Fig. 7. Two models of the molecular motility machinery in apicomplexan zoites. The diagrams show the periphery of a Plasmodium sporozoite and the possible arrangements of identified and hypothetical intracellular components of the motor complex. (A) In the currently prevailing model, actin filaments are tethered to the outer membrane of the IMC by a hypothetical protein. The cytoplasmic domain of TRAP directly or indirectly interacts with the tail of MyoA. The MyoA head domain interacts with actin filaments and moves towards the plus end of the filaments. This leads to displacement of the MyoA/TRAP complex from anterior to posterior and results in a forward movement of the zoite. (B) In the alternative model the N-terminal portion of MTIP anchors it to the outer membrane of the IMC by interaction with a hypothetical protein at the IMC. MTIP binds the tail domain of MyoA, immobilizing it, and this determines MyoA orientation with the head domain projecting outward. The head domain interacts with short actin filaments that are directly or indirectly linked to the cytoplasmic domain of TRAP. The MyoA head domain interacts with actin filaments and moves towards the plus end. Because MyoA is fixed to the IMC, the actin/TRAP complex is displaced from anterior to posterior resulting in a forward movement of the zoite. Note that in model A, the plus end of the actin filaments is oriented towards the posterior of the zoite. In model B the plus end of the actin filaments is oriented towards the anterior end of the zoite. IMC, inner membrane complex; MTIP, MyoA tail domain interacting protein; MyoA, myosin A; TRAP, thrombospondin-related anonymous protein.