Leukocytes drop anchor

B urkart et al. describe how a secreted protease helps egg cells avoid being fertilized by more than one sperm. Because polyspermy disrupts em-bryonic development, oocytes take several steps to ensure they only fuse with a single sperm. One key step is to prevent additional sperm from binding to the surface of an already-fertilized egg, a blockade that involves the release of secretory granules and cleavage of the glycoprotein ZP2, a component of the zona pellucida matrix that surrounds eggs. ZP2 is cleaved upstream of two acidic amino acids, a cleavage site recognized by the astacin family of met-alloendoproteases. Burkart et al. therefore investigated the function of ovastacin, an astacin family member expressed in oocytes. Ovastacin localized to cortical granules that were exocytosed after fertilization, and recombinant ovastacin cleaved ZP2 when added to zonae pellucidae. Mice lacking ovastacin failed to cleave ZP2 after fertilization, allowing sperm to continue to bind to the surface of early embryos. Ovastacin-null female mice had slightly fewer offspring than wild-type animals but otherwise appeared normal. The researchers found that ovastacin targeted several sites in ZP2's N terminus. Senior author Jurrien Dean now wants to investigate how this proteolysis blocks sperm binding, a critical question because the molecular interactions between sperm and egg cells remain unknown. He also wants to examine how ovasta-cin is packaged into oocyte cortical granules and to identify other components of these secretory organelles. E rlemann et al. count the number of ␥-tu-bulin complexes that assemble together to nucleate microtubules in S. cerevisiae. Budding yeast microtubules are nucleated from spindle pole bodies (SPBs) by ␥-tubulin small complexes (␥-TuSCs), which consist of two ␥-tubulin subunits and one molecule each of Spc97 and Spc98. In vitro, ␥-TuSCs form a spiral with 13 ␥-tubulin molecules per turn, which could serve as a template for the 13 tubu-lin protofi laments of a microtubule. But whether similar numbers of ␥-TuSCs nucleate microtubules in vivo was unknown. By comparing the fl uorescence of GFP-tagged ␥-TuSC proteins to known fl uorescent standards, Erlemann et al. counted the number of ␥-TuSC components at yeast SPBs and at the minus ends of individual microtubules, which had been released into the cytoplasm by a genetic trick. The quantification revealed that approximately seven ␥-TuSCs nucleate each microtubule in vivo, with an additional three ␥-tubulin and two Spc98 molecules also incorporating into each nucleation site. Budding yeast probably only nucleate microtubules during G1, but …