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¹ Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
² Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8562, Japan
³ The Institute for Advanced Research, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
⁴ The Core Research for Evolutional Science and Technology (CREST), Japan Science Technology Corporation (JST), Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan

During mating of budding yeast, Saccharomyces cerevisiae, two haploid nuclei fuse to produce a diploid nucleus. The process of nuclear fusion requires two J proteins, Jem1p in the endoplasmic reticulum (ER) lumen and Sec63p, which forms a complex with Sec71p and Sec72p, in the ER membrane. Zygotes of mutants defective in the functions of Jem1p or Sec63p contain two haploid nuclei that were closely apposed but failed to fuse. Here we analyzed the ultrastructure of nuclei in jem1 and sec71 mutant zygotes using electron microscope with the freeze-substituted fixation method. Three-dimensional reconstitution of nuclear structures from electron microscope serial sections revealed that Jem1p facilitates nuclear inner-membrane fusion and spindle pole body (SPB) fusion while Sec71p facilitates nuclear outer-membrane fusion. Two haploid SPBs that failed to fuse could duplicate, and mitotic nuclear division of the unfused haploid nuclei started in jem1 and sec71 mutant zygotes. This observation suggests that nuclear inner-membrane fusion is required for SPB fusion, but not for SPB duplication in the first mitotic cell division.

^* Correspondence: shuh{at}biochem.chem.nagoya-u.ac.jp