction, requiring the lumenal system to be distinct in several ways, as described in the following.
The need for transthylakoid redox exchange . To fulfill its role in chemiosmotic energy transduction, the thylakoid lumen must be an electrochemically sealed compartment, to prevent the leakage of energy stored in the pmf . There must then be machinery to allow for transmembrane thiol-disulfide exchange. While such systems have been extensively studied in bacteria, how this occurs in chloroplasts is only beginning to emerge. For example, in the well-characterized oxidative protein folding disulfide bond isomerization pathway in Gram-negative bacteria, the Dsb family (Ito & Inaba 2008; Reardon-Robinson & Ton-That 2015) protein has been imported into the periplasm by the Sec transport apparatus, is facilitated by protein oxidation by DsbA (Grauschopfet al. 1995). Subsequently, the reduced DsbA is oxidized by DsbB in the inner membrane (IM), which transfers reducing equivalents to quinones (Bader, Muse, Ballou, Gassner & Bardwell 1999). However, when a newly imported protein is subjected to oxidation, this can lead to misfolding and inactivation. To prevent or repair these situations, Trx in the cytoplasm transfers reducing power to DsbD in the inner membrane which then transfers this reducing power to DsbC, which maintains proteins in reduced (active or foldable) states (Krupp, Chan & Missiakas 2001; Herrmann, Kauff & Neuhaus 2009) (Fig 2A).
An analogous system likely operates in thylakoid membranes, but may also function in reversible thiol regulation of enzymes. The protein components needed to establish a functional thiol-disulfide exchange system across the thylakoid membrane, while not complete, are slowly being identified, as discussed in the following sections. Unlike the thiol-disulfide redox regulation in the stroma, stromal soluble electron carriers, such as Trx or Trx-like, proteins have not yet been identified in the lumen (Buchanan 2016b). However, several thiol-modulated enzymes or proteins have been identified (Ströher & Dietz 2008; Hall et al. 2010; Kieselbach 2013; Yu, Lu, Du, Peng & Wang 2014; Simionato et al. 2015; Hallin, Guo & Åkerlund 2015; Wang et al. 2017a; Wu et al. 2021) and multiple H-carriers (redox transporters) have been reported to transfer reducing equivalents from the stroma across the thylakoid membrane to the thylakoid lumen (Motohashi & Hisabori 2006, 2010; Brooks, Jansson & Niyogi 2014; Motohashi & Hisabori 2006, 2010; Karamoko, Cline, Redding, Ruiz & Hamel 2011; Brooks et al. 2014).