Import of nuclear‐encoded proteins into carotenoid‐deficient young etioplasts

Abstract

Young etioplasts with different carotenoid contents were assayed for their ability to import in vitro synthesized nuclear‐encoded proteins. The plastids were isolated from the basal 1. 5cm of dark‐grown wheat seedlings developed from seeds imbibed with 4 different concentrations of Norflurazon. an inhibitor of the carotenoid biosynthesis. Plastids isolated from plants treated with the two highest concentrations. 2. 8 and 28 mg l−1, of Norflurazon contained approximately 10 and 5% of the carotenoid contents, respectively, compared to the control. The total amounts of proteins in these plastids were approximately 68 and 60% compared to control plastids. Translocation assays employing the precursors of the small subunit of ribulose 1. 5‐bisphosphate carboxylase/oxygenase (pSS), and the non‐Photosynthetic heat‐shock protein 21 (pHSP21), showed that the rate of protein import was considerably lower in plastids with low carotenoid contents. The amounts of imported, processed SS were 11 and 10% after 2. 8 and 28 mg 1−1, respectively, compared to the control, whereas the amounts of HSP21 at these herbicide concentrations were 20 and 18%, respectively. The low apparent import in plastids of Norflurazon‐treated leaves was not an effect of intraorganellar degradation of imported proteins, nor were there any differences in the amounts of processed, protease‐protected protein when Norflurazon was added to the import reaction using control plastids. The low import capabilities are therefore discussed in relation to the possible role of the carotenoids in the translocation of cytosolically synthesized proteins into the plastidic compartment.

Fibril Assembly and Carotenoid Overaccumulation in Chromoplasts: A Model for Supramolecular Lipoprotein Structures

“…It is known that low levels of carotenoids are essential for the light-harvesting mechanism of chloroplasts (Plumley and Schmidt, 1987;Humbecketal., 1989;Herrinetal., 1992), The core is occupied by carotenoids (CAR) that interact with the acyl residues of the more polar galactolipids and phospholipids (LIP), whose polar head groups (dark circles) interact with fibrillin molecules (FIB), which are directly in contact with the plastid stroma. and it has also been noted that inhibition of carotenoid synthesis can inhibit transport of nuclear-encoded proteins destined for the plastid (Dahlin, 1993). Our data showed that disruption of carotenoid synthesis following CPTA treatment also influences membrane function and affects the uptake and maturation of at least one nuclear-encoded plastid protein.…”

Characterisation of the assembly pathway of the pea NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR), with emphasis on the role of its substrate, Pchlide

“…Import of radiolabelled proteins into isolated plastids (corresponding to 50 μg chlorophyll) was performed with different concentrations of ATP and NADPH for 2×10 min, as described earlier (Dahlin 1993), if not otherwise stated. To perform import assays without endogenous ATP, plastids were put in darkness for 45 min at room temperature prior to the import reaction.…”

Protochlorophyllide‐independent import of two NADPH:Pchlide oxidoreductase proteins (PORA and PORB) from barley into isolated plastids

“…The samples containing radiolabelled precursor proteins pPORA and pPORB were adjusted to 30 mM unlabelled leucine prior to use. All steps except for the import reaction were run at 4°C according to Dahlin (1993), if not otherwise stated. Briefly, the import mixture consisted of precursor protein, Mg-ATP (final concentration 6 mM), plastids (50× 10 6 ) and import buffer to a final volume of 100 ml.…”