The several-micrometer-sized Toxoplasma gondii protozoan parasite invades virtually any type of nucleated cell from a warm-blooded animal within seconds. Toxoplasma initiates the formation of a tight ring-like junction bridging its apical pole with the host cell membrane. The parasite then actively moves through the junction into a host cell plasma membrane invagination that delineates a nascent vacuole. Recent high resolution imaging and kinematics analysis showed that the host cell cortical actin dynamics occurs at the site of entry while gene silencing approaches allowed motor-deficient parasites to be generated, and suggested that the host cell could contribute energetically to invasion. In this study we further investigate this possibility by analyzing the behavior of parasites genetically impaired in different motor components, and discuss how the uncovered mechanisms illuminate our current understanding of the invasion process by motor-competent parasites.
By simultaneously tracking host cell membrane and cortex dynamics at the site of interaction with myosin A-deficient Toxoplasma, the junction assembly step could be decoupled from the engagement of the Toxoplasma invasive force. Kinematics combined with functional analysis revealed that myosin A-deficient Toxoplasma had a distinct host cell-dependent mode of entry when compared to wild-type or myosin B/C-deficient Toxoplasma. Following the junction assembly step, the host cell formed actin-driven membrane protrusions that surrounded the myosin A-deficient mutant and drove it through the junction into a typical vacuole. However, this parasite-entry mode appeared suboptimal, with about 40 % abortive events for which the host cell membrane expansions failed to cover the parasite body and instead could apply deleterious compressive forces on the apical pole of the zoite.
This study not only clarifies the key contribution of T. gondii tachyzoite myosin A to the invasive force, but it also highlights a new mode of entry for intracellular microbes that shares early features of macropinocytosis. Given the harmful potential of the host cell compressive forces, we propose to consider host cell invasion by zoites as a balanced combination between host cell membrane dynamics and the Toxoplasma motor function. In this light, evolutionary shaping of myosin A with fast motor activity could have contributed to optimize the invasive potential of Toxoplasma tachyzoites and thereby their fitness.