Crystal structure of the Psb28 accessory factor of Thermosynechococcus elongatus photosystem II at 2.3 Å.

Members of the Psb28 family of proteins are accessory factors implicated in the assembly and repair of the photosystem II complex. We present here the crystal structure of the Psb28 protein (Tlr0493) found in the thermophilic cyanobacterium Thermosynechococcus elongatus at a resolution of 2.3 Å. Overall the crystal structure of the Psb28 monomer is similar to the solution structures of C-terminally His-tagged Psb28-1 from Synechocystis sp. PCC 6803 obtained previously by nuclear magnetic resonance spectroscopy. One new aspect is that Escherichia coli-expressed T. elongatus Psb28 is able to form dimers in solution and packs as a dimer of dimers in the crystal. Analysis of wild type and mutant strains of Synechocystis 6803 by blue native-polyacrylamide gel electrophoresis suggests that Psb28-1, the closest homologue to T. elongatus Psb28 in this organism, also exists as an oligomer in vivo, most likely a dimer. In line with the prediction based on the crystal structure of T. elongatus Psb28, the addition of a 3× Flag-tag to the C-terminus of Synechocystis 6803 Psb28-1 interferes with the accumulation of the Psb28-1 oligomer in vivo. In contrast, the more distantly related Psb28-2 protein found in Synechocystis 6803 lacks the residues that stabilize dimer formation in the T. elongatus Psb28 crystal and is detected as a monomer in vivo. Overall our data suggest that the dimer interface in the Psb28 crystal might be physiologically relevant.