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J. Cell Biol.,
Volume 144, Number 5, March 8, 1999 947-961

* Department of Pediatric Oncology, The Dana-Farber Cancer Institute and Department of Pediatric Hematology, The
Children's Hospital, Harvard Medical School, Boston, Massachusetts 02115; Alignment of the mitotic spindle with the
axis of cell division is an essential process in Saccharomyces cerevisiae that is mediated by interactions between cytoplasmic microtubules and the cell cortex. We
found that a cortical protein, the yeast formin Bni1p,
was required for spindle orientation. Two striking abnormalities were observed in bni1
Department of Molecular Biology, Princeton
University, Princeton, New Jersey 08544; § Institut für Biochemie, Universität Stuttgart, 70569 Stuttgart, Germany; and ¶ Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6
cells. First, the initial movement of the spindle pole body (SPB) toward
the emerging bud was defective. This phenotype is similar to that previously observed in cells lacking the kinesin Kip3p and, in fact, BNI1 and KIP3 were found to be
in the same genetic pathway. Second, abnormal pulling
interactions between microtubules and the cortex appeared to cause preanaphase spindles in bni1
cells to
transit back and forth between the mother and the bud.
We therefore propose that Bni1p may localize or alter
the function of cortical microtubule-binding sites in the
bud. Additionally, we present evidence that other bipolar bud site determinants together with cortical actin
are also required for spindle orientation.
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