Novel sfi1Alleles Uncover Additional Functions for Sfi1p in Bipolar Spindle Assembly and Function

A variety of spindle and kinetochore defects have been shown to induce a mitotic delay through activation of the spindle checkpoint. With the aim of identifying novel mitotic defects we carried out a mad1synthetic lethal screen in budding yeast. In this screen, four novel alleles of sfi1were isolated. SFI1is an essential gene, previously identified through its interaction with centrin/CDC31and shown to be required for spindle pole body (SPB) duplication. The new mutations were all found in the C-terminal domain of Sfi1p, which has no known function, but it is well conserved among budding yeasts. Analysis of the novel sfi1mutants, through a combination of light and electron microscopy, revealed duplicated SPBs <0.3 μm apart. Importantly, these SPBs have completed duplication, but they are not separated, suggesting a possible defect in splitting of the bridge. We discuss possible roles for Sfi1p in this step in bipolar spindle assembly.