Previous results (TJ Buckhout, Planta [1989] 178: 393-399) indicated that the structural specificity of the H+-sucrose symporter on the plasma membrane from sugar beet leaves (Beta vulgaris L.) was specific for the sucrose molecule. To better understand the structural features of the sucrose molecule involved in its recognition by the symport carrier, the inhibitory activity of a variety of phenylhexopyranosides on sucrose uptake was tested. Three competitive inhibitors of sucrose uptake were found, phenyl-a-D-glucopyranoside, phenyl-a-D-thioglucopyranoside, and phenyl-a-D-4-deoxythioglucopyranoside (PDTGP; Ki = 67, 180, and 327 micromolar, respectively). The Km for sucrose uptake was approximately 500 micromolar. Like sucrose, phenyla-D-thioglucopyranoside and to a lesser extent, PDTGP induced alkalization of the external medium, which indicated that these derivatives bound to and were transported by the sucrose symporter. Phenyl-a-D-3-deoxy-3-fluorothioglucopyranoside, phenyla-D-4-deoxy-4-fluorothioglucopyranoside, and phenyl-a-D-thioallopyranoside only weakly but competively inhibited sucrose uptake with K1 values ranging from 600 to 800 micromolar, and phenyl-a-D-thiomannopyranoside, phenyl-,6-D-glucopyranoside, and phenylethyl-,8-D-thiogalactopyranoside did not inhibit sucrose uptake. Thus, the hydroxyl groups of the fructose portion of sucrose were not involved in a specific interaction with the carrier protein because phenyl and thiophenyl derivatives of glucose inhibited sucrose uptake and, in the case of phenyl-a-D-thioglucopyranoside and PDTGP, were transported.The active transport of sucrose into the phloem tissues of higher plants drives the export of photosynthate carbon and energy to nonphotosynthetic tissues. Numerous studies of sucrose uptake into intact tissues suggest that uptake is carriermediated and catalyzed by a H+-sucrose symport (9). Confirmation of this inechanism of sucrose uptake has been ob- ' Supported by a grant from the Deutsche Forschungsgemeinschaft to T.J.B.2 Present address: Biology Department, Francis Marion College, Florence, SC 29501. tained in PM3 preparations from sugar beet leaves (2, 3, 13), Ricinus cotyledons (19), and spinach leaves (18). In these examples, sucrose uptake into PM vesicles is dependent on a pH gradient, alkaline inside, is electrogenic, and is specific for the sucrose molecule. It also has been demonstrated that sucrose uptake induces a transient alkalization of the exterior medium and that the characteristics of sucrose-induced alkalization and pH-dependent sucrose uptake are identical (4,17). The stoichiometry of sucrose to H+ transport is 1:1 (4, 17). At this time, however, equating the mechanism of sucrose uptake in isolated vesicles with that of phloem loading in vivo is not possible.The chemical features on the sucrose molecule that are involved in the specific interaction of sucrose with the protein carrier are critical to the understanding of the molecular mechanism of sucrose transport. Phloem loading in sugar beets (8) and maize...
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