Sfi1p has multiple roles in the spindle pole body cycle

• Main Author: Anderson, Victoria Elizabeth
• Published: University of Edinburgh 2006
• Subjects: 572.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.640504

In order to identify new mitotic defects a screen for mutants synthetic lethal with a <i>mad1Δ</i> deletion strain was performed in <i>S. cerevisiae. </i>Mad1p is a component of the spindle assembly checkpoint. Four of the mutants isolated in the screen were novel mutations in the essential <i>sfi1</i> gene. In this study, Sfi1p is shown to be a spindle pole body (SPB) protein that contains a conserved set of 211 amino acid repeats. Its phosphorylation is cell cycle regulated, with most phosphorylation being present in an alpha factor (G0/g1) arrest, the time of spindle pole body duplication. The mutants from the screen all had mutations C-terminal to the conserved 21 amino acid repeats. This C terminal region is not conserved outwith the budding yeast, but within the budding yeasts family is the most conserved region. Deletion of the C terminal region is lethal, and results in mis-localisation of the protein. The <i>sfi1-CT</i> mutants are lethal with a range of spindle checkpoint proteins, indicating that the defect is recognised by the spindle assembly checkpoint. At all temperatures the <i>sfi1-CT</i> mutants showed varying levels of large budded cells delayed in mitosis with spindle pole bodies very close together (0.2 – 0.4 μm) and few normal metaphase (1-1.15 μm) spindles. Examination of these abnormal SPB pairs by electron microscopy revealed many large budded cells containing sets of paired spindle pole bodies still attached by a half bridge, as well as some that had partially separated and had some microtubules between them. In all cases, both spindle pole bodies appear morphologically normal and can nucleate nuclear and cytoplasmic microtubules. This is in contrast to other Sfi1p mutants in the conserved repeats that result in a failure of spindle pole body duplication, and suggests that Sfi1p has multiple functions in both SPB duplication and separation.