We consider the cytoskeletal structure, function, and motility of the invasive zoites of the Apicomplexa. This monophyletic group possess a prominent microtubular cytoskeleton, with a very distinct polarity. It is associated with a non-actin based filamentous system, and with a cisternal double membrane assembly beneath the plasma membrane. The origin of the microtubular cytoskeleton is a set of apical rings. Its role in motility is still unclear, but the present knowledge of apicomplexan tubulins' molecular biology and chemistry is outlined. Actin and accessory proteins are present, and it is apparent that actin polymerisation is tightly controlled in zoites. It does not contribute to the cytoskeleton ordinarily, but is crucial in the acto-myosin linear motor which drives gliding, capping, and invasion, the best understood aspects of zoite motility. Several myosins distinct from the primary linear motor myosin are also found, but not yet well understood functionally. Many of the myosins fall into a class of the superfamily so far seen only in this phylum. The possible relationships of the actin, myosin, cytoskeletal linkage proteins, and external force-transducing adherent proteins are discussed.
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