High-resolution structure of monomeric photosystem I from Thermosynechococcus vestitus

Photosystem I (PSI) exists in trimeric (tPSI) and monomeric (mPSI) forms within the thylakoid membrane of cyanobacteria. PSI functions as a light-driven electron pump, enabling the transfer of electrons from cytochrome c₆ (Cyt c₆) or plastocyanin to ferredoxin. Here, we have developed a novel protocol to obtain large amounts of intact, active mPSI from Thermosynechococcus vestitus (Tv) by dissociating highly purified tPSI using the detergent octaethylene glycol monododecyl ether (C₁₂E₈) at elevated temperatures. We analysed the stability of the protein-detergent complex employing a combination of spectroscopic methods, such as fluorescence at 77 K and circular dichroism (CD), and investigated the detergent's influence on delipidation and monomerisation. We determined the structure of mPSI to a resolution of 1.8 Å using cryogenic electron microscopy (cryo-EM). This structure retains all subunits except PsaL and we identified a sulfoquinovosyl diacylglycerol (SQD) molecule and 16 n-dodecyl ß-maltoside (βDM) molecules arranged in a ring around the complex. The mPSI structure was verified in solution at physiological temperature by applying small angle neutron scattering. Comparing the cofactor inventory of mPSI to that of tPSI, we suggest the assignment of a red-shifted chlorophyll pigment and discuss the influence of detergents and lipids on stability and oligomeric state of PSI. Finally, when adding native Tv Cyt c₆ under reducing conditions, additional density was observed in close proximity of the P₇₀₀ pigments, which indicates the presence of a transiently bound Cyt c₆.