A cDNA encoding a 224-kDa Dictyostelium discoideum

From an evolutionary perspective, the Dictyostelium centrosome is an intriguing organelle. Like its counterpart in yeast, the spindle pole body, it exhibits a compact, layered structure that lacks the centrioles typical of metazoan cells, but as in the latter, the core is surrounded by an electron-dense, amorphous matrix. With vertebrate centrosomes it shares a cytoplasmic localization in interphase, but it inserts itself into an opening of the nuclear envelope during mitosis like the yeast spindle pole body, which is a permanent resident of the nuclear envelope. This centrosome with its hybrid features resides in a cell type that resembles mammalian cells in terms of behavioral repertoire and motile properties, making Dictyostelium amoebae an important model system for the analysis of centrosome structure, function, and evolution. The Dictyostelium centrosome is located in close proximity to the nucleus (Roos, 1975) to which it is tightly connected via a fibrous linkage (Omura and Fukui, 1985). It consists of a boxshaped core structure with three major layers surrounded by a corona, which is composed of regularly spaced, dense nodules embedded in an amorphous matrix (Moens, 1976; Roos, 1975). The nodules seem to be the sites of microtubule nucleation since they contain γ-tubulin, and since all interphase microtubules emanate from these nodules (Euteneuer et al., 1998). Thus the corona seems to be the functional equivalent of the pericentriolar matrix of animal cells. Recently, the mode of centrosome duplication in Dictyostelium was elucidated in detail by a combination of electron microscopic analysis of fixed cells and observation of living cells tranformed with γtubulin-GFP, which possess green fluorescing centrosomes (Ueda et al., 1999). In contrast to animal cells and yeast, the entire duplication process occurs during mitosis. Centrosome duplication starts in prophase with an enlargement of the threelayered core. In late prophase, the corona dissociates and the interphase microtubules are lost. The central layer disappears at the transition to prometaphase, and the two outer layers peel apart and become the mitotic centrosomes, or spindle pole plaques. Microtubules are nucleated at the formerly inner surfaces of the two layers to form a nascent spindle that starts to separate the spindle pole plaques. During the separation process the edges of the plaques bend away from the nucleus until, in telophase, each plaque folds back onto itself. As a result of this folding process the microtubule-nucleating surface turns into the outside of the new daughter centrosome while the former outer surface becomes buried inside (Ueda et al., 1999). These morphological changes during centrosome duplication are unique among eukarytic cells. At present, relatively little is known about the centrosomal components that orchestrate this intriguing duplication process and carry out all other centrosomal activities. So far, molecular analyses were based on the expected similarity of 1747 Journal of Cell Science 113, 1747-1758 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JCS1194