Edible mushroom (Agaricus bisporus) lectin, which reversibly inhibits epithelial cell proliferation, blocks nuclear localization sequence-dependent nuclear protein import

The Gal beta 1-3GalNAc alpha (TF antigen)-binding lectin (ABL) from the com mon edible mushroom (Agaricus bisporus) has a potent anti-proliferative eff ect without any apparent cytotoxicity, This unusual combination of properti es prompted investigation of its mechanism of action. In contrast to solubl e lectin, agarose-immobilized, and hence noninternalizable ABL had no effec t on proliferation of HT29 colon cancer cells. Electron microscopy of HT29 cells incubated with fluorescein- and gold-conjugated ABL showed internaliz ation of the lectin into endocytotic vesicles and multivesicular bodies. Co nfocal microscopy showed perinuclear accumulation of fluorescein isothiocya nate-conjugated lectin, which also inhibits HT29 cell proliferation, raisin g the possibility that the lectin might interfere with nuclear pore functio n. Transport of heat shock protein 70 into the nucleus in response to heat shock was blocked by preincubation of HT29 cells for 6 h with 40 mu g/ml AB L, In digitonin-permeabilized cells, nuclear uptake of bovine albumin conju gated to a nuclear localization sequence (NLS)-containing peptide was also inhibited by a 15-min preincubation with 40-100 mu g/ml ABL, In contrast, s erum-stimulated nuclear translocation of mitogen-activated protein kinase, which is NLS-independent, was not affected by pretreatment of cells with th e lectin, These results suggest that the anti-proliferative effect of ABL i s Likely to be a consequence of the lectin trafficking to the nuclear perip hery, where it blocks NLS-dependent protein uptake into the nucleus.