Current limits of structural biology: The transient interaction between cytochrome c(6) and photosystem I

Trimeric photosystem I from the cyanobacterium Thermosynechococcus elongatus (TePSI) is an intrinsic membrane protein, which converts solar energy into electrical energy by oxidizing the soluble redox mediator cytochrome c(6) (Cyt c(6)) and reducing ferredoxin. Here, we use cryo-electron microscopy and small angle neutron scattering (SANS) to characterize the transient binding of Cyt c(6) to TePSI. The structure of TePSI cross-linked to Cyt c(6) was solved at a resolution of 2.9 Å and shows additional cofactors as well as side chain density for 84% of the peptide chain of subunit PsaK, revealing a hydrophobic, membrane intrinsic loop that enables binding of associated proteins. Due to the poor binding specificity, Cyt c(6) could not be localized with certainty in our cryo-EM analysis. SANS measurements confirm that Cyt c(6) does not bind to TePSI at protein concentrations comparable to those for cross-linking. However, SANS data indicate a complex formation between TePSI and the non-native mitochondrial cytochrome from horse heart (Cyt c(HH)). Our study pinpoints the difficulty of identifying very small binding partners (less than 5% of the overall size) in EM structures when binding affinities are poor. We relate our results to well resolved co-structures with known binding affinities and recommend confirmatory methods for complexes with K(M) values higher than 20 μM.