Export of galectin-3 from nuclei of digitonin-permeabilized mouse 3T3 fibroblasts

Galectin-3 is a galactose-/lactose-binding protein (M-r similar to 30,000), identified as a required factor in the splicing of pre-mRNA. Immunofluores cence staining revealed that galectin-3 distributes differentially between the nucleus and the cytoplasm, depending on the proliferative state of the cells under analysis. Using digitonin-permeabilized mouse 3T3 fibroblasts, we provide evidence that galectin-3 is rapidly and selectively exported fro m the nucleus. Although both phosphorylated and nonphosphorylated isoforms of galectin-3 are found in the nuclear fraction, only phosphorylated galect in-3 is identified in the exported fraction, implying that phosphorylation is important for the nuclear export of the protein. The rate of galectin-3 export is temperature dependent and is decreased by the addition of wheat g erm agglutinin. More strikingly, galectin-3 export can be inhibited by the addition of lepto-mycin B, a drug that disrupts the interaction between the leucine-rich nuclear export signal and its receptor, CRM1 (chromosome main tenance region 1). Indeed, a putative leucine-rich nuclear export signal ca n be found in residues 241-249 of the murine galectin-3 sequence. Finally, gel filtration of the exported material showed that galectin-3 can be found in at least two high molecular weight complexes (similar to 650 and simila r to 60 kDa), both of which can be disrupted by lactose. (C) 1999 Academic Press.