Fractionation and reconstitution of the sarcoplasmic reticulum Ca2+ pump solubilized and stabilized by CHAPS/lipid micelles

Metabolic cross talk between cytosolic and plastidial pathways of isoprenoid biosynthesis: unidirectional transport of intermediates across the chloroplast envelope membrane

In higher plants, two independent pathways are responsible for the biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate, the central five-carbon precursors of all isoprenoids. The cytosolic pathway, which involves mevalonate (MVA) as a key intermediate, provides the precursor molecules for sterols, ubiquinone, and certain sesquiterpenes, whereas the plastidial MVA-independent pathway is involved in the formation of precursors for the biosynthesis of isoprene, monoterpenes, diterpenes, carotenoids, abscisic acid, and the side chains of chlorophylls, tocopherols, and plastoquinone. Recent experiments provided indirect evidence for the presence of an export system for isoprenoid intermediates from the plastids to the cytosol in Arabidopsis thaliana. Here we report that isolated chloroplasts (from spinach, kale, and Indian mustard), envelope membrane vesicles, and proteoliposomes prepared from the solubilized proteins of envelope membranes (from spinach) are capable of the efficient transport of isopentenyl diphosphate and geranyl diphosphate. Lower rates of transport were observed with the substrates farnesyl diphosphate and dimethylallyl diphosphate, whereas geranylgeranyl diphosphate and mevalonate were not transported with appreciable efficiency. Our data suggest that plastid membranes possess a unidirectional proton symport system for the export of specific isoprenoid intermediates involved in the metabolic cross talk between cytosolic and plastidial pathways of isoprenoid biosynthesis.

Liposome solubilization and membrane protein reconstitution using Chaps and Chapso

The process of liposome solubilization and reconstitution of two transport proteins have been studied using Chaps and Chapso (3-[(3-cholamidopropyl)dimethylammonio]-2- hydroxy-1-propanesulfonate). The solubilization of unilamellar liposomes was followed by absorption experiments and the process was shown to fit well to the three-stage model previously proposed for other detergents. The solubilization parameters have been determined and the detergent to phospholipid ratios at which the lamellar-to-micellar transition initiates and ends were estimated to be 0.21 mol/mol and 0.74 mol/mol, for Chapso and 0.4 mol/mol and 1.04 mol/mol for Chaps, respectively. The best conditions for the incorporation of two membrane proteins, bacteriorhodopsin and the H(+)-ATP synthase from chloroplasts, were analyzed at each step of the solubilization process. After detergent removal, the activities of the resulting proteoliposomes were measured indicating that the most efficient reconstitutions were obtained by addition of the proteins to completely solubilized lipid-detergent micelles. The use of Chapso and Chaps for membrane protein reconstitution studies provides a reproducible method of achieving active proteoliposomes, homogeneous in size, with a low permeability and thus, well suited for transport measurements.