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

Abstract

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 were exchanged with those domains of StSUT1 and vice versa. Chimeras containing the N‐terminus of AtSUT2 showed significantly lower affinity for sucrose compared to chimeras containing the N‐terminus of StSUT1. The results indicate a significant function of the N‐terminus but not the central cytoplasmic loop in determining substrate affinity. Expression of AtSUT2 in major veins of source leaves and in flowers is compatible with a role as a second low‐affinity sucrose transporter or as a sucrose sensor.