Sucrose transport into plasma membrane vesicles from tobacco leaves by H+ symport or counter exchange does not display a linear component

Uptake of [C-14] sucrose by plasma membrane vesicles from leaves of tobacco (Nicotiana tabacum L.) was measured after the imposition of an inwardly directed proton gradient (DeltapH = 2) and an electrical gradient (Deltapsi = -68 mV, inside negative) across the vesicle membrane. The vesicles were isolated from a microsomal fraction by two-phase partitioning using media that contained 330 mm of either sorbitol or sucrose. Sucrose transport into vesicles isolated using the sorbitol-containing media showed the hallmarks of electrogenic H+-symport, as it was highly dependent on DeltapH, could be increased three- to four-fold by A, and was abolished by carbonylcyanide m-chlorophenylhydrazone (CCCP). Transport of [C-14]sucrose into vesicles that were isolated using the sucrose-containing media apparently occurred by counter exchange. Its initial influx also depended on a low external pH, but it was insensitive to CCCP and hardly stimulated by A. Both symport and counter exchange obeyed simple Michaelis-Menten kinetics. Transport that depends linearly on the external sucrose concentration could not be detected, indicating that the 'linear' component that has been observed in sucrose uptake by leaf tissues does not represent a transport route that is provided by the sucrose symporter. The potential role of H+/sucrose-symporters in phloem unloading is briefly discussed.