Membrane preparation has been widely used for characterization the membrane proteins. Membrane fractions can be separated by a combination of differential and density-gradient centrifugation techniques (Hodges et al., 1972; Leonard and Vanderwoude, 1976). Here we firstly describe a method to isolate total microsomal fractions including plasma membrane, intracellular vesicles, Golgi membranes, endoplasma reticulum, and tonoplast (vacuolar membrane) from 5-7 days old seedlings, which is often analyzed for auxin transporters in Arabidopsis (Leonard and Vanderwoude, 1976; Titapiwatanakun, et al., 2009; Yang et al., 2013; Blakeslee et al., 2007). After homogenization, plant debris including cell walls, chloroplasts and nucleus were removed by low speed centrifugation (8,000 x g), then total microsomal membranes were pelleted by high speed centrifugation (10,000 x g) and separated from soluble fractions. We secondly describe a method to separate microsomal fractions according to size or density in a sucrose density-gradient system by centrifugation. The linear sucrose gradient from 20%-55% (1.09-1.26 g cm-3) were used to separate membranes with different densities: tonoplast, 1.10-1.12 cm-3, Golgi membranes, 1.12-1.15 cm-3, rough endoplasmic reticulum 1.15-1.17 cm-3, thylakoids, 1.16-1.18 cm-3, plasma membrane, 1.14-1.17 g cm-3, and mitochondrial membranes, 1.18-1.20 cm-3 (Leonard and Vanderwoude, 1976; Larsson et al., 1987; Briskin and Leonard, 1980). However, the plasma membrane can also be isolated according to its outer surface properties which are very different from intracellular membrane surfaces. Thus, the right-side-out plasma membrane vesicles can be separated in an aqueous Dextran-polyethylene glycol two-phase system. The plasma membranes can be purified to > 90% in the upper phase (Larsson et al., 1987; Alexandersson et al., 2008). Two-phase systems for Arabidopsis seedlings were described in the section 3. Sucrose density gradient membrane fractionation followed by western blot is often used to analyze the distribution of certain membrane protein, while Two-phase separation is used when high purity of plasma membrane or intracellular membrane is required.
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