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).