Structural and functional characterization of two conserved lumenal TPM-domain proteins involved in the maturation of Photosystem II.

Many proteins conserved across oxygenic phototrophs play essential roles in photosynthetic function and acclimation, yet many remain unidentified or poorly characterized. In the green alga Chlamydomonas reinhardtii we identified two paralogous thylakoid lumenal proteins, encoded by Cre15.g636050 (LMTP1) and Cre03.g154600 (TLP26), belonging to the conserved TPM-domain (TLP18.3-Psb32-MOLO1) PF04536 family. We generated C. reinhardtii knock-out mutants for LMTP1 and TLP26 and produced recombinant proteins to assess their biochemical properties. The crystal structures of both proteins reveal the presence of a conserved redox-responsive cysteine pair, novel in TPM-domain proteins, and show that LMTP1 binds manganese. Double mutants lacking both LMTP1 and TLP26 show reduced photosynthetic performance due to effects at the acceptor-side of Photosystem II (PSII); however, these mutants accumulate more chlorophyll and photosynthetic proteins due to increased synthesis rates, a phenotype not observed for the single lmtp1 and tlp26 mutants. We propose that these two TPM-domain proteins, LMTP1 and TLP26, are functionally redundant in maturing nascent PSII intermediates during assembly and repair.