Apical organelles of Apicomplexa: biology and isolation by subcellular fractionation

The apical organelles are characteristic secretory vesicles of Plasmodium, Toxoplasma, Cryptosporidium and other apicomplexan organisms. They consist of rhoptries, micronemes and dense granules. Recent research has provided m uch new data concerning their structure, contents, functions and developmen t. Ail of these organelles contain complex mixtures of proteins, with broad homologies as well as differences iii molecular structure between species and genera. Many of the proteins interact with host cell membranes, and are thought to mediate selective adhesion to host cells as well as membrane mo dification during intracellular invasion. Micronemal proteins are important in the initial selection of host cells, and in enabling gliding motility o f the parasites, while rhoptries appear to be more important in parasitopho rous vacuole formation. Dense granules are involved predominantly in modify ing the host cell after invasion. Research into apical organellar compositi on and function depends on accurate assignment of molecular identity. This requires the simultaneous application of several complementary approaches i ncluding immunolocalisation by light- and electron-microscopy, subcellular fractionation, and transgene expression. The merits and limitations of thes e different types of approach are discussed, and the importance of cell fra ctionation methods in characterising apical organelle proteins is stressed. (C) 2001 Elsevier Science B.V. All rights reserved.