Cellular localization of cytochrome c550. Its specific association with cyanobacterial photosystem II.

Cellular localization of cytochrome (cyt) c550, a low potential, c-type monoheme cytochrome, in a thermophilic cyanobacterium Synechococcus vulcanus was investigated by systematic fractionation of the cells followed by its enzymatic and immunological detection. While cyt c-553, a soluble cyt that donates an electron to P700, was detected in the supernatant of osmotic disruption of lysozyme-treated cells, cyt c550 was detected only in the thylakoid membrane fraction, being tightly bound to thylakoids, and its removal required sonication in the presence of 1 M CaCl2. Upon further fractionation of the thylakoids into photosystem (PS) I and PSII by lauryl dimethylamine N-oxide solubilization, cyt c550 was exclusively concentrated in the crude PSII fraction together with cyt f, with no significant amount being detected in any of the soluble and PSI fractions. Upon further fractionation of the crude PSII by n-dodecyl beta-D-maltoside solubilization followed by column chromatography, cyt c550 was detected exclusively in the purified PSII core complex fraction but not in any other fractions. A 12-kDa protein, one of the extrinsic components of cyanobacterial PSII, exhibited completely the same behavior as that of cyt c550 during these fractionation procedures. These results, coupled with our previous results that cyt c550 binds stoichiometrically to the cyanobacterial PSII core complex and enhances O2 evolution (Shen, J.-R., and Inoue, Y. (1993) Biochemistry 32, 1825-1832), indicate that there is only one species of cyt c550 in cyanobacterial cells and that this cyt is exclusively associated with PSII as a functional component for O2 evolution.