The Most Abundant Glycoprotein of Amebic Cyst Walls (Jacob) Is a Lectin with Five Cys-Rich, Chitin-Binding Domains

<jats:title>ABSTRACT</jats:title> <jats:p> The infectious stage of amebae is the chitin-walled cyst, which is resistant to stomach acids. In this study an extraordinarily abundant, encystation-specific glycoprotein (Jacob) was identified on two-dimensional protein gels of cyst walls purified from <jats:italic>Entamoeba invadens</jats:italic> . Jacob, which was acidic and had an apparent molecular mass of ∼100 kDa, contained sugars that bound to concanavalin A and ricin. The <jats:italic>jacob</jats:italic> gene encoded a 45-kDa protein with a ladder-like series of five Cys-rich domains. These Cys-rich domains were reminiscent of but not homologous to the Cys-rich chitin-binding domains of insect chitinases and peritrophic matrix proteins that surround the food bolus in the insect gut. Jacob bound purified chitin and chitin remaining in sodium dodecyl sulfate-treated cyst walls. Conversely, the <jats:italic>E. histolytica</jats:italic> plasma membrane Gal/GalNAc lectin bound sugars of intact cyst walls and purified Jacob. In the presence of galactose, <jats:italic>E. invadens</jats:italic> formed wall-less cysts, which were quadranucleate and contained Jacob and chitinase (another encystation-specific protein) in secretory vesicles. A galactose lectin was found to be present on the surface of wall-less cysts, which phagocytosed bacteria and mucin-coated beads. These results suggest that the <jats:italic>E. invadens</jats:italic> cyst wall forms when the plasma membrane galactose lectin binds sugars on Jacob, which in turn binds chitin via its five chitin-binding domains. </jats:p>