Ycf12 is a core subunit in the photosystem II complex

The latest crystallographic model of the cyanobacterial photosystem II (PS II) core complex added one transmembrane low molecular weight (LMW) component to the previous model, suggesting the presence of an unknown transmembrane LMW component in PS II. We have investigated the polypeptide composition in highly purified intact PS II core complexes from Thermosynechococcus elongatus, the species which yielded the PS II crystallographic models described above, to identify the unknown component. Using an electrophoresis system specialized for separation of LMW hydrophobic proteins, a novel protein of ∼ 5 kDa was identified as a PS II component. Its N-terminal amino acid sequence was identical to that of Ycf12. The corresponding gene is known as one of the ycf (hypothetical chloroplast reading frame) genes, ycf12, and is widely conserved in chloroplast and cyanobacterial genomes. Nonetheless, the localization and function of the gene product have never been assigned. Our finding shows, for the first time, that ycf12 is actually expressed as a component of the PS II complex in the cell, revealing that a previously unidentified transmembrane protein exists in the PS II core complex.