The Thylakoid Protease Against LHCII

Abstract

LHCII (Light-harvesting pigment-protein complex serving PSII) is the most abundant protein in mature thylakoid membranes. During chloroplast development and under conditions of limited Chl accumulation in Intermittent light plants, thylakoids possess very small amount of LHCII protein (1), despite the existence of in vitro translatable mRNA (2). Similarly, in plants briefly exposed to continuous light and then transferred to the dark, the preaccumulated LHCII is degraded (3, 4). These results implicate a protease in the control of LHCII accumulation. Indeed, thylakoids possess proteolytic activity against azocoll and LHCII (5–7). Here we describe our attempts to study the role of this protease in the regulation of LHCII stabilization during thylakoid biogenesis, its characteristics and initial purification steps. The protease is mainly located in the membrane fraction of etioplasts (8) and chloroplasts; the activity is enhanced after exposure of plants to a flash of light, to intermittent light or after brief exposure to continuous light and it is developmentally regulated (6, 8). Chlorophyll accumulation rescues LHCII from proteolysis. In vitro studies show that high light (HL) enhances LHCII proteolysis in TX-100 solubilized thylakoids and stroma, while ATP further enhances the activity in HL. NaBr washing of the thylakoid membranes removes the activity in the wash. This activity is heat stable up to 60°C. Thylakoid proteins resolved by SDS-PAGE reveal 5 peaks of activity against azocoll and one peak of activity against exogenously added LHCII, located at the molecular weight range above 180 kDa. A similar peak is obtained after SDS-PAGE analysis of the NaBr wash proteins.