Clustering of host N-glycans by the microneme MIC1/4/6 complex licenses Toxoplasma rhoptry discharge.

To initiate the process of invasion, apicomplexan parasites must discharge the contents of specialized organelles called rhoptries into host cells. This process requires prior recognition and binding of the host cell by proteins released from another set of parasite organelles, the micronemes. However, the host-parasite interactions required for rhoptry discharge are largely unknown. Here, we performed a host cell-directed genome-wide screen for factors that mediate rhoptry discharge from Toxoplasma gondii, the causative agent of toxoplasmosis. The screen identified host N-glycosylation and cholesterol biosynthesis as pathways required for normal rhoptry discharge. A trimeric microneme complex, MIC1/4/6, interfaces with both pathways by binding host N-glycans to cluster proteins in a process dependent on host plasma membrane cholesterol. The process can be inhibited by depletion of host cholesterol or competition with exogenous glycans. This clustering of host factors by MIC1/4/6 likely prepares the host membrane for rhoptry discharge, delineating a new step in the Toxoplasma invasion process.