Integration of nuclear‐encoded proteins into pea thylakoids with different pigment contents

Abstract

The in vitro membrane integration of the light‐harvesting protein of photosystem II (LHCP), the Rieske FeS protein of the cytochrome (Cyt) blf‐complex, and the NADPH:protochlorophyllide oxidoreductase (Pchlide reductase) into pea thylakoids with different pigment composition was studied. Pea plants (Pisum sativum L. cv. Kelvedon Wonder) with different contents of chlorophyll (Chl) and carotenoids were obtained by growing the seedlings in a greenhouse or in weak red light with or without the herbicide Norflurazon, an inhibitor of carotenoid biosynthesis. Chloroplasts from untreated and Norflurazon‐treated plants grown in weak red light contained approximately 29 and 14% of Chl compared to chloroplasts from untreated plants grown in the greenhouse. The corresponding carotenoid contents were 66 and 5%. Following an integration reaction using LHCP precursor protein and chloroplast lysate, thylakoids from untreated and Norflurazon‐treated plants grown in weak red light contained approximately 30 and 5% of protease‐protected LHCP, respectively, compared to thylakoids of untreated plants grown in a greenhouse. In contrast to LHCP, the in vitro assembly of the Pchlide reductase was only sligthly reduced in chloroplast lysates of plants grown in weak red light compared to greenhouse‐grown plants. In chloroplast lysates of Norflurazon‐treated plants, however, the amount of membrane associated, protease‐protected Pchlide reductase was reduced to 32% of the amount in untreated plants grown under the same light conditions. In contrast, the integration of the Rieske FeS protein occurred to almost similar levels irrespective of light conditions and herbicide treatments.Reconstitution assays where stroma from Norflurazon‐treated plants was added to thylakoids from untreated plants, showed that the herbicide did not affect any stromal component(s) vital for the insertion reaction. Removal of samples during the integration reaction of LHCP showed that no degradation of the protein occurred during the assay. Neither was the assembled protein degraded up to 24 h after the termination of the assay. This indicates that growing plants in weak red light, with or without Norflurazon treatment, mainly affected the primary step in thylakoid assembly of LHCP, i.e. the insertion reaction into the membrane. The results further indicate that proteins normally bound to pigments also require pigments for membrane recognition or integration.