Expression of protein complexes and individual proteins upon transition of etioplasts to chloroplasts in pea (Pisum sativum).

The protein complexes of pea (Pisum sativum L.) etioplasts, etio-chloroplasts and chloroplasts were examined using 2D Blue Native/SDS-PAGE. The most prominent protein complexes in etioplasts were the ATPase and the Clp and FtsH protease complexes which probably have a crucial role in the biogenesis of etioplasts and chloroplasts. Also the cytochrome b(6)f (Cyt b(6)f) complex was assembled in the etioplast membrane, as well as Rubisco, at least partially, in the stroma. These complexes are composed of proteins encoded by both the plastid and nuclear genomes, indicating that a functional cross-talk exists between pea etioplasts and the nucleus. In contrast, the proteins and protein complexes that bind chlorophyll, with the PetD subunit and the entire Cyt b(6)f complex as an exception, did not accumulate in etioplasts. Nevertheless, some PSII core components such as PsbE and the luminal oxygen-evolvong complex (OEC) proteins PsbO and PsbP accumulated efficiently in etioplasts. After 6 h de-etiolation, a complete PSII core complex appeared with 40% of the maximal photochemical efficiency, but a fully functional PSII was recorded only after 24 h illumination. Similarly, the core complex of PSI was assembled after 6 h illumination, whereas the PSI-light-harvesting complex I was stably assembled only in chloroplasts illuminated for 24 h. Moreover, a battery of proteins responsible for defense against oxidative stress accumulated particularly in etioplasts, including the stromal and thylakoidal forms of ascorbate peroxidase, glutathione reductase and PsbS.

Light regulation of CaS, a novel phosphoprotein in the thylakoid membrane of Arabidopsis thaliana.

Exposure of Arabidopsis thaliana plants to high levels of light revealed specific phosphorylation of a 40 kDa protein in photosynthetic thylakoid membranes. The protein was identified by MS as extracellular calcium-sensing receptor (CaS), previously reported to be located in the plasma membrane. By confocal laser scanning microscopy and subcellular fractionation, it was demonstrated that CaS localizes to the chloroplasts and is enriched in stroma thylakoids. The phosphorylation level of CaS responded strongly to light intensity. The light-dependent thylakoid protein kinase STN8 is required for CaS phosphorylation. The phosphorylation site was mapped to the stroma-exposed Thr380, located in a motif for interaction with 14-3-3 proteins and proteins with forkhead-associated domains, which suggests the involvement of CaS in stress responses and signaling pathways. The knockout Arabidopsis lines revealed a significant role for CaS in plant growth and development.