Binding and internalization of rat colonic mucins by the galactose/N-acetyl-D-galactosamine adherence lectin of Entamoeba histolytica.

Purified rat colonic mucins inhibit Entamoeba histolytica in vitro adherence to and lysis of colonic epithelial cells by binding to the amoebic galactose/N-acetyl-D-galactosamine (Gal/GalNAc)-inhibitable adherence lectin. We found that 125I-labeled mucins demonstrated saturable Gal-specific binding to E. histolytica trophozoites (strain HM1:IMSS), with 2.8 x 10(3) binding sites per amoeba and a dissociation constant of 8.20 x 10(-11) M-1. Inhibition of parasite protein synthesis completely abrogated mucin binding; elevation of amoebic vesicle pH with ammonium chloride (10 mM) had no effect. Surface-bound 125I-labeled mucins were rapidly internalized and released from amoebae without evidence of proteolytic degradation. Three avirulent HM1 clones contained immunoreactive Gal/GalNAc lectin molecules with high-affinity binding of 125I-labeled mucins but exhibited markedly reduced rates of uptake and exocytosis of bound mucins. High-affinity binding by the Gal/GalNAc adherence lectin was followed by rapid internalization and eventual release of the colonic mucins. Additionally, defects in lectin surface expression and endocytosis were found in the avirulent clones.