C-terminal processing of reaction center protein D1 is essential for the function and assembly of photosystem II inArabidopsis

Abstract

Photosystem II (PSII) reaction center protein D1 is synthesized as a precursor (pD1) with a short C-terminal extension. The pD1 is processed to mature D1 by carboxyl-terminal peptidase A to remove the C-terminal extension and form active protein. Here we report functional characterization of the Arabidopsis gene encoding D1 C-terminal processing enzyme (AtCtpA) in the chloroplast thylakoid lumen. Recombinant AtCtpA converted pD1 to mature D1 and a mutant lacking AtCtpA retained all D1 in precursor form, confirming that AtCtpA is solely responsible for processing. As with cyanobacterial ctpa, a knockout Arabidopsis atctpa mutant was lethal under normal growth conditions but was viable with sucrose under low-light conditions. Viable plants, however, showed deficiencies in PSII and thylakoid stacking. Surprisingly, unlike its cyanobacterial counterpart, the Arabidopsis mutant retained both monomer and dimer forms of the PSII complexes that, although nonfunctional, contained both the core and extrinsic subunits. This mutant was also essentially devoid of PSII supercomplexes, providing an unexpected link between D1 maturation and supercomplex assembly. A knock-down mutant expressing about 2% wild-type level of AtCtpA showed normal growth under low light but was stunted and accumulated pD1 under high light, indicative of delayed C-terminal processing. Although demonstrating the functional significance of C-terminal D1 processing in PSII biogenesis, our study reveals an unsuspected link between D1 maturation and PSII supercomplex assembly in land plants, opening an avenue for exploring the mechanism for the association of light-harvesting complexes with the PSII core complexes.photosynthesis | photoinhibition P hotosystem II (PSII) consists of more than 20 subunits. Assembly of this photosystem is a multistep process that functions in a highly coordinated fashion (1-3). The process starts with PSII initiation complexes (D2, PsbE, PsbF, and PsbI), and then D1 and CP47 are sequentially recruited to form CP47-RC complexes, followed by addition of PsbH, PsbM, PsbTc, and PsbR subunits. Next, CP43 and other subunits are added to generate PSII monomers, which develop into PSII dimers. Finally, lightharvesting complex (LHC) II is attached to form PSII supercomplexes. The D1 protein of PSII is prone to photodamage under excessive light conditions (4). To sustain photosynthesis, damaged D1 protein is degraded and replaced with a newly synthesized copy via PSII repair-a highly complex and critical process whose mechanism remains unclear (3, 4).In most oxygen-evolving photosynthetic organisms, D1 protein is synthesized as a precursor (pD1) with a C-terminal tail. The pD1 protein is integrated into the thylakoid membrane and forms the initial PSII reaction center combined with other PSII

A Novel Protective Function for Cytokinin in the Light Stress Response Is Mediated by the ARABIDOPSIS HISTIDINE KINASE2 and ARABIDOPSIS HISTIDINE KINASE3 Receptors

“…However, the corresponding loss-of-function mutant (Yin et al, 2008) showed a wild-type-like HL response under our conditions. This indicates that At3g57680 is not indispensable under HL stress, which is consistent with the report of Che et al (2013). Together, the transcript data suggest that cytokinin does not act mainly through transcriptional regulation of the analyzed genes but may act on a different level to influence the efficiency of the D1 repair cycle.…”

“…At the end of the D1 repair cycle, newly synthesized preD1 needs to be processed to mature D1 by CTP activity (Anbudurai et al, 1994;Roose and Pakrasi, 2004). Recently, mutant analysis has identified one of the three predicted Arabidopsis CTP homologs (At4g17740) to be required for an efficient repair of D1 under HL (Che et al, 2013). Our experiments showed that HL caused no major differences in the expression of genes encoding FTSH and DEG proteases between wild-type and cytokinin-deficient plants (Fig.…”

Chloroplast Proteases: Updates on Proteolysis within and across Suborganellar Compartments

“…C-terminal processing protease, a monomeric Sertype protease that cleaves off a C-terminal extension for maturation of the D1 precursor protein, is another type of processing peptidase in the thylakoid lumen (Anbudurai et al, 1994;Fujita et al, 1995;Oelmüller et al, 1996;Satoh and Yamamoto, 2007;Che et al, 2013). Its basic structure contains a PDZ domain for D1 C-terminal binding and a protease domain with an SK dyad for proteolysis (Liao et al, 2000).…”

MET1 Is a Thylakoid-Associated TPR Protein Involved in Photosystem II Supercomplex Formation and Repair in Arabidopsis

“…More than a dozen higher plant PSII-specific biogenesis/repair factors have been reported, including HCF136 (Meurer et al, 1998;Covshoff et al, 2008), LPA1 (Peng et al, 2006), FKBP-2 (Lima et al, 2006), CYP38 (Fu et al, 2007;Sirpiö et al, 2008), TLP18.3 (Sirpiö et al, 2007), LPA2 (Ma et al, 2007), LPA3 (Cai et al, 2010), PAM68 (Armbruster et al, 2010), HCF243 (Zhang et al, 2011), LTO1 (Karamoko et al, 2011), TERC (Schneider et al, 2014), LQY1 (Lu et al, 2011), HHL1 (Jin et al, 2014), and psbN (Torabi et al, 2014). Additionally, the lumenal peptidase CtpA is specifically required for C-terminal processing of the D1 protein (Anbudurai et al, 1994;Oelmüller et al, 1996;Yamamoto et al, 2001); in the absence of this C-terminal processing, no active PSII complex can be formed (Che et al, 2013). Thylakoid bound FtsH and Deg proteases play an important role in degrading damaged D1 (Kapri-Pardes et al, 2007;Sun et al, 2010aSun et al, , 2010bChi et al, 2012b;Kato et al, 2012), even if these proteases are not specific to PSII.…”