Condensation-decondensation of the gamma-tubulin containing material in the absence of a structurally visible organelle during the cell cycle of Physarum plasmodia

Genetic evidence has shown the presence of a common spindle pole organiser in Physarum amoebae and plasmodia. But the typical centrosome and mitosis o bserved in amoebae are replaced in plasmodia by an intranuclear mitosis dev oid of any structurally defined organelle. The fate of gamma-tubulin and of another component (TPH17) of the centrosome of Physarum amoebae was invest igated in the nuclei of synchronous plasmodia. These two amoebal centrosoma l elements were present in the nuclear compartment during the entire cell c ycle and exhibited, similar relocalisation from metaphase to telophase. Thr ee preparation methods showed that gamma-tubulin containing material was di spersed in the nucleoplasm during interphase. It constituted an intranuclea r thread-like structure. Ln contrast, the TPH17 epitope exhibited a localis ation close to the nucleolus. In late G2-phase, the gamma-tubulin containin g elements condensed in a single organelle which further divided. Intranucl ear microtubules appeared before the condensation of the gamma-tubulin mate rial and treatment with microtubule poisons suggested that microtubules wer e required in this process. The TPH17 epitope relocalised in the intranucle ar spindle later than the gamma-tubulin containing material suggesting a ma turation process of the mitotic poles. The decondensation of the gamma-tubu lin material and of the material containing the TPH17 epitope occurred imme diately after telophase. Hence in the absence of a structurally defined cen trosome homologue, the microtubule nucleating material undergoes a cycle of condensation and decondensation during the cell cycle. (C) 1999 Editions s cientifiques et medicales Elsevier SAS.