SPO21 is required for meiosis-specific modification of the spindle pole body in yeast

During meiosis II in the yeast Saccharomyces cerevisiae, the cytoplasmic fa ce of the spindle pole body changes from a site of microtubule initiation t o a site of de novo membrane formation. These membranes are required to pac kage the haploid meiotic products into spores. This functional change in th e spindle pole body involves the expansion and modification of its cytoplas mic face, termed the outer plaque. We report here that SPO21 is required fo r this modification. The Spo21. protein localizes to the spindle pole in me iotic cells. In the absence of SPO21 the structure of the outer plaque is a bnormal, and prospore membranes do not form. Further, decreased dosage of S PO21 leaves only two of the four spindle pole bodies competent to generate membranes. Mutation of CNM67, encoding a known component of the mitotic out er plaque, also results in a meiotic outer plaque defect but does not block membrane formation, suggesting that Spo21p may play a direct role in initi ating membrane formation.