Fus2 localizes near the site of cell fusion and is required for both cell fusion and nuclear alignment during zygote formation

Cell fusion during yeast mating requires high levels of a-factor mating pheromone

During conjugation, two yeast cells fuse to form a single zygote. Cell fusion requires extensive remodeling of the cell wall, both to form a seal between the two cells and to remove the intervening material. The two plasma membranes then fuse to produce a continuous cytoplasm. We report the characterization of two cell fusion defective (Fus-) mutants, fus5 and fus8, isolated previously in our laboratory. Fluorescence and electron microscopy demonstrated that the fus5 and fus8 mutant zygotes were defective for cell wall remodeling/removal but not plasma membrane fusion. Strikingly, fus5 and fus8 were a specific; both mutations caused the mutant phenotype when present in the MATa parent but not in the MAT alpha parent. Consistent with an a-specific defect, the fus5 and fus8 mutants produced less a-factor than the isogenic wild-type strain. FUS5 and FUS8 were determined to be allelic to AXL1 and RAM1, respectively, two genes known to be required for biogenesis of a-factor. Several experiments demonstrated that the partial defect in a-factor production resulted in the Fus- phenotype. First, overexpression of a-factor in the fus mutants suppressed the Fus- defect. Second, matings to an MAT alpha partner supersensitive to mating pheromone (sst2 delta) suppressed the Fus- defect in trans. Finally, the gene encoding a-factor, MFA1, was placed under the control of a repressible promoter; reduced levels of wild-type a-factor caused an identical cell fusion defect during mating. We conclude that high levels of pheromone are required as one component of the signal for prezygotes to initiate cell fusion.

Role of the ABC transporter Ste6 in cell fusion during yeast conjugation

Though early stages of yeast conjugation are well-mimicked by treatment with pheromones, the final degradation of the cell wall and membrane fusion of mating that leads to cytoplasmic mixing may require separate signals. Mutations that blocked cell fusion during mating in Saccharomyces cerevisiae were identified in a multipartite screen. The three tightest mutations proved to be partial-function alleles of the ABC-transporter gene STE6 required for transport of a-factor. The ste6(cefl-1) allele was recovered and sequenced. The ste6(cefl-1) allele contained a stop codon predicted to truncate Ste6 at amino acid residue 862 (of 1290). The ste6(cef) mutations reduced, but did not eliminate, expression of a-factor. Light and electron microscopy revealed that unlike ste6 null mutations which block mating before the formation of mating pairs, the ste6(cef) (cell fusion) alleles permitted early steps in mating to proceed normally but blocked at a late stage in conjugation where mating partners were encased by a single cell wall and separated by only a thin layer of cell wall material we term the fusion wall. Morphologically the prezygotes appeared symmetrical with successful cell wall fusion at the periphery of the region of cell contact. Responses to a-factor were efficiently induced in partner cells under mating conditions as expected given the symmetric appearance of the prezygotes. A strain expressing a ste6(K1093A) mutation that conferred export of a twofold to fourfold higher level of a-factor than ste6(cef) did not accumulate prezygotes during mating which could indicate a tight threshold of a-factor signaling required for mating. However, mating to an sst2 partner which has a greatly increased sensitivity to a-factor did not suppress the fusion defect of a ste6(cef) strain. Overexpression of the structural gene for a-factor also did not suppress the fusion defect. It is possible that a-factor or STE6 play more complex roles in cell fusion.

Nuclear fusion in the yeast Saccharomyces cerevisiae

Karyogamy, or nuclear fusion, is the process during mating by which two haploid yeast nuclei fuse to produce a single diploid nucleus. Karyogamy occurs in two major steps: microtubule-dependent nuclear congression followed by fusion of the nuclear envelope membranes. Many of the proteins required for karyogamy have been discovered to act in related processes during mitotic growth. Accordingly, yeast karyogamy has become an important model system to investigate critical functions of the cytoplasmic microtubules and the microtubule organizing center, the nuclear envelope, and the endoplasmic reticulum.

Virus-cell and cell-cell fusion

Significant progress has been made in elucidating the mechanisms of viral membrane fusion proteins; both those that function at low, as well as those that function at neutral, pH. For many viral fusion proteins evidence now suggests that a triggered conformational change that exposes a previously cryptic fusion peptide, along with a rearrangement of the fusion protein oligomer, allows the fusion peptide to gain access to the target bilayer and thus initiate the fusion reaction. Although the topologically equivalent process of cell-cell fusion is less well understood, several cell surface proteins, including members of the newly described ADAM gene family, have emerged as candidate adhesion/fusion proteins.

The SH3-domain protein Bem1 coordinates mitogen-activated protein kinase cascade activation with cell cycle control in Saccharomyces cerevisiae

The mating mitogen-activated protein kinase (MAPK) cascade has three major outputs prior to fusion: transcriptional activation of many genes, cell cycle arrest in the G1 phase, and polarized growth. Bem1 localizes near the cortical actin cytoskeleton and is essential for polarized growth during mating. Here we show that Bem1 is required for efficient signal transduction and coordinates MAPK cascade activation with G1 arrest and mating. bem1delta null mutants are defective in G1 arrest and transcriptional activation in response to mating pheromone. Bem1 protein stimulates Fus3 (MAPK) activity and associates with Ste5, the tethering protein essential for activation of the MAPK kinase kinase Ste11. Bem1-Ste5 complexes also contain Ste11, Ste7 (MAPK kinase), and Fus3, suggesting that Ste5 localizes the MAPK cascade to Bem1. Strikingly, Bem1 also copurifies with Far1, a Fus3 substrate required for G1 arrest and proper polarized growth during mating. These and other results suggest that Bem1 may cross-link the Ste5-MAPK cascade complex to upstream activators and specific downstream substrates at the shmoo tip, thus enabling efficient circuitry for G1 arrest and mating.