B. DEPARTMENT OF CELL GENETICS
B-c. Division of Cytoplasmic Genetics - Masayuki Yamamoto Group

RESEARCH ACTIVITIES

(1) Fission yeast Mes1p ensures the onset of meiosis II by blocking degradation of cyclin Cdc13p²⁾

Masayuki Yamamoto

--Meiosis is a special form of nuclear division to generate eggs, sperm, and spores in eukaryotes. Meiosis consists of the first (MI) and the second (MII) meiotic divisions, which occur consecutively. MI is reductional, in which homologous chromosomes derived from parents segregate. MII is equational, in which replicated sister-chromatids separate as in mitosis. MII is generally considered to mimic mitosis in mechanism. However, fission yeast Mes1p is essential for MII but dispensable for mitosis. We have demonstrated that Mes1p is a factor that suppresses destruction of cyclin Cdc13p at anaphase I. Mes1p inhibits the activity of APC/C (anaphase promoting complex/cyclosome) to degrade Cdc13p, thereby playing a key role in saving a sufficient level of MPF (M-phase promoting factor) activity required for the execution of MII²⁾.

(2) The p150-Glued Ssm4p regulates microtubular dynamics and nuclear movement in fission yeast⁴⁾

Masayuki Yamamoto

--During vegetative growth of the fission yeast, microtubules nucleate from multiple MTOCs in the vicinity of the nucleus, polymerizing until they reach the end of the cell and then shrinking back to the cell middle. In response to mating pheromone, fission yeast undergoes a morphological switch from a vegetative to a shmooing growth pattern. The switch in growth mode is paralleled by a switch in microtubular dynamics. Microtubules nucleate mostly from a single MTOC and pull on the ends of the cell to move the nucleus back and forth. This movement continues after cellular and nuclear fusion in the zygote and is important to ensure correct chromosome pairing, recombination and segregation during meiosis. We have demonstrated that Ssm4p, a p150-glued protein, is induced specifically in response to pheromone and is required for this nuclear movement. Ssm4p is associated with the cytoplasmic dynein complex and regulates dynein heavy chain localization. We have also shown that Ssm4p functions in establishing the shmooing microtubular array⁴⁾.

(3) C. elegans DAZ-1 directs proper nuclear organization and cytoplasmic core formation during oogenesis³⁾

Masayuki Yamamoto

--The Deleted in Azoospermia (DAZ) family genes encode potential RNA-binding proteins that are expressed exclusively in germ cells in a wide range of metazoans. Mutations in daz-1, the only DAZ family gene in Caenorhabditis elegans, cause pachytene stage arrest of female germ cells, but do not affect spermatogenesis. We have demonstrated that DAZ-1 protein is most abundantly expressed in proliferating female germ cells. DAZ-1 is dispensable in males, but it is expressed also in male mitotic germ cells. By detailed phenotypic analyses we have found that loss of daz-1 function causes multiple abnormalities as early as the onset of meiotic prophase, which include aberrant chromatin structure, small nucleoli, absence of the cytoplasmic core, and precocious cellularization³⁾. It appears that DAZ-1 in C. elegans plays essential roles in female pre-meiotic and early meiotic germ cells, probably via regulating the translational activity of specific target genes required for the progression of oogenesis.

PUBLICATIONS

Papers
1. Inoue, T., Sugimoto, A., Suzuki, Y., Yamamoto, M., Tsujimoto, M., Inoue, K., Aoki, J. and Arai, H. (2004). Type II platelet-activating factor-acetylhydrolase is essential for epithelial morphogenesis in Caenorhabditis elegans. Proc. Natl. Acad. Sci. USA. 101, 13233-13238.
2. Izawa, D., Goto, M., Yamashita, A., Yamano, H. and Yamamoto, M. (2005). Fission yeast Mes1p ensures the onset of meiosis II by blocking degradation of cyclin Cdc13p. Nature 434, 529-533.
3. Maruyama, R., Endo, S., Sugimoto, A. and Yamamoto, M. (2005). C. elegans DAZ-1 is expressed in proliferating germ cells and directs proper nuclear organization and cytoplasmic core formation during oogenesis. Dev. Biol. 277, 142-154.
4. Niccoli, T., Yamashita, A., Nurse, P. and Yamamoto, M. (2004). The p150-Glued Ssm4p regulates microtubular dynamics and nuclear movement in fission yeast. J. Cell Sci. 117, 5543-5556.
5. Yamashita, A., Sato, M., Fujita, A., Yamamoto, M. and Toda, T. (2005). The roles of fission yeast Ase1 in mitotic cell division, meiotic nuclear oscillation and cytokinesis checkpoint signaling. Mol. Biol. Cell 16, 1378-1395.

SOCIAL CONTRIBUTIONS AND OTHERS

President: The Molecular Biology Society of Japan
Editor: Genes to Cells
Editorial Board: Current Genetics, YEAST