Amino acid deletions in the cytosolic domains of the chlorophyll a-binding protein CP47 slow Q_A ⁻ oxidation and/or prevent the assembly of Photosystem II

Abstract

The Photosystem II (PSII) core antenna chlorophyll a-binding protein, CP47, contains six membrane-spanning α-helices separated by five hydrophilic loops: A–E. To identify important hydrophilic cytosolic regions, oligonucleotide-directed mutagenesis was employed to introduce short segment deletions into loops B and D, and the C-terminal domain. Four strains carrying deletions of between three and five residues were created in loop B. Two strains, with deletions adjacent to helices II and III, did not assemble PSII; however, the mutants Δ(F123–D125) and Δ(R127–S131) remained photoautotrophic with near wild-type levels of assembled reaction centers. In contrast, all deletions introduced into loop D, connecting helices IV and V, failed to assemble significant levels of PSII and were obligate photoheterotrophic mutants. However, deletions in the C-terminal domain did not prevent the assembly of PSII reaction centers although the mutant Δ(S471–T473), with a deletion adjacent to helix VI, exhibited retarded Q_A ⁻ oxidation kinetics and the PSII-specific herbicide, atrazine, bound less tightly in the Δ(S471–T473) and Δ(F475–D477) strains. Deletions in the C-terminal domain also created mutants with large protein aggregates that were recognized by an antibody raised against the PSII reaction center D1 protein. Low-temperature fluorescence emission spectra of photoautotrophic strains carrying deletions in either the C-terminal domain or loop B did not provide evidence for impaired energy transfer from the phycobilisomes to the PSII reaction center. The data therefore suggest an important structural role for loop D in the assembly of PSII and a potential interaction between the C-terminal domain of CP47 and the PSII reaction center that, when perturbed, results in photoinduced protein aggregates involving the D1 protein.