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We have identified a Schizosaccharomyces pombe gene with homology to the budding yeast gene CDCS, the Drosophila gene polo, and the mammalian family of genes encoding polo-like kinases. Disruption of this gene, plol +, indicates that it is essential. Loss of plol + function leads to a mitotic arrest in which condensed chromsomes are associated with a monopolar spindle or to the failure of septation following the completion of nuclear division. In the latter case, cells show a failure both in the formation of an F-actin ring and in the deposition of septal material, suggesting that plol + function is required high in the regulatory cascade that controls septation. The overexpression of plol + in wild-type cells also results in the formation of monopolar spindles but also induces the formation of multiple septa without nuclear division. Septation can also be induced in the absence of mitotic commitment and concomitant spindle formation by the overexpression of plol + in cdc25-22 or cdc2-33 cells arrested in G2; in GI cells arrested at Start by the cdclO-V50 mutation, or in cells lacking the cyclin B homolog cdcl3 that undergo repeated S phases in the absence of mitosis.[Key Words: S. pombe; polo homolog; bipolar spindle; actin ring; septum; plol kinase] Received January 20, 1995; revised version accepted March 16, 1995.The polo ~ mutation of Drosophila was first identified as a maternal-effect mutant giving rise to embryos that show abnormal networks of microtubules associated with condensed chromosomes, in which the centrosomal antigen CP190 {formerly called Bx63; Frasch et al. 1986;Whitfield et al. 1988) fails to become organized into centrosomes with microtubule-organizing capability (Sunkel and Glover 1988). The mutation results in multipolar spindles and nondisjunction in male meiosis leading to reduced fertility. There is a high mitotic index in larval neuroblasts, with a wide range of defects including monopolar spindles and spindles with broad poles (Sunkel and Glover 1988;Llamazares et al. 1991).polo encodes a 576-amino-acid protein that has an amino-terminal putative kinase domain and a 300-residue carboxy-terminal domain (Llamazares et al. 1991). The polo kinase is highly conserved. The budding yeast and murine genes, CDC5 (Kitada et al. 1993) and mouse Plk (polo-like kinase; Clay et al. 1993;Lake and Jelinek 1993;Hamanaka et al. 1994), for example, show 52% and 65% identity in the kinase domain and 33% and 43% in the carboxy-terminal domain, with the latter homolo3Corresponding author. gies lying in distinct blocks. The human Plk has also been identified (Golsteyn et al. 1994;Hamanaka et al. 1994;Holtrich et al. 1994), and studies in the mouse suggest that there is another closely related kinase, Snk (serum-induced kinase; Simmons et al. 1992), that falls into the same gene family. These mammalian genes are expressed only in proliferating cells or tissues suggesting a cell cycle role.In mutants of the budding yeast gene CDC5, nuclear division is arrested at a late stage and the spindle is elongated (Ha...

Cells in a variety of developmental contexts sense extracellular cues that are given locally on their surfaces, and subsequently amplify the initial signal to achieve cell polarization. Drosophila wing cells acquire planar polarity along the proximal-distal (P-D) axis, in which the amplification of the presumptive cue involves assembly of a multiprotein complex that spans distal and proximal boundaries of adjacent cells. Here we pursue the mechanisms that place one of the components, Frizzled (Fz), at the distal side. Intracellular particles of GFP-tagged Fz moved preferentially toward distal boundaries before Fz::GFP and other components were tightly localized at the P/D cortex. Arrays of microtubules (MTs) were approximately oriented along the P-D axis and these MTs contributed to the formation of the cortical complex. Furthermore, there appeared to be a bias in the P-D MTs, with slightly more plus ends oriented distally. The hypothesis of polarized vesicular trafficking of Fz is discussed.

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