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.