3.1 HCF164, CcdA and SOQ1 transfer reducing equivalent from
stroma to lumen
HCF164 was first identified by
Meurer,
Meierhoff & Westhoff as a gene that when mutated in plants resulted in
high chlorophyll fluorescence phenotype
(Meureret al. 1996; Meurer, Plücken, Kowallik & Westhoff 1998). This
phenotype was later found to be caused by a defect in the assembly of
cytochrome b6f(Lennartzet al. 2001). HCF164 is anchored in the thylakoid membrane via a
single transmembrane domain (TMD) with the bulk of HCF164 orientated
towards the thylakoid lumen (See Fig. 2B). HCF164 possesses a Trx-like
domain, localized to the thylakoid lumen with disulfide reductase
activity
(Motohashi
& Hisabori 2006, 2010). Further, HCF164 can interact with potential
target proteins such as PSI-N, Cytf, and Rieske FeS (a subunit of the
Cyt b6f ) through Trx affinity chromatography
experiments
(Motohashi
& Hisabori 2006, 2010; Brooks et al. 2014). Intriguingly, it
has also shown that Trx-m type is an electron donor for HCF164
(Motohashi
& Hisabori 2006, 2010; Brooks et al. 2014). However, because
HCF164 has its Cys residues on the lumenal side, and not on the
transmembrane side, it was proposed that chloroplasts must possess a
system for transferring redox equivalents across the thylakoid membrane
from the stroma to the lumen.
Motohashi
& Hisabori found evidence that CcdA serves precisely this function: a
thylakoid membrane protein that acts to transfer reducing equivalents
from the stroma to the thylakoid lumen. CcdA is a homolog of the
prokaryotic thiol-disulfide transporter, and it was previously reported
to be required for the assembly of the Cytb6fcomplex
(Pageet al. 2004). Motohashi and Hisabori (2010) further showed that
CcdA’s redox state is modulated in the thylakoids by stromal m-type
thioredoxins.
The suppressor of quenching (SOQ1) was identified during the process of
a suppressor screening of non-photochemical quenching (NPQ)
(Brooks,
Sylak-Glassman, Fleming & Niyogi 2013). SOQ1 is anchored in the
thylakoid membrane via a single transmembrane domain (TMD) with the bulk
of SOQ1 localized within the thylakoid lumen (Fig. 2B). SOQ1 appears to
transfer reducing equivalents from the stroma to the plastid lipocalin
(LCNP), thereby suppressing the formation of a sustainable form of NPQ
called qH
(Malnoëet al. 2018) (see Section 4.3 for more details). The lumenal
thioredoxin-like domain and a β-propeller NCL-1, HT2A, and LIN-41 (NHL)
domain of SOQ1 have been shown to be required for the transfer of
reducing equivalents from the stroma to the lumen
(Slack
& Ruvkun 1998; Brooks et al. 2013). Finally, another domain of
SOQ1, the stromal-located halo-acid dehalogenase-like hydrolase (HAD)
domain with a transmembrane helix, has been shown to be not involved in
the suppression of qH
(Brookset al. 2013). Interestingly, recent work from Yu et al
(Yuet al. 2022) showed that the independent β-stranded C-terminal
domain (CTD), which has structural homology to the N-terminal domain of
DsbD, is essential for the regulation of qH, suggesting that it is
involved in transferring redox equivalents from the stroma side to the
lumen
(Yuet al. 2022). The electron donor to SOQ1 has not been specified
yet, but it is possible that it is either CcdA or another unknown
mediator.
Taken together, these examples suggest a mechanism for redox regulation
of lumen proteins as illustrated in Fig. 2B in which disulfide-thiol
redox control across the thylakoid membrane, in which reducing
equivalents are transferred from the stroma (e.g., from Trx-m) to CcdA
(or other, yet to be identified) carriers, and then to HCF164, which
then delivers them to target proteins.