Abstract
C-terminal peptidase (Ctp) cleaves the C-terminal extension of the D1 precursor (pD1) to form mature D1. Among the three homologs CtpA, CtpB, and CtpC in photosynthetic organisms only the first is capable of processing pD1 while the roles of CtpB and CtpC remain elusive. Phylogenetic analysis of Ctps from photosynthetic organisms revealed that CtpA has diverged early from CtpB and CtpC during evolution implying distinct roles for the Ctps. Analysis of Arabidopsis Ctp-deficient mutants revealed that pD1 processing was not affected in atctpb, atctpc, or atctpbatctpc mutants, demonstrating that AtCtpA, not AtCtpB or AtCtpC, is responsible for cleaving the pD1 C-terminal extension. Ectopic expression of CtpAs from Synechococcus elongatus, Chlamydomonas reinhardtii, and Physcomitrella patens in atctpa rescued the lethal phenotype of the mutant indicating that SeCtpA, CrCtpA, and PpCtpA could process pD1 in Arabidopsis. Enzyme activity assays showed that PpCtpA and CrCtpA could convert pD1 into mature D1 in vitro. In contrast, expressing CtpB or CtpC from Arabidopsis, C. reinhardtii, or P. patens in atctpa did not rescue its D1 maturation deficiency, and enzyme activity assays also showed that neither CtpB nor CtpC could process pD1 in vitro. Taken together, we conclude that the function of pD1 processing by CtpA is conserved in photosynthetic organisms. It is possible that among other factors CtpA developed this function to initiate the formation of the oxygenic D1/D2 type PSII complex during evolution whereas CtpB or CtpC have other roles that are still unclear.
Highlights
Photosystem II (PSII), a multimeric protein complex in the thylakoid membrane, catalyzes the light-driven oxidation of water and the reduction of plastoquinone (Ferreira et al, 2004; Wei et al, 2016)
It is interesting that most oxygenic photosynthetic organisms encode three C-terminal peptidase (Ctp) but only CtpA is critical for cleaving the pD1 C-terminal extension (Ivleva et al, 2002; Satoh and Yamamoto, 2007; Che et al, 2013)
The phylogenetic analysis indicated that Ctp proteins evolved from a single ancestor and followed a complex evolutionary pathway to generate the paralogs in photosynthetic organisms (Figure 1 and Supplementary Figure 1)
Summary
Photosystem II (PSII), a multimeric protein complex in the thylakoid membrane, catalyzes the light-driven oxidation of water and the reduction of plastoquinone (Ferreira et al, 2004; Wei et al, 2016). The pD1 C-terminal extension usually comprises 8 or 9 amino acids (aa) in eukaryotes while it is longer (usually 16 aa) in prokaryotes and requires two-step cleavage (Inagaki et al, 2001b). It is often absent in D1 from photosynthetic organisms that arouse through a secondary or tertiary endosymbiosis (Satoh and Yamamoto, 2007). D1 is highly conserved in oxygenic photosynthetic organisms while the C-terminal extension of pD1 is quite divergent (Nixon et al, 1991; Satoh and Yamamoto, 2007). PCC 6803 strain lacking psbA yielded a functional PSII, showing that pD1 from higher plants can be correctly processed and play a normal function in cyanobacteria (Nixon et al, 1991)
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