The lumenal α-helical loop j of photosystem I (PSI) reaction center subunit PsaB modulates electron transfer between PSI and plastocyanin or cytochome c6 and is important for proper orientation of PsaF

Abstract

PsaB is one of the heterodimeric reaction center subunits of photosystem I. We started a combinatorial site directed mutagenesis of conserved residues within the luminal loop j of PsaB connecting the transmembrane helices IX and X in Chlamydomonas reinhardti. Mutant strains D624K, E613K/D624K, D624K/W627F showed no accumulation of PSI while strains E613N, E613K, E613K/W627F and W627F assembled PSI to >50 % compared to wild type but were sensitive to high light. The interaction between plastocyanin (pc) or cytochrome c6 (cyt c6) and PSI isolated from wild type and different mutants was analyzed using cross linking techniques and flash absorption spectroscopy. Electron transfer kinetics showed a faster electron transfer and a stronger binding of the donors to PSI in mutant E613N as compared to wild type. In contrast no stable complex formation between electron donors and PSI isolated from mutant E613K was observed. Further investigations revealed that this strong effect on electron transfer in the latter mutant can be explained by a disarrangement of the positively charged N-terminal domain of PsaF in PSI. The mutation of an uncharged residue (W627F) has also a strong effect on electron transfer between electron donors and mutant PSI. Our experiments suggest that loop j is important for both modulating binding of pc and cyt c6 to PSI and to keep PsaF in a propper orientation for efficient electron transfer.