Apicomplexa are a group of parasites that includes Toxoplasma gondii (T. gondii) and Plasmodium falciparum (P. falciparum), which cause the major human diseases toxoplasmosis and malaria. For apicomplexan parasites, the secretion of specialised organelles called micronemes, and a unique form of motion called gliding motility are a pre-requisite for parasitic host-cell invasion, egress and dissemination within the host. A lipid signalling pathway leading to the generation of phosphatidic acid (PA) at the parasitic plasma membrane, has emerged as an important pathway for regulating microneme secretion. A protein called APH is anchored to the surface of micronemes and has previously been thought to act as an effector for lipid signalling to promote microneme secretion. This thesis describes the structural characteristics of APH from T. gondii (TgAPH) and P. falciparum (PfAPH). NMR solution state structures of TgAPH and PfAPH C-terminal region reveal a PH domain-like fold that preferentially binds PA-enriched membranes. Results from membrane mimetic binding studies indicate that the membrane interacting interface of TgAPH and PfAPH PH domains incorporates two major PA-binding surfaces, which may bind multiple PA lipids. Further studies with TgAPH suggest that the linker sequence upstream of the C-terminal PH domain exhibits conformational variability, and may adopt an extended structure that protrudes the PH domain away from the micronemal surface. Parasitic gliding is powered by an acto-myosin motor contained within a multi-protein assembly called the glideosome. A protein called GAC can function as a critical component of the glideosome by connecting adhesin tails to the acto-myosin motor. Like APH, GAC is thought to contain a C-terminal PH domain that is capable of binding PA. Results from studies presented in this thesis describe a mechanism for GAC C-terminal PH domain PA-sensing and demonstrate that TgGAC C-terminal PH domain does not form a high-affinity interaction with TgMIC2 adhesin tail.