Glycinebetaine stabilizes photosystem 1 and photosystem 2 electron transport in spinach thylakoid membranes against heat inactivation

Glycinebetaine, a compatible osmolyte of halotolerant plants and bacteria, partially protected photosystem (PS) 1 and PS2 electron transport reactions against thermal inactivation but with different efficiencies. In its prese nce, the temperature for half-maximal inactivation (t(1/2)) was generally s hifted downward by 3-12 degrees C. Glycinebetaine stabilized photoinduced o xygen evolving reactions of PS2 by protecting the tetranuclear Mn cluster a nd the extrinsic proteins of this complex. A weaker, although noticeable, s tabilizing effect was observed in photoinduced PS2 electron transport react ions that did not originate in the oxygen-evolving complex (OEC). This weak er protection by glycinebetaine was probably exerted on the PS2 reaction ce ntre. Glycinebetaine protected also photoinduced electron transport across PS1 against thermal inactivation. The protective effect was exerted on plas tocyanin, the mobile protein in the lumen that carries electrons from the i ntegral cytochrome b(6)f complex to the PS1 complex.