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

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.Photosystem 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, light-harvesting 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 subunits. The C-terminal tail of pD1 must be cleaved by an endopeptidase named the carboxyl terminal peptidase (Ctp) to produce mature D1, the functional form (5). In the cyanobacterium Synechocystis PCC 6803, there are three Ctp homologs (CtpA, CtpB, and CtpC), but only one, CtpA, is responsible for cleavage of the pD1 C-terminal extension (5). Disruption of CtpA leads to a loss of PSII activity and oxygen evolution from failure to form the manganese cluster (4, 6). The processing of pD1 is also critical for the association of extrinsic proteins on the luminal side to stabilize the PSII complexes (6, 7).In contrast to cyanobacteria, our knowledge of the significance of Ctp enzymes and D1 C-terminal processing is limited in land plants. Previous researchers reported the purification of CtpA-like protein from pea (8) and spinach (9). The spinach study further showed that the recombinant Ctp protein expressed from Escherichia coli displays activity against pD1 (9). However, because we lack a genetic approach, the functional significance of CtpA and C-terminal processing remains unknown in those and other land plants. In this study, we applied genetics to identify a gene (At4g17740) encoding a CtpA enzyme in Arabidopsis and showed that it is required for PSII function and chloroplast development. We found that Arabidopsis CtpA is essential for assembling functional PSII core complexes, dimers, and PSII supercomplexes. The enzyme is also critical for the PSII damage–repair cycle during the photoinhibition process.