Identification, characterization, and resolution of the in vivo phosphorylated form of the D1 photosystem II reaction center protein

The chloroplast-encoded D1 protein of oxygenic photosynthetic organisms is a component of the photosystem II reaction center. Previously, we detected an electrophoretic variant of D1 which was generated in vivo in granal-localized reaction centers in a light dependent manner (Callahan, F. E., Ghirardi, M. L., Sopory, S. K., Mehta, A. M., Edelman, M., and Mattoo, A. K. (1990) J. Biol Chem. 265, 15357-15360). In the present study, we identify this modified form as phosphorylated D1. The in vivo phosphorylation occurs on a threonine residue(s) localized within 1 kDa from the N terminus and is identical to the phosphorylation of D1 catalyzed in vitro by a redox-regulated thylakoid-bound protein kinase. While virtually all of the D1 protein present in thylakoids can be phosphorylated in vitro, the steady-state level of phosphorylated D1 in vivo varies with light intensity and did not exceed of the total D1 under the conditions of this study.