‘Epixenosomes’: Peculiar epibionts of the protozoon ciliate Euplotidium itoi: Do their cytoplasmic tubules consist of tubulin?

Abstract

Epixenosomes, epibiontic organisms living on the protozoon ciliate Euplotidium itoi, like prokaryotes, lack a true nucleus surrounded by a nuclear envelope. However, their cytoplasm, at least in the mature state, is somehow compartmentalized by the presence of a complex extrusive apparatus surrounded by a form of basket consisting of bundles of tubules (BT), 20–24 nm in diameter, delineated by a wall made of globular subunits. The nature of these tubules was investigated by means of in vivo experiments, ultrastructural analysis and immunocytochemistry by both fluorescence and electron microscopy. It has been found that BT, besides being well preserved when fixed in the presence of tannic acid, are sensitive to factors known to depolymerize cytoplasmic microtubules in a variety of cells such as nocodazole and cold and show a positive immunoreaction against different antitubulin antibodies. On the basis of all the results obtained, the hypothesis that the epixenosomal BT consist of tubulin acquires a strong credibility. This is particularly interesting if we consider that tubulin in prokaryotes has never been conclusively demonstrated.