Addition of C-terminal histidyl tags to PsaL and PsaK1 proteins of cyanobacterial photosystem I.
In vitro mutagenesis was used to produce two photosystem I mutants of the cyanobacterium Synechocystis sp. PCC 6803. The mutant HK and HL contained hexahistidyl tags at the C-termini of the PsaK1 and PsaL subunits, respectively. The HK mutant contained wild-type amounts of trimeric PS I complexes, but the level of hexahistidine-tagged PsaK1 was found only ten per cent in the PS I complexes and membranes of the wild type level. Therefore, attachment of a tag at the C-terminus interferes with the expression or assembly of PsaK1. In contrast, the HL mutant contained a similar level of tagged PsaL as that in the wild type. However, trimeric PS I complexes could not be obtained from this strain, indicating that the C-terminus of PsaL is involved in the formation of PS I trimers. Hexahistidine-tagged complexes of the HL and HK strains could not be purified with Nickel-affinity chromatography, unless photosystem I was denatured with urea, demonstrating that tagged C-termini of PsaK1 and PsaL were embedded inside of the PS I complex. Protection of the C-terminus from trypsin cleavage further supported this conclusion. Thus, histidine tagging allowed us to demonstrate role of C-termini of two proteins of photosystem I.