The PsaD Subunit of Photosystem I (Mutations in the Basic Domain Reduce the Level of PsaD in the Membranes)

Abstract

The PsaD subunit of photosystem I (PSI) is a peripheral protein that provides a docking site for ferredoxin and interacts with the PsaB, PsaC, and PsaL subunits of PSI. We used site-directed mutagenesis to determine the function of a basic region in PsaD of the cyanobacterium Synechocystis sp. PCC 6803. We generated five mutant strains in which one or more charged residues were altered. Western blotting showed that replacement of lysine (Lys)-74 with glutamine or glutamic acid led to a substantial decrease in the level of PsaD in the membranes. The mutant PSI complexes showed reduced NADP+ photoreduction activity mediated by ferredoxin; the decrease in activity correlated with the reduced level of PsaD. Using protein synthesis inhibitors we showed that the degradation rates of the mutant and wild-type PsaD were similar, indicating a defect in the assembly of the mutant protein. Treatment of the mutant PSI complexes with a different concentration of Nal showed that the mutations decreased affinity between PsaD and the transmembrane components of PSI. With glutaraldehyde, the mutant and wild-type PsaD proteins could be cross-linked with PsaC, but the PsaD-PsaL cross-linked product was reduced drastically when arginine-72, Lys-74, and Lys-76 were mutated simultaneously. These studies demonstrate that the basic residues in the central region of PsaD, especially Lys-74, are crucial in the assembly of PsaD into the PSI complex.

Mutants for photosystem I subunit D of Arabidopsis thaliana: effects on photosynthesis, photosystem I stability and expression of nuclear genes for chloroplast functions

“…Black boxes indicate strictly conserved amino acids, shaded boxes closely related amino acids. The symbol ‘+’ refers to amino acid residues involved in the binding of ferredoxin (Bottin et al ., 2001; Hanley et al ., 1996; Lagoutte et al ., 2001); circles highlight lysine residues of the basic domain, which are important for the interaction of PSI‐D with other PSI subunits (Chitnis et al ., 1997).…”

Low red/far-red ratio as a signal promotes carbon assimilation of soybean seedlings by increasing the photosynthetic capacity

“…The PsaD subunit of PS I is a peripheral protein that provides a docking site for ferredoxin and interacts with other PS I subunits [35]. PsaD is highly sensitive to the light environment, and high light intensity significantly decreases the PsaD content [36].…”

Structure of cyanobacterial Photosystem I

“…It is essential for the electron transfer from PS I to ferredoxin (Chitnis 1996(Chitnis , 1997Barth et al 1998) and allows PsaC to adjust its final position in the complex.…”