The photosystem 1 subunit PsaF is involved in the docking of the electron-donor proteins plastocyanin and cytochrome c(6) in eukaryotic photosynthetic organisms. Here we report the expression, purification and basic characterization of the luminal domain of spinach PsaF, encompassing amino-acid residues 1-79. The recombinant protein was expressed in Escherichia coil BL21 (DE3) using a pET32 Xa/LIC thioredoxin fusion system. The thioredoxin fusion protein contained a His(6) tag and was removed and separated from PsaF through proteolytic digestion by factor Xa followed by immobilized metal affinity chromatography. Further purification with size-exclusion chromatography resulted in a final yield of approximately 6 mg PsaF from one liter growth medium. The correct identity after the factor Xa treatment of PsaF was verified by FT-ICR mass spectrometry which also showed that the purified protein contains an intact disulfide bridge between Cys residues 6 and 38. Secondary structure and folding was further explored using far-UV CD spectroscopy indicating a alpha-helical content in agreement with the 3.3 angstrom-resolution crystal structure of photosystem I Ref. [5] and a helix-coil transition temperature of 29 degrees C. Thermofluorescence studies showed that the disulfide bridge is necessary to keep the overall fold of the protein and that hydrophobic regions become exposed at 50-65 degrees C depending on the ionic strength. The described expression and purification procedure can be used for isotopic labeling of the protein and N-15-HSQC NMR studies indicated a slow or intermediate exchange between different conformations of the prepared protein and that it belongs to the molten-globule structural family. Finally, by using a carboxyl- and amine-reactive zero-length crosslinker, we have shown that the recombinant protein binds to plastocyanin by a specific, native-like, electrostatic interaction, hence, confirming its functionality. (C) 2011 Elsevier Inc. All rights reserved.