Glucokinase and glucokinase regulatory protein: mutual dependence for nuclear localization

Conditional expression of the glucokinase regulatory protein in insulinoma cells, under control of the reverse tetracycline-dependent transactivator, was used to investigate whether expression of this protein de novo would alter the intracellular distribution of glucokinase. The regulatory protein, which was undetectable in the basal state, could be induced by doxycycline to levels comparable to those of liver and was detected mostly in the nucleus. Concomitantly, glucokinase accumulated in the nucleus. Human embryonic kidney cells were transiently transfected to express glucokinase and the regulatory protein, either separately or together. Each protein localized predominantly to the cytoplasm when expressed alone. On co-expression, however, both proteins localized virtually entirely to the nucleus. The enzymic activity of glucokinase was not required for promoting nuclear import of the two proteins, as shown with a glucose-phosphorylation-deficient mutant. Finally, in embryonic kidney cells expressing the regulatory protein alone, treatment with leptomycin B resulted in a partial redistribution of the protein from the cytoplasm to the nucleus, suggesting that this protein can shuttle between the two compartments.