Gliding motility and cell invasion by Apicomplexa: insights from the Plasmodium sporozoite

Apicomplexa constitute one of the largest phyla of protozoa. Most Apicomple xa, including those pathogenic to humans, are obligate intracellular parasi tes. Their extracellular forms, which are highly polarized and elongated ce lls, share two unique abilities: they glide on solid substrates without cha nging their shape and reach an intracellular compartment without active par ticipation from the host cell. There is now ample ultrastructural evidence that these processes result from the backward movement of extracellular int eractions along the anteroposterior axis of the parasite. Recent work in se veral Apicomplexa, including genetic studies in the Plasmodium sporozoite, has provided molecular support for this 'capping' model. It appears that th e same machinery drives both gliding motility and host cell invasion. The c ytoplasmic motor, a transmembrane bridge and surface ligands essential for cell invasion are conserved among the main apicomplexan pathogens.