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.