CUT11(- A GENE REQUIRED FOR CELL CYCLE-DEPENDENT SPINDLE POLE BODY ANCHORING IN THE NUCLEAR-ENVELOPE AND BIPOLAR SPINDLE FORMATION IN SCHIZOSACCHAROMYCES-POMBE() )

The ''cut'' mutants of Schizosaccharomyces pombe are defective in spin dle formation and/or chromosome segregation, but they proceed through the cell cycle, resulting in lethality. Analysis of temperature-sensit ive alleles of cut11(+) suggests that this gene is required for the fo rmation of a functional bipolar spindle. Defective spindle structure w as revealed with fluorescent probes for tubulin and DNA. Three-dimensi onal reconstruction of mutant spindles by serial sectioning and electr on microscopy showed that the spindle pole bodies (SPBs) either failed to complete normal duplication or were free floating in the nucleopla sm. Localization of Cut11p tagged with the green fluorescent protein s howed punctate nuclear envelope staining throughout the cell cycle and SPBs staining from early prophase to mid anaphase. This SPB localizat ion correlates with the time in the cell cycle when SPBs are inserted into the nuclear envelope. Immunoelectron microscopy confirmed the loc alization of Cut11p to mitotic SPBs and nuclear pore complexes. Clonin g and sequencing showed that cut11(+) encodes a novel protein with sev en putative membrane-spanning domains and homology to the Saccharomyce s cerevisiae gene NDC1. These data suggest that Cut11p associates with nuclear pore complexes and mitotic SPBs as an anchor in the nuclear e nvelope; this role is essential for mitosis.