Mutants of interest in studies on host cell invasion by apicomplexans.

<p>Mutants of interest in studies on host cell invasion by apicomplexans.</p

Molecular models of apicomplexan gliding and invasion.

<p><b>A.</b> The parasite motor (glideosome) is located in the space between the parasite plasma membrane (PPM) and the inner membrane complex (IMC) apposed to the microtubules. Gliding motility is mediated by the binding of the ectodomain of transmembrane TRAP-family proteins to a solid substrate, while the cytoplasmic tail of the protein is linked to the parasite motor. The integrity of the glideosome is maintained by the gliding-associated protein 45 (GAP45), which is anchored to the PPM at one end and to the IMC, via GAPs 40 and 50, at the other end. The link between the GAPs, and ultimately the IMC, to actin is provided by Myosin A (MyoA) and the MyoA Light Chain 1 (MLC1). The movement of the cell is the consequence of the capping, by myosin-actin activity, of the TRAP-family protein. <b>B.</b> The model of invasion seen as the junction structured by the AMA1-RON complex. The figure on the left shows a <i>Toxoplasma</i> tachyzoite invading a host cell. The arrow indicates the direction of movement. Immunostaining of surface MIC2 (sMIC2) stains the part of the zoite cell still extracellular (blue), while the rest of the cell, already internalized, is not stained. Immunostaining of total RON4 (tRON4, red) marks the junction as a ring at the point of constriction, indicated by the circle, and the rhoptries at the apical pole of the zoite cell. After a first step of adhesion to the host cell plasma membrane (HCPM) mediated by parasite surface adhesins and host cell surface receptors, the binding of the transmembrane protein AMA1 to RON2, inserted at the host cell membrane and complexed with RONs 4 and 5, forms the junction. The link to the parasite motor is as in (A), while host actin recruited at the junction provides the link to the host cell cytoskeleton. The movement of the zoite towards the interior of the newly formed parasitophorous vacuole membrane (PVM) is thus a consequence of the capping of AMA1, which would be anchored at the junction by binding to RON2. <b>C.</b> Models of zoite invasion in which the functions of AMA1 and RONs are dissociated. Color codes and acronyms are as in (A) and (B). After a first step of adhesion mediated by parasite surface adhesins and host cell surface receptors, AMA1 binding to a host cell receptor provides a strong attachment between the zoite and host cell membranes, possibly leading to reorientation of the zoite to allow junction formation. Three different hypotheses could then explain junction formation: 1. A still-unknown transmembrane parasite protein binds to the motor and to RON2, taking the place previously assigned to AMA1. 2. Unknown proteins structure the junction and connect the parasite motor to the host cell cortical actin, in which case the role of the RONs at the junction is not structural. 3. Unknown proteins structure the junction without a role of the parasite motor during invasion.</p