Tropomyosin is required for the cell fusion process during conjugation in fission yeast.
BACKGROUND
Tropomyosin is an actin-binding protein, which is thought to stabilize actin filaments and influence many aspects of F-actin. In fission yeast, the cdc8 gene encodes tropomyosin, and the gene product Cdc8p is known to be essential for the formation of the F-actin contractile ring and hence for cytokinesis in the mitotic cell cycle.


RESULTS
We isolated fission yeast mutants that were defective in cell fusion during conjugation. One of them turned out to carry a point mutation in cdc8. We found that the original temperature-sensitive cdc8 mutant frequently failed to undergo cell fusion when mated at a semi-permissive temperature. Additional cdc8 mutants isolated by targeted mutagenesis also showed defects in both cell fusion and cytokinesis. A decrease in the amount of intracellular Cdc8p also affected both, but cell growth was more severely blocked than cell fusion in this case. Immunostaining revealed that Cdc8p was localized as a spot at the cell-to-cell attachment site during conjugation, without overlapping with F-actin patches.


CONCLUSIONS
Tropomyosin Cdc8p is indispensable for cell fusion during conjugation in fission yeast. However, cell fusion appears to require fewer tropomyosin molecules than cytokinesis. We speculate that tropomyosin may organize a small F-actin-containing organelle at the cell-to-cell contact site in each mating cell, which plays a key role in cell fusion.
Kurahashi
Hiroshi
H
Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Hongo, Tokyo 113-0033, Japan.
Imai
Yoshiyuki
Y
Yamamoto
Masayuki
M
D000199:Actins / Q000378:metabolism; D000595:Amino Acid Sequence; D018797:Cell Cycle Proteins / Q000235:genetics / Q000502:physiology*; D003227:Conjugation, Genetic; D005656:Fungal Proteins / Q000378:metabolism / Q000502:physiology*; D008969:Molecular Sequence Data; D009154:Mutation; D011401:Promoter Regions, Genetic; D012568:Schizosaccharomyces / Q000166:cytology* / Q000235:genetics; D029702:Schizosaccharomyces pombe Proteins; D014335:Tropomyosin / Q000502:physiology
D000199:Actins; C410737:Cdc8 protein, S pombe; D018797:Cell Cycle Proteins; D005656:Fungal Proteins; D029702:Schizosaccharomyces pombe Proteins; D014335:Tropomyosin; C490025:fus1 protein, S pombe