3-DIMENSIONAL ANALYSIS AND ULTRASTRUCTURAL DESIGN OF MITOTIC SPINDLESFROM THE CDC20 MUTANT OF SACCHAROMYCES-CEREVISIAE

The three-dimensional organization of mitotic microtubules in a mutant strain of Saccharomyces cerevisiae has been studied by computer-assis ted serial reconstruction. At the nonpermissive temperature, cdc20 cel ls arrested with a spindle length of similar to 2.5 mu m. These spindl es contained a mean of 81 microtubules (range, 56-100) compared with 2 3 in wild-type spindles of comparable length. This increase in spindle microtubule number resulted in a total polymer length up to four time s that of wild-type spindles. The spindle pole bodies in the cdc20 cel ls were similar to 2.3 times the size of wild-type, thereby accommodat ing the abnormally large number of spindle microtubules. The cdc20 spi ndles contained a large number of interpolar microtubules organized in a ''core bundle''. A neighbor density analysis of this bundle at the spindle midzone showed a preferred spacing of similar to 35 nm center- to-center between microtubules of opposite polarity. Although this is evidence of specific interaction between antiparallel microtubules, mu tant spindles were less ordered than the spindle of wild-type cells. T he number of noncore microtubules was significantly higher than that r eported for wild-type, and these microtubules did not display a charac teristic metaphase configuration. cdc20 spindles showed significantly more cross-bridges between spindle microtubules than were seen in the wild type. The cross-bridge density was highest between antiparallel m icrotubules. These data suggest that spindle microtubules are stabiliz ed in cdc20 cells and that the CDC20 gene product may be involved in c ell cycle processes that promote spindle microtubule disassembly.