Abstract
Photosystem I (PSI) interacts with plastocyanin or cytochrome c6 on the luminal side. To identify sites of interaction between plastocyanin/cytochrome c6 and the PSI core, site-directed mutations were generated in the luminal 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 membranes. The other mutant strains contained different levels of PSI proteins. Among the mutant strains that contained PSI proteins, the H595C/L596I, Q627H/L628C/I629S, and N638C/N639S mutants showed similar levels of PSI-mediated electron transfer activity when either cytochrome c6 or an artificial electron donor was used. In contrast, cytochrome c6 could not function as an electron donor to the W622C/A623R mutant, even though the PSI activity mediated by an artificial electron donor was detected in this mutant. Thus, the W622C/A623R mutation affected the interaction of the PSI complex with cytochrome c6. Biotin-maleimide modification of the mutant PSI complexes indicated that His-595, Trp-622, Leu-628, Tyr-632, and Asn-638 in wild-type PsaB may be exposed on the surface of the PSI complex. The results presented here demonstrate the role of an extramembrane loop of a PSI core protein in the interaction with soluble electron donor proteins.
Highlights
Photosystem I (PSI) interacts with plastocyanin or cytochrome c6 on the luminal side
Cytochrome c6 could not function as an electron donor to the W622C/A623R mutant, even though the PSI activity mediated by an artificial electron donor was detected in this mutant
Examination of the structures of plastocyanin and cytochrome c6 revealed the presence of a flat hydrophobic surface around the redox center in both proteins [44, 45]
Summary
EVIDENCE FOR INTERACTION BETWEEN THE ELECTRON DONOR PROTEINS AND A LUMINAL SURFACE HELIX OF THE PsaB SUBUNIT*. To identify sites of interaction between plastocyanin/cytochrome c6 and the PSI core, site-directed mutations were generated in the luminal J loop of the PsaB protein from Synechocystis sp. Among the mutant strains that contained PSI proteins, the H595C/L596I, Q627H/L628C/I629S, and N638C/N639S mutants showed similar levels of PSI-mediated electron transfer activity when either cytochrome c6 or an artificial electron donor was used. Photosystem I (PSI) is a multisubunit membrane-protein complex that catalyzes electron transfer from the reduced plastocyanin in the thylakoid lumen to the oxidized ferredoxin in the chloroplast stroma or cyanobacterial cytoplasm [1,2,3,4]. The luminal extramembrane loops of PsaA and PsaB are likely candidates for interaction with plastocyanin during electron transfer. The l and lЈ helices are in the extramembrane J loops that are located between the
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have