The isolation and characterization of right-side-out plasma membrane vesicles from barley aleurone cells

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Department of Chemistry


Examination of organelle- and membrane-specific processes such as signal transduction necessitates the use of plasma membrane vesicles with cytoplasmic side-in orientation. We are interested in the structural identity and subcellular localization of in vivo [32P]phosphoric acid ([32Pi])- labeled phosphoinositides, including the recently discovered phosphatidyl- scyllo-inositol, for signal transduction studies. In the first part of this investigation, plasma membrane vesicles from barley aleurone cells were isolated employing the aqueous polymer (Dextran and polyethylene glycol) two- phase partition method. The membrane vesicles that partitioned into the upper and lower phases of the aqueous polymer two-phase system were characterized and the purity of the vesicles ascertained by assaying for two marker enzymes, K+-stimulated, Mg2+-dependent adenosine triphosphatase (EC, ATPase), localized in the plasma membranes, and cytochrome c oxidase, localized in the mitochondria. Inhibitors for ATPases such as azide, molybdate, and vanadate were used to distinguish between plasma membrane- associated and intracellular membrane-associated ATPases. These inhibitor studies suggest that the plasma membrane preparation contained about 7% of intracellular membrane vesicles and the intracellular membrane fraction contained about 6% of plasma membrane vesicles. Orientation of the plasma membrane vesicles was ascertained by measuring the latent ATPase activity. These latency studies suggest that about 95% of the plasma membrane vesicles were of cytoplasmic side-in orientation. In the second part of this investigation, intracellular distribution and in vivo [32Pi] labeling of phosphoinositides in the plasma membranes and intracellular membranes were investigated. Preferential accumulation of [32Pi-labeled phosphatidyl-myo- inositol monophosphate (myo-PIP) and phosphatidyl-myo-inositol bisphosphate (myo-PIP2) was observed in the plasma membrane. However, scyllo- phosphatidylinositol (scyllo-PI) was detected in both the plasma membrane and the intracellular membranes. The cellular concentration of myo- phosphoinositides was determined, and, after 24 h of labeling with [32Pi], the ratio of radiolabel in myo-PI, PIP, and PIP2 paralleled the relative concentrations in aleurone cells.

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