Submitted to: Physiologia Plantarum
Publication Type: Review Article
Publication Acceptance Date: August 16, 1999
Publication Date: N/A
Interpretive Summary: Protein turnover is an indispensable feature of any living system. Its main functions are adaptive, regulatory and housekeeping (replacement of the damaged components). The photosynthetic apparatus represents a crucial contact point between the plant and the environment. Its flexibility on the one hand and stability on the other are instrumental to plant survival. Breakdown and replacement of the pigment-protein complexes are tightly coordinated for maximizing photosynthesis. This review summarizes recent literature on the dynamics of the protein and pigment (chlorophyll and beta-carotene) components of an essential reaction center complex of the plastid membranes. Details of the time-dependence and light requirements for the degradation of individual protein components are provided. Also summarized are the protection mechanisms that plant's employ to prevent disassembly of this protein complex. This information should interest students and scientists involved in understanding the limiting factors that affect photosynthesis and protein turnover.
Photosystem II (PSII) reaction center is an intrinsic membrane-protein complex in the chloroplast made of D1, D2, alpha and beta subunits of cytochrome b-559, the psbI gene product and a few low molecular weight proteins. One of the major outcomes of light-mediated photochemistry is the fact that, in the light, D1 protein is rapidly turned over compared to the other proteins of the reaction center; the relative lability of proteins being: D1>>D2>Cyt b-559. The presence of a physiologically relevant level of ultraviolet-B radiation (UV-B) in a background of photosynthetically active radiation stimulates D1/D2 heterodimer degradation in a synergistic manner. The role(s) of protein modifications in D1/D2 assembly, turnover or function are still a matter of conjecture. Nor do we yet know about the fate of the liganded pigments, such as the chlorophyll and carotenoids bound to the reaction center proteins. Environmental extremes that negatively impact photosynthesis seem to involve D1 metabolism. Thus, D1 protein is a major factor of PSII instability and, its replacement after its degradation, a primary component of the PSII repair cycle.