Oxidizing side of the cyanobacterial photosystem I - Evidence for interaction between the electron donor proteins and a luminal surface helix of the PsaB subunit

Photosystem I (PSI) interacts with plastocyanin or cytochrome c(6) on the l uminal side. To identify sites of interaction between plastocyanin/cytochro me c(6) and the PSI core, site-directed mutations were generated in the lum inal J loop of the PsaB protein from Synechocystis sp, PCC 6803. The eight mutant strains differed in their photoautotrophic growth. Western blotting with subunit-specific antibodies indicated that the mutations affected the PSI level in the thylakoid membranes. PSI proteins could not be detected in the S600R/G601C/ N602I, N609K/S610C/T611I, and M614I/G615C/W616A mutant me mbranes. The other mutant strains contained different levels of PSI protein s. Among the mutant strains that contained PSI proteins, the H595C/L596I, Q 627H/L628C/I629S, and N638C/N639S mutants showed similar levels of PSI-medi ated electron transfer activity when either cytochrome c(6) or an artificia l electron donor was used. In contrast, cytochrome c(6) could not function as an electron donor to the W622C/A623R mutant, even though the PSI activit y mediated by an artificial electron donor was detected in this mutant. Thu s, the W622C/A623R mutation affected the interaction of the PSI complex wit h cytochrome c(6). Biotin-maleimide modification of the mutant PSI complexe s indicated that His-595, Trp-622, Leu-628, Tyr-632, and Asn-638 in wildtyp e PsaB may be exposed on the surface of the PSI complex. The results presen ted here demonstrate the role of an extramembrane loop of a PSI core protei n in the interaction with soluble electron donor proteins.