An Hg-sensitive channel mediates the diffusional component of glucose transport in olive cells

In several organisms solute transport is mediated by the simultaneous operation of saturable and non-saturable (diffusion-like) uptake, but often the nature of the diffusive component remains elusive. The present work investigates the nature of the diffusive glucose transport in Olea europaea cell cultures. In this system, glucose uptake is mediated by a glucose-repressible, H+-dependent active saturable transport system that is superimposed on a diffusional component. The latter represents the major mode of uptake when high external glucose concentrations are provided. In glucose-sufficient cells, initial velocities of d- and l-[U-14C]glucose uptake were equal and obeyed linear concentration dependence up to 100 mM sugar. In sugar starved cells, where glucose transport is mediated by the saturable system, countertransport of the sugar pairs 3-O-methyl-d-glucose/d-[U-14C]glucose and 3-O-methyl-d-glucose/3-O-methyl-d-[U-14C]glucose was demonstrated. This countertransport was completely absent in glucose-sufficient cells, indicating that linear glucose uptake is not mediated by a typical sugar permease. The endocytic inhibitors wortmannin-A and NH4Cl inhibited neither the linear component of d- and l-glucose uptake nor the absorption of the nonmetabolizable glucose analog 3-O-methyl-d-[U-14C]glucose, thus excluding the involvement of endocytic mediated glucose uptake. Furthermore, the formation of endocytic vesicles assessed with the marker FM1–43 proceeded at a very slow rate. Activation energies for glucose transport in glucose sufficient cells and plasma membrane vesicles were 7 and 4 kcal mol− 1, respectively, lower than the value estimated for diffusion of glucose through the lipid bilayer of phosphatidylethanolamine liposomes (12 kcal mol− 1). Mercury chloride inhibited both the linear component of sugar uptake in sugar sufficient cells and plasma membrane vesicles, and the incorporation of the fluorescent glucose analog 2-NBDG, suggesting protein-mediated transport. Diffusive uptake of glucose was inhibited by a drop in cytosolic pH and stimulated by the protein kinase inhibitor staurosporine. The data demonstrate that the low-affinity, high-capacity, diffusional component of glucose uptake occurs through a channel-like structure whose transport capacity may be regulated by intracellular protonation and phosphorylation/dephosphorylation.