Exportin 1-independent nuclear export of GAPDH.

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme of the glycolytic pathway. Recent studies have demonstrated an additional role in apoptosis: GAPDH is targeted to the nucleus during apoptotic signalling. This nuclear transport has also been observed in serum-depleted cells, but it is reversible in fibroblasts, in contrast to apoptotic-induced transport (Eur J Cell Biol 80 (2001) 419). Here, we analyse the serum depletion-induced transport processes of GAPDH in NIH 3T3 cells. Prolonged serum depletion did not cause cell death, nuclear fragmentation (hoechst staining) or a significant increase in DNA strand-breaks (comet assay). Using cells expressing green fluorescent protein (GFP)-tagged GAPDH allowed us to monitor its intracellular localisation by confocal laser scanning microscopy (CLSM). Treatment of cells with the exportin1 inhibitor leptomycin B (LMB) did not influence cytoplasmic localisation of GFP-GAPDH, indicating that nuclear targeting of GAPDH is not constitutive and may be altered via a serum-dependent regulatory export process. Suprisingly, the export of nuclear GFP-GAPDH after re-addition of serum to starved cells was not prevented by LMB. Thus, nuclear export of GAPDH upon serum depletion is not mediated by exportin1.