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 Factin 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. Abbreviation: CDCytochalasin-D, CLEM corelative light and electron microscopy, TJ tight junctional ring, SR-SIM Super Resolution -Structure Illumination Microscopy, Cb Chromobody, FP Fluorescent protein,

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