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Kinetic Characteristics of the Two Glucose Transport Systems in Neurospora crassa

Glucose is transported across the cell membrane of Neurospora crassa by two physiologically and kinetically distinct transport systems. System II is repressed by growth of the cells in 0.1 m glucose. System I is synthesized constitutively. The apparent Km for glucose uptake by system I and system II are 25 and 0.04 mm, respectively. Both uptake systems are temperature dependent, and are inhibited by NaN3 and 2,4-dinitrophenol. Glucose uptake by system II was not inhibited by fructose, galactose, or lactose. However, glucose was shown to be a noncompetitive inhibitor of fructose and galactose uptake. The transport rate of [14C]3-0-methyl-d-glucose (3-0-MG) was higher in cells preloaded with unlabeled 3-0-MG than in control cells. The rate of entry of labeled 3-0-MG was only slightly inhibited by the presence of NaN3 in the medium. Further, NaN3 caused a rapid efflux of accumulated [14C]3-0-MG. These data imply that the energetic step in the transport process prevents efflux.