Regulation of thylakoid protein phosphorylation by the thiol redox state ¿ the role of thioredoxin.

Abstract

Redox controlled thylakoid protein phosphorylation plays an important role in the regulation of PSII function and several PSII proteins are reversibly phosphorylated. In vivo, protein phosphorylation of LHCII has been found to decrease as the light is increased. In contrast, the phosphorylation of PSII remains high also at high light intensities (Rintamäki et al.1997, JBC 272, 30476). The mechanism for the specific down regulation of LHCII phosphorylation in high light is not yet known. We have earlier shown that thylakoid protein phosphorylation in vitro is strongly affected by the thiol redox state. In spite of the fact that the phosphorylation is generally activated by reducing conditions, LHCII phosphorylation is strongly inhibited by reduced DTT. This indicates the importance of a correct redox state of thiol groups, but also points to the possible existence of multiple levels of redox regulation. The PSII phosphorylation, on the other hand, is much less sensitive and even increases in the presence of low concentrations of DTT and it was suggested that the decrease in LHCII phosphorylation under high light could be mediated via the thiol redox state (Carlberg et al. 1999, Biochemistry 38, 3197). In the present work, we have investigated the possible participation of the wellknown redox regulator thioredoxin in this process.