Characterization of low- and high-affinity glucose transports in the yeast Kluyveromyces marxianus

Glucose transport in the yeast Kluyveromyces marxianus proceeds by two functionally and presumably structurally distinct transporters depending on the carbon source of the culture medium. In lactose-grown cells, glucose was taken up through a high-affinity H+-sugar symporter (Km = 0.09 mM), whereas a low-affinity transporter (Km = 3.5 mM) was utilized in glucose-grown cells. The two transporters exhibited different substrate specificities. Galactose was demonstrated to be a selective substrate of the H+-glucose symporter (Km = 0.14 mM) and did not significantly enter glucose-grown cells. Fructose was a preferential substrate of the low-affinity carrier (Km = 3.5 mM), but it entered lactose-grown cells through a high-affinity H+-fructose symporter distinct from the H+-glucose one. Other putative substrates of the two glucose transporters were identified by competition experiments. 2-Deoxyglucose recognized both carriers with a similar affinity, while the non-phosphorylatable analogues 6-deoxyglucose, 3-O-methylglucose and D-fucose exhibited a 10-30 fold preference for the high-affinity transporter.