Characterization of spermidine binding to solubilized plasma membrane proteins from zucchini hypocotyls

In this work [14C]spermidine binding to total proteins solubilized from plasma membrane purified from zucchini (Cucurbita pepo I.) hypocotyls was investigated. Proteins were solubilized using octyl glucosidc as a detergent. Specific polyamine binding was thermolabile, reversible, pH dependent with an optimum at pH 8.0, and had a Kd value of 5 μM, as determined by glass-fiber-filter assays. Sephadcx G-25 M gel-filtration assays confirmed the presence of a spermidine-protein(s) complex with a specific binding activity. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis and native polyacrylamide gel electrophoresis of collected fractions having the highest specific spermidine-binding activity, several protein bands (113, 75, 66, and 44 kD) were identified. The specificity of spermidine binding was examined by gel-filtration competition experiments performed using other polyamines and compounds structurally related to spermidine. Partial purification on Sephadex G-200 led to the identification of 66- and 44-kD protein bands, which may represent the putative spermidine-binding protein(s) on the plasmalemma.