Sucrose transporters in plants: update on function and structure

In plants, sucrose is the major transport form for photoassimilated carbon and is both a source of carbon skeletons and energy for plant organs unable to perform photosynthesis (sink organs). As a molecule translocated over d istance, sucrose has to pass through a number of membranes. Membrane transp ort of sucrose has therefore been considered for a long time as a major det erminant of plant productivity. After several decades of physiological and biochemical experiments measuring the activity of sucrose carriers, unequiv ocal evidence came from the first identification of a cDNA coding a sucrose carrier (SoSUT1, Riesmeier et al. (1992) EMBO J. 11, 4705-4713). At presen t 20 different cDNAs encoding sucrose carriers have been identified in diff erent plant species, in both dicots and monocots (one case). The total numb er is increasing rapidly and most importantly, it can be guessed from the r esults obtained for Arabidopsis, that in each species, sucrose transporters represent a gene family. The sequences are highly conserved and those carr iers display the typical 12 transmembrane alpha-helices of members of the M ajor Facilitator superfamily. Yeast expression of those carriers indicate t hat they are all influx carriers, all cotransport sucrose and proton and th at their affinity for sucrose is surprisingly similar (0.2-2 mM). All their characteristics are in agreement with those demonstrated at the physiologi cal level in plants. These characteristics are discussed in relation to the function in plants and the few data available on the structure of those tr ansporters in relation to their function are presented. (C) 2000 Elsevier S cience B.V. All rights reserved.