Virulence of apicomplexan parasites relies on actomyosin-based motility to traverse through host tissue, invade and ultimately egress host cells. Motility is initiated in response to intracellular calcium signaling events and accomplished through the generation of mechanical force by a unique molecular motor, termed the glideosome. The glideosome consists of a Myosin heavy chain (MyoA), together with its Myosin Light Chain (MLC1) and several scaffold proteins, the Gliding Associated Proteins (GAPs). While the role of MyoA is to convert chemical energy in the form of ATP to mechanical energy in the form of movement, the mechanisms that regulate MyoA activity and thus force production are unknown. We have recently identified two Myosin Essential Light Chains (ELC1 and ELC2) in Toxoplasma gondii and show that both proteins interact specifically with the tail region of MyoA. Using a conditional gene regulation system we investigate their roles in vivo and observe significant impairment of motility, invasion and egress, when ELC1 and ELC2 are simultaneously suppressed. Utilising a structural model of the MyoAtail-Light Chain complex to inform mutational analyses of the ELCs, we identify the key residues required for ELC function in vivo. These results demonstrate the importance to parasite motility, of calcium binding to ELC and intermolecular interactions between the glideosome components. To understand the mechanism by which these interactions promote force production, we have tagged MyoA for affinity purification of the native glideosome. Using this, we will reveal the effect on glideosome function by kinetic analyses and determine the macromolecular structure of the glideosome by single-particle imaging.