Structural and functional analyses of the host cell invasion motor in Toxoplasma parasites

© 2018 Dr. Melanie Joy WilliamsKey to the virulence of apicomplexan parasites is their ability move through tissue, invade and subsequently egress from host cells. Apicomplexan motility is initiated in response to calcium signalling events and accomplished through the generation of mechanical force by an actin-myosin motor, termed the glideosome. The glideosome consists of a Myosin heavy chain (MyoA), together with its Myosin Light Chain (MLC1) and several scaffold proteins, which act together to displace actin filaments. While the role of MyoA is to convert chemical energy to mechanical energy, the mechanisms that regulate MyoA activity and thus force production are unknown. This thesis presents the identification of a novel glideosome component, ELC2, and the functional characterisation of the two Essential Light Chains (ELC1 and ELC2) of MyoA in Toxoplasma gondii. Results show these proteins are important for invasion, egress and motility. Furthermore, a molecular model of the MyoA-light chain complex is presented and molecular simulations are used to identify a role for a divalent cation such as calcium, in promoting intermolecular contacts between components, which likely stabilises the domain for productive movement. Using point mutations predicted to ablate either the interaction with calcium or the interface between the two light chains, their contribution to the quality, displacement and speed of gliding Toxoplasma is demonstrated. Therefore this work delineates the importance of the MyoA lever arm in vivo and highlights a mechanism by which this domain could be stabilised in order to promote invasion, egress and gliding motility in these important pathogens