Identification of Membrane Protein Associated with Sucrose Transport Into Cells of Developing Soybean Cotyledons

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Abstract
The photolyzable sucrose derivative 6''-deoxy-6''-(4-azido-2-hydroxy)-benzamidosucrose (6''-HABS), competitively inhibited the influx of [14C) sucrose into protoplasts from developing soybean (Glycine max L. Merr cv Wye) cotyledons. Photolysis of 125I-labeled 6''-HABS in the presence of 10 millimolar dithiothreitol and microsomal preparations from developing soybean cotyledons led to label incorporation into a moderately abundant membrane protein with an apparent molecular mass of about 62 kilodalton (kD) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 62 kD protein was partially protected from labeling by the inclusion of 100 millimolar sucrose in the photolysis medium and also by the inclusion of 10 millimolar phenyl .alpha.-D-thioglucopyranoside. Glucose, raffinose, or phenyl .alpha.-D-3-deoxy-3-fluoroglucopyranoside did not afford even partial protection from labeling. When the photolyzable moiety of 6''-HABS was attached to 6-deoxy-6-aminoglucose and 125I labeled, the resulting photoprobe did not label the 62 kD protein above background. The labeled protein at 62 kD is therefore apparently a specific, sucrose binding protein. Sucrose influx into cotlyedons of less than 25 milligrams fresh weight (approximately 10 days after flowering) occurred by passive processes, but metabolically dependent uptake became dominant over the next 5 to 7 days of development. Both the Coomassive staining protein at 62 kD and label incorporation at that position in analysis of membrane proteins appeared concomitant with the onset of active sucrose influx. Polyclonal antibodies to the purified 62 kD protein bound specifically to a protein in the plasmalemma of thin sections prepared from cotyledons and density stained with colloidal gold-protein A. The results suggest that the 62 kD membrane protein is associated with sucrose transport and may be the plasmalemma sucrose transporter.