Invasion by Toxoplasma gondii establishes a moving junction that selectively excludes host cell plasma membrane proteins on the basis of their membrane anchoring

The protozoan parasite Toxoplasma gondii actively penetrates its host cell by squeezing through a moving junction that forms between the host cell pla sma membrane and the parasite. During invasion, this junction selectively c ontrols internalization of host cell plasma membrane components into the pa rasite-containing vacuole. Membrane lipids flowed past the junction, as sho wn by the presence of the glycosphingolipid G(M1) and the cationic lipid la bel 1.1'-dihexadecyl-3-3'-3-3'-tetramethylindocarbocyanine (DiIC(16)). Glyc osylphosphatidylinositol (GPI)-anchored surface proteins, such as Sca-1 and CD55, were also readily incorporated into the parasitophorous vacuole (PV) . In contrast, host cell transmembrane proteins, including CD44, Na+/K+ ATP ase, and beta 1-integrin, were excluded from the vacuole. To eliminate pote ntial differences in sorting due to the extracellular domains, parasite inv asion was examined in host cells transfected with recombinant forms of inte rcellular adhesion molecule 1 (ICAM-1, CD54) that differed in their mechani sm of membrane anchoring. Wild-type ICAM-1, which contains a transmembrane domain, was excluded from the PV, whereas both GPI-anchored ICAM-1 and a mu tant of ICAM-1 missing the cytoplasmic tail (ICAM-1-Cyt(-)) were readily in corporated into the PV membrane. Our results demonstrate that during host c ell invasion, Toxoplasma selectively excludes host cell transmembrane prote ins at the moving junction by a mechanism that depends on their anchoring i n the membrane, thereby creating a nonfusigenic compartment.