Function of the cytosolic N-terminus of sucrose transporter AtSUT2 in substrate affinity

AtSUT2 was found to be a low-affinity sucrose transporter (K-M = 11.7 mM at pH 4), Chimeric proteins between AtSUT2 and the high-affinity StSUT1 were constructed in which the extended N-terminus and central loop of AtSUT2 wer e exchanged with those domains of StSUT1 and vice versa. Chimeras containin g the N-terminus of AtSUT2 showed significantly lower affinity for sucrose compared to chimeras containing the N-terminus of StSUT1, The results indic ate a significant function of the N-terminus but not the central cytoplasmi c loop in determining substrate affinity. Expression of AtSUT2 in major vei ns of source leaves and in flowers is compatible with a role as a second lo w-affinity sucrose transporter or as a sucrose sensor. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All r ights reserved.