Centrosomes competent for parthenogenesis in Xenopus eggs support procentriole budding in cell-free extracts

Heterologous centrosomes from diversed species including humans promote egg cleavage when injected into metaphase-arrested Xenopus eggs. We have recently isolated centrosomes from calf thymocytes and shown that they were unable to induce egg cleavage, an inability that was apparently correlated with the peculiar structure of these centrosomes rather than with a lack of microtubule-nucleating activity: the two centrioles were associated in a colinear orientation by their proximal ends. To promote cleavage, a heterologous centrosome probably is required to duplicate, although this has not yet been demonstrated. Therefore, we designed an in vitro assay that would enable us to directly observe the duplication process. We show that competent centrosomes from KE37 cells synchronized in G1 phase initiate procentriole budding in interphasic extracts from Xenopus eggs in the absence of protein synthesis, whereas calf thymocyte centrosomes do not. Since calf thymocyte centrosomes do not support parthenogenesis, the present results suggest that duplication of the foreign centrosome is required for centrosome-induced parthenogenesis. Furthermore, procentriole budding takes place in the absence of protein synthesis in egg extracts arrested in S phase. This in vitro assay should contribute to the identification of molecular mechanisms involved in the initiation of centrosome duplication.