Light- and Redox-Dependent Force Spectroscopy Reveals that the Interaction between Plastocyanin and Plant Photosystem I Is Favored when One Partner Is Ready for Electron Transfer

Abstract

Photosynthesis is a fundamental process that converts photons into chemical energy, driven by large protein complexes at the thylakoid membranes of plants, cyanobacteria, and algae. In plants, water-soluble plastocyanin (Pc) is responsible for shuttling electrons between cytochrome b6f complex and the photosystem I (PSI) complex in the photosynthetic electron transport chain (PETC). For an efficient turnover, a transient complex must form between PSI and Pc in the PETC, which implies a balance between specificity and binding strength. Here, we studied the binding frequency and the unbinding force between suitably oriented plant PSI and Pc under redox control using single molecule force spectroscopy (SMFS). The binding frequency (observation of binding-unbinding events) between PSI and Pc depends on their respective redox states. The interaction between PSI and Pc is independent of the redox state of PSI when Pc is reduced, and it is disfavored in the dark (reduced P700) when Pc is oxidized. The frequency of interaction between PSI and Pc is higher when at least one of the partners is in a redox state ready for electron transfer (ET), and the post-ET situation (PSIRed-PcOx) leads to lower binding. In addition, we show that the binding of ET-ready PcRed to PSI can be regulated externally by Mg2+ ions in solution.

The protein matrix of plastocyanin supports long-distance charge transport with photosystem I and the copper ion regulates its spatial span and conductance

“…For Pc/PSI, the binding probability depends on the proteins' redox state and is higher when at least one of the partners is in an ET-ready state. 33 Those results pave the way for disentangling the contribution of binding from the overall charge exchange process between PSI and Pc. However, characterization of the ET step between Pc/PSI pairs is lacking.…”

The Protein Matrix of Plastocyanin Supports Long-Distance Charge Transport with Photosystem I and the Copper Ion Regulates Its Spatial Span and Conductance

“…For Pc/PSI, the binding probability depends on the proteins' redox state and is higher when at least one of the partners is in an ET-ready state. 33 Those results reveal the importance of disentangling the contribution of binding from the overall charge exchange process between PSI and Pc. However, the characterization of the ET step between Pc/PSI pairs is lacking.…”

“…We have recently studied the binding of single Pc/PSI pairs using force spectroscopy. 33 Controlling the redox state of PSI and Pc, we quantified the binding probability as the number of adhesion events. For Pc/PSI, the binding probability depends on the proteins' redox state and is higher when at least one of the partners is in an ET-ready state.…”

“…Transient spectroscopic measurements − ,− tracking Pc/PSI ET exhibit a fast μs phase associated with the ET of preformed complexes and a slower ms phase, dependent on Pc concentration and related to the Pc/PSI complex formation. We have recently studied the binding of single Pc/PSI pairs using force spectroscopy . Controlling the redox state of PSI and Pc, we quantified the binding probability as the number of adhesion events.…”

“…Controlling the redox state of PSI and Pc, we quantified the binding probability as the number of adhesion events. For Pc/PSI, the binding probability depends on the proteins’ redox state and is higher when at least one of the partners is in an ET-ready state . Those results reveal the importance of disentangling the contribution of binding from the overall charge exchange process between PSI and Pc.…”