Restricted sugar uptake by sugar-induced internalization of the yeast lactose/galactose permease Lac12

Kluyveromyces lactis Lac12 permease mediates lactose and low-affinity galactose transports. In this study we investigated the effects of carbon sources on internalization of Lac12 using a LAC12-GFP fusion construct. When galactose- or lactose-grown cells are shifted to a fresh sugar medium, Lac12-GFP is removed from the plasma membrane and is localized intracellularly. Surprisingly, either galactose or lactose in the new media caused the internalization, and cells responded differently to these two sugars. Our results reveal that this process is dependent on sugar species and also sugar concentration. Lac12-GFP internalization causes reduction of C(14) ]lactose uptake rates and also occurs in a Klsnf1 mutant strain; it is therefore independent of KlSnf1 activity. We suggest that glucose-6-phosphate is the intracellular signal, as internalization was induced by 2-deoxyglucose, and inhibition of phosphoglucomutase by lithium prevented galactose- but not lactose- or glucose-induced internalization. Lac12-GFP internalization was not triggered by 6-deoxyglucose, and was irreversible in the absence of protein synthesis.