Evidence for a "Wattle and Daub" Model of the Cyst Wall of Entamoeba

Abstract

The cyst wall of Entamoeba invadens (Ei), a model for the human pathogen Entamoeba histolytica, is composed of fibrils of chitin and three chitin-binding lectins called Jacob, Jessie3, and chitinase. Here we show chitin, which was detected with wheat germ agglutinin, is made in secretory vesicles prior to its deposition on the surface of encysting Ei. Jacob lectins, which have tandemly arrayed chitin-binding domains (CBDs), and chitinase, which has an N-terminal CBD, were each made early during encystation. These results are consistent with their hypothesized roles in cross-linking chitin fibrils (Jacob lectins) and remodeling the cyst wall (chitinase). Jessie3 lectins likely form the mortar or daub of the cyst wall, because 1) Jessie lectins were made late during encystation; 2) the addition to Jessie lectins to the cyst wall correlated with a marked decrease in the permeability of cysts to nucleic acid stains (DAPI) and actin-binding heptapeptide (phalloidin); and 3) recombinant Jessie lectins, expressed as a maltose-binding proteins in the periplasm of Escherichia coli, caused transformed bacteria to agglutinate in suspension and form a hard pellet that did not dissociate after centrifugation. Jessie3 appeared as linear forms and rosettes by negative staining of secreted recombinant proteins. These findings provide evidence for a "wattle and daub" model of the Entamoeba cyst wall, where the wattle or sticks (chitin fibrils likely cross-linked by Jacob lectins) is constructed prior to the addition of the mortar or daub (Jessie3 lectins). Author SummaryParasitic protists, which are spread by the fecal-oral route, have cyst walls that resist environmental insults (e.g. desiccation, stomach acids, bile, etc.). Entamoeba histolytica, the cause of amebic dysentery and liver abscess, is the only protist characterized to date that has chitin in its cyst wall. We have previously characterized Entamoeba chitin synthases, chitinases, and multivalent chitin-binding lectins called Jacob. Here we present evidence that the Entamoeba Jessie3 lectin contributes to the mortar or daub, which makes the cyst wall impenetrable to small molecules. First, the Jessie3 lectin was made after chitin and Jacob lectins had already been deposited onto the surface of encysting Entamoeba. Second, cysts became impenetrable to small molecules at the same time that Jessie3 was deposited into the wall. Third, recombinant Jessie3 lectins self-aggregated and caused transfected bacteria to agglutinate. These results suggest a "wattle and daub" model of the Ei cyst wall, where the wattle or sticks (chitin fibrils likely cross-linked by Jacob lectins) is constructed prior to the addition of the mortar or daub (Jessie3 lectins).