Functional characterization of two ripening-related sucrose transporters from grape berries

The ripening of grape (Vitis vinifera L.) berries is associated with a larg e accumulation of glucose and fructose in the vacuoles of the fruit cells. These hexoses are derived from sucrose, which is released from the phloem a nd may be taken up by parenchyma cells prior to hydrolysis. We have express ed two putative ripening-related sucrose transporters from grape berries, V vSUC11 (synonymous with VvSUT1) and VvSUC12, in an invertase deficient yeas t strain to characterize their transport activities. Sucrose was taken up b y yeast transformed with either transporter at an optimum pH of < 4.5 and w ith a Michaelis constant (K-m) of 0.9-1.4 mM. The uptake of sucrose through VvSUC11 and VvSUC12 was inhibited by protonophores and by vanadate. This i s consistent with an active uptake mechanism involving proton cotransport, typical of sucrose/H+ symporters. The transporters from grape berries were functionally similar to Scr1, a sucrose transporter from Ricinus cotyledons . It is likely that in grape berries VvSUC11 and VvSUC12 facilitate the loa ding of sucrose from the apoplast into the parenchyma cells. (C) 2001 Annal s of Botany Company.