Multiple functions for the C terminus of the PsaD subunit in the cyanobacterial photosystem I complex

PsaD subunit of Synechocystis sp PCC 6803 photosystem I (PSI) plays a criti cal role in the stability of the complex and is part of the docking site fo r ferredoxin (Fd). In the present study we describe major physiological and biochemical effects resulting from mutations in the accessible C-terminal end of the protein. Four basic residues were mutated: R111, K117, K131, and K135, and a large 36-amino acid deletion was generated at the C terminus. PSI from R111C mutant has a 5-fold decreased affinity for Fd, comparable wi th the effect of the C terminus deletion, and NADP(+) is photoreduced with a 2-fold decreased rate, without consequence on cell growth. The K117A muta tion has no effect on the affinity for Fd, but decreases the stability of P saE subunit, a loss of stability also observed in R111C and the deletion mu tants. The double mutation K131A/K135A does not change Fd binding and reduc tion, but decreases the overall stability of PSI and impairs the cell growt h at temperatures above 30 degreesC. Three mutants, R111C, K117A, and the C -terminal deleted exhibit a higher content of the trimeric form of PSI, in apparent relation to the removal of solvent accessible positive charges. Va rious regions in the C terminus of cyanobacterial PsaD thus are involved in Fd strong binding, PSI stability, and accumulation of trimeric PSI.