Oxidizing side of the cyanobacterial Photosystem I: Mutational analysis ofthe luminal H loop of the PsaB subunit

PS I core proteins are expected to interact with the electron donor protein s plastocyanin or cytochrome c(6). To investigate the role of the luminal H loop of PsaB in the assembly and function of the PS I complex, we generate d 15 deletion and repetition mutations in the H loop of the PsaB protein fr om Synechocystis sp. PCC 6803. The mutant strains differed in their photoau totrophic growth. The PS I proteins could not be detected in the membranes of mutants in which the N438-E448, I453-T464, or S500-G512 region was delet ed from the PsaB protein, indicating the essential role of these segments i n proper folding of the PsaB protein. Mutants with partial or complete dele tion of the L469-D496 segment contained the PS I proteins. These results in dicate that the regions near the transmembrane helices are more important f or the assembly of PsaB than the middle region of the H loop. The L469-D496 segment in the H loop of PsaB is dispensable in the interaction between th e PS I complex and the soluble donor proteins. These results suggested that sections of the H loop of PsaB are crucial for the structural integrity of the PsaB protein.