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.