Isolation of the cyanobacterial YFP-tagged photosystem I using GFP-Trap®

A strain of Synechocystis sp. PCC 6803 expressing the yellow fluorescent protein (YFP) fused to the C-terminus of the PsaF subunit of PSI has been constructed and used to isolate native PSI complexes employing the GFP-Trap®, an efficient immunoprecipitation system which recognizes the green fluorescent protein (GFP) and its variants. The protein analysis and spectroscopic characterization of the preparation revealed an isolate of trimeric and monomeric PSI complexes, which showed minimal unspecific contamination as demonstrated by comparison with the wild type control. Interestingly, we detected CP43 subunits of PSII and small amounts of PSII core complexes specifically pulled-down with the YFP-PSI, supporting the association of PSII assembly modules and intermediate assembly complexes with PSI, as observed in our previous studies. The results demonstrate that the GFP-Trap® system represents an excellent tool for studies of PSI biogenesis and interconnection of PSI and PSII assembly processes.