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J. Cell Biol.,
Volume 140, Number 3, February 9, 1998 461-483
Department of Biology, Yale University, New Haven, Connecticut 06520-8103
Yeast cells mate by an inducible pathway
that involves agglutination, mating projection formation, cell fusion, and nuclear fusion. To obtain insight
into the mating differentiation of Saccharomyces cerevisiae, we carried out a large-scale transposon tagging
screen to identify genes whose expression is regulated
by mating pheromone. 91,200 transformants containing
random lacZ insertions were screened for 
-galactosidase (-gal) expression in the presence and absence of
factor, and 189 strains containing pheromone-regulated lacZ insertions were identified. Transposon insertion alleles corresponding to 20 genes that are novel or
had not previously been known to be pheromone regulated were examined for effects on the mating process.
Mutations in four novel genes, FIG1, FIG2, KAR5/
FIG3, and FIG4 were found to cause mating defects.
Three of the proteins encoded by these genes, Fig1p,
Fig2p, and Fig4p, are dispensible for cell polarization in
uniform concentrations of mating pheromone, but are
required for normal cell polarization in mating mixtures, conditions that involve cell-cell communication.
Fig1p and Fig2p are also important for cell fusion and
conjugation bridge shape, respectively. The fourth protein, Kar5p/Fig3p, is required for nuclear fusion. Fig1p
and Fig2p are likely to act at the cell surface as Fig1::
-gal and Fig2::-gal fusion proteins localize to the periphery of mating cells. Fig4p is a member of a family of
eukaryotic proteins that contain a domain homologous
to the yeast Sac1p. Our results indicate that a variety of
novel genes are expressed specifically during mating
differentiation to mediate proper cell morphogenesis,
cell fusion, and other steps of the mating process.
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