Apicomplexan F-actin is required for efficient nuclear entry during host cell invasion

The obligate intracellular parasites Toxoplasma gondii and Plasmodium spp . invade host cells by injecting a protein complex into the membrane of the targeted cell that bridges the two cells through the assembly of a ring-like junction. This circular junction stretches while the parasites applies a traction force to pass through; a step that typically concurs with transient constriction of the parasite body. Here we show that the junction can oppose resistance to the passage of the parasite’s nucleus. Super-resolution microscopy and real time imaging highlighted an F-actin pool at the apex of pre-invading parasite, an F-actin ring at the junction area during invasion but also networks of perinuclear and posteriorly localized F-actin. Mutant parasites with dysfunctional acto-myosin showed significant decrease of junctional and perinuclear F-actin and are coincidently affected in nuclear passage through the junction. We propose that the F-actin machinery eases nuclear passage by stabilising the junction and pushing the nucleus through the constriction, providing first evidence for a dual contribution of actin-forces during host cell invasion by apicomplexan parasites.