An essential gene pair in Saccharomyces cerevisiae with a potential role in mating

Translation initiation factor eIF4G1 modulates assembly of the polypeptide exit tunnel region in yeast ribosome biogenesis

eIF4G is an important eukaryotic translation initiation factor. In this study, eIF4G1, one of the eIF4G isoforms, was shown to directly participate in biogenesis of the large (60S) ribosomal subunit in Saccharomyces cerevisiae cells. Mutation of eIF4G1 decreased the amount 60S ribosomal subunits significantly. The C-terminal fragment of eIF4G1 could complement the function in 60S biogenesis. Analyses of its purified complex with mass spectrometry indicated that eIF4G1 associated with the pre-60S form directly. Strong genetic and direct protein-protein interactions were observed between eIF4G1 and Ssf1 protein. Upon deletion of eIF4G1, Ssf1, Rrp15, Rrp14 and Mak16 were abnormally retained on the pre-60S complex. This purturbed the loading of Arx1 and eL31 at the polypeptide exit tunnel (PET) site and the transition to a Nog2 complex. Our data indicate that eIF4G1 is important in facilitating PET maturation and 27S processing correctly. This article has an associated First Person interview with the first author of the paper.

Ppan is essential for preimplantation development in mice†

PETER PAN (PPAN), located to nucleoli and mitochondria, is a member of the Brix domain protein family, involved in rRNA processing through its rRNA binding motif and mitochondrial apoptosis by protecting mitochondria structure and suppressing basal autophagic flux. Ppan is important for cell proliferation and viability, and mutation of Ppan in Drosophila caused larval lethality and oogenesis failure. Yet, its role in mammalian reproduction remains unclear. In this study, we explored the function of Ppan in oocyte maturation and early embryogenesis using conditional knockout mouse model. Deficiency of maternal Ppan significantly downregulated the expression level of 5.8S rRNA, 18S rRNA, and 28S rRNA, though it had no effect on oocyte maturation or preimplantation embryo development. However, depletion of both maternal and zygotic Ppan blocked embryonic development at morula stage. Similar phenotype was obtained when only zygotic Ppan was depleted. We further identified no DNA binding activity of PPAN in mouse embryonic stem cells, and depletion of Ppan had minimum impact on transcriptome but decreased expression of 5.8S rRNA, 18S rRNA, and 28S rRNA nevertheless. Our findings demonstrate that Ppan is indispensable for early embryogenesis in mice.

Increased RNA production in Saccharomyces cerevisiae by simultaneously overexpressing FHL1, IFH1, and SSF2 and deleting HRP1

Ribonucleic acid (RNA) and its degradation products are widely used in the food industry. In this study, we constructed Saccharomyces cerevisiae mutants with FHL1, IFH1, SSF1, and SSF2 overexpression and HRP1 deletion, individually to evaluate the effect on RNA production. The RNA content of recombinant strains W303-1a-FHL1, W303-1a-SSF2, and W303-1a-ΔHRP1 was increased by 14.94%, 24.4%, and 19.36%, respectively, compared with the RNA content of the parent strain. However, W303-1a-IFH1 and W303-1a-SSF1 showed no significant change in RNA production compared with the parent strain. IFH1 and FHL1 encode Ifh1p and Fhl1p, respectively, which combine to form a complex that plays a key role in the transcription of the ribosomal protein (RP) gene. Ssf2p, encoded by SSF2, plays an important role in ribosome biosynthesis and Hrp1p is a negative regulator of cell growth in S. cerevisiae. Subsequently, a high RNA production strain, W112, was constructed by simultaneously overexpressing FHL1, IFH1, and SSF2 and deleting HRP1. The RNA content of W112 was 38.8% higher than the parent strain. The growth performance, RP transcription levels, and rRNA content were also investigated in the recombinant strains. This study provides a new strategy for the construction of S. cerevisiae strains containing large amounts of RNA, and it will make a significant contribution to progress in the nucleic acid industry. KEY POINTS: • Simultaneously overexpressing FHL1, IFH1, and SSF2 and deleting HRP1 can significantly increases RNA production. • The production of RNA increased by 38.8% in Saccharomyces cerevisiae. • The cell size and growth rate of the strains with higher RNA content also increased.

SNAIL1 is essential for female gametogenesis in Arabidopsis thaliana

Two yeast Brix family members Ssf1 and Ssf2, involved in large ribosomal subunit synthesis, are essential for yeast cell viability and mating efficiency. Their putative homologs exist in the Arabidopsis genome; however, their role in plant development is unknown. Here, we show that Arabidopsis thaliana SNAIL1 (AtSNAIL1), a protein sharing high sequence identity with yeast Ssf1 and Ssf2, is critical to mitosis progression of female gametophyte development. The snail1 homozygous mutant was nonviable and its heterozygous mutant was semi-sterile with shorter siliques. The mutation in SNAIL1 led to absence of female transmission and reduced male transmission. Further phenotypic analysis showed that the synchronic development of female gametophyte in the snail1 heterozygous mutant was greatly impaired and the snail1 pollen tube growth, in vivo, was also compromised. Furthermore, SNAIL1 was a nucleolar-localized protein with a putative role in protein synthesis. Our data suggest that SNAIL1 may function in ribosome biogenesis like Ssf1 and Ssf2 and plays an important role during megagametogenesis in Arabidopsis.

Interaction network of the ribosome assembly machinery from a eukaryotic thermophile

Ribosome biogenesis in eukaryotic cells is a highly dynamic and complex process innately linked to cell proliferation. The assembly of ribosomes is driven by a myriad of biogenesis factors that shape pre-ribosomal particles by processing and folding the ribosomal RNA and incorporating ribosomal proteins. Biochemical approaches allowed the isolation and characterization of pre-ribosomal particles from Saccharomyces cerevisiae, which lead to a spatiotemporal map of biogenesis intermediates along the path from the nucleolus to the cytoplasm. Here, we cloned almost the entire set (∼180) of ribosome biogenesis factors from the thermophilic fungus Chaetomium thermophilum in order to perform an in-depth analysis of their protein-protein interaction network as well as exploring the suitability of these thermostable proteins for structural studies. First, we performed a systematic screen, testing about 80 factors for crystallization and structure determination. Next, we performed a yeast 2-hybrid analysis and tested about 32,000 binary combinations, which identified more than 1000 protein-protein contacts between the thermophilic ribosome assembly factors. To exemplary verify several of these interactions, we performed biochemical reconstitution with the focus on the interaction network between 90S pre-ribosome factors forming the ctUTP-A and ctUTP-B modules, and the Brix-domain containing assembly factors of the pre-60S subunit. Our work provides a rich resource for biochemical reconstitution and structural analyses of the conserved ribosome assembly machinery from a eukaryotic thermophile.

Protein expression patterns of the yeast mating response

Microfluidics, in combination with time-lapse microscopy, is a transformative technology that significantly enhances our ability to monitor and probe biological processes in living cells. However, high-throughput microfluidic devices mostly require sophisticated preparatory and setup work and are thus hard to adopt by non-experts. In this work, we designed an easy-to-use microfluidic chip, which enables tracking of 48 GFP-tagged yeast strains, with each strain under two different stimulus conditions, in a single experiment. We used this technology to investigate the dynamic pattern of protein expression during the yeast mating differentiation response. High doses of pheromone induce cell cycle arrest and the shmoo morphology, whereas low doses of pheromone lead to elongation and chemotrophic growth. By systematically analyzing the protein dynamics of 156 pheromone-regulated genes, we identified groups of genes that are preferentially induced in response to low-dose pheromone (elongation during growth) or high-dose pheromone (shmoo formation and cell cycle arrest). The protein dynamics of these genes may provide insights into the mechanisms underlying the differentiation switch induced by different doses of pheromone.

Pescadillo homologue 1 and Peter Pan function during Xenopus laevis pronephros development

BACKGROUND INFORMATION

pes1 (pescadillo homologue 1) and ppan (Peter Pan) are multifunctional proteins involved in ribosome biogenesis, cell proliferation, apoptosis, cell migration and regulation of gene expression. Both proteins are required for early neural development in Xenopus laevis, as previously demonstrated.

RESULTS

We show that the expression of both genes in the developing pronephros depends on wnt4 and fzd3 (frizzled homologue 3) function. Loss of pes1 or ppan by MO (morpholino oligonucleotide)-based knockdown approaches resulted in strong malformations during pronephric tubule formation. Defects were already notable during specification of pronephric progenitor cells, as shown by lhx1 expression. Moreover, we demonstrated that Xenopus pes1 and ppan interact physically and functionally and that pes1 and ppan can cross-rescue the loss of function phenotype of one another. Interference with rRNA synthesis, however, did not result in a similar early pronephros phenotype.

CONCLUSION

These results demonstrate that pes1 and ppan are required for Xenopus pronephros development and indicate that their function in the pronephros is independent of their role in ribosome biosynthesis.

Yeast Rrp14p is required for ribosomal subunit synthesis and for correct positioning of the mitotic spindle during mitosis

Here we report that Rrp14p/Ykl082p is associated with pre-60S particles and to a lesser extent with earlier 90S pre-ribosomes. Depletion of Rrp14p inhibited pre-rRNA synthesis on both the 40S and 60S synthesis pathways. Synthesis of the 20S precursor to the 18S rRNA was largely blocked, as was maturation of the 27SB pre-rRNA to the 5.8S and 25S rRNAs. Unexpectedly, Rrp14p-depleted cells also showed apparently specific cell-cycle defects. Following release from synchronization in S phase, Rrp14p-depleted cells uniformly arrested in metaphase with short mitotic spindles that were frequently incorrectly aligned with the site of bud formation. In the absence of Bub2p, which is required for the spindle orientation checkpoint, this metaphase arrest was not seen in Rrp14p-depleted cells, which then arrested with multiple buds, several SPBs and binucleate mother cells. These data suggest that Rrp14p may play some role in cell polarity and/or spindle positioning, in addition to its function in ribosome synthesis.

Crystal structure of Mil (Mth680): internal duplication and similarity between the Imp4/Brix domain and the anticodon-binding domain of class IIa aminoacyl-tRNA synthetases

Proteins of the Imp4/Brix superfamily are involved in ribosomal RNA processing, an essential function in all cells. We report the first structure of an Imp4/Brix superfamily protein, the Mil (for Methanothermobacter thermautotrophicus Imp4-like) protein (gene product Mth680), from the archaeon M. thermautotrophicus. The amino- and carboxy-terminal halves of Mil show significant structural similarity to one another, suggesting an origin by means of an ancestral duplication. Both halves show the same fold as the anticodon-binding domain of class IIa aminoacyl-tRNA synthetases, with greater conservation seen in the N-terminal half. This structural similarity, together with the charge distribution in Mil, suggests that Imp4/Brix superfamily proteins could bind single-stranded segments of RNA along a concave surface formed by the N-terminal half of their beta-sheet and a central alpha-helix. The crystal structure of Mil is incompatible with the presence, in the Imp4/Brix domain, of a helix-turn-helix motif that was proposed to comprise the RNA-binding moiety of the Imp4/Brix proteins.

Genetic evidence of a role for membrane lipid composition in the regulation of soluble NEM-sensitive factor receptor function in Saccharomyces cerevisiae

SEC9 and SPO20 encode SNARE proteins related to the mammalian SNAP-25 family. Sec9p associates with the SNAREs Sso1/2p and Snc1/2p to promote the fusion of vesicles with the plasma membrane. Spo20p functions with the same two partner SNAREs to mediate the fusion of vesicles with the prospore membrane during sporogenesis. A chimeric molecule, in which the helices of Sec9p that bind to Sso1/2p and Snc1/2p are replaced with the homologous regions of Spo20p, will not support vesicle fusion in vegetative cells. The phosphatidylinositol-4-phosphate-5-kinase MSS4 was isolated as a high-copy suppressor that permits this chimera to rescue the temperature-sensitive growth of a sec9-4 mutant. Suppression by MSS4 is specific to molecules that contain the Spo20p helical domains. This suppression requires an intact copy of SPO14, encoding phospholipase D. Overexpression of MSS4 leads to a recruitment of the Spo14 protein to the plasma membrane and this may be the basis for MSS4 action. Consistent with this, deletion of KES1, a gene that behaves as a negative regulator of SPO14, also promotes the function of SPO20 in vegetative cells. These results indicate that elevated levels of phosphatidic acid in the membrane may be required specifically for the function of SNARE complexes containing Spo20p.

Role of the yeast Rrp1 protein in the dynamics of pre-ribosome maturation

The Saccharomyces cerevisiae gene RRP1 encodes an essential, evolutionarily conserved protein necessary for biogenesis of 60S ribosomal subunits. Processing of 27S pre-ribosomal RNA to mature 25S rRNA is blocked and 60S subunits are deficient in the temperature-sensitive rrp1-1 mutant. We have used recent advances in proteomic analysis to examine in more detail the function of Rrp1p in ribosome biogenesis. We show that Rrp1p is a nucleolar protein associated with several distinct 66S pre-ribosomal particles. These pre-ribosomes contain ribosomal proteins plus at least 28 nonribosomal proteins necessary for production of 60S ribosomal subunits. Inactivation of Rrp1p inhibits processing of 27SA(3) to 27SB(S) pre-rRNA and of 27SB pre-rRNA to 7S plus 25.5S pre-rRNA. Thus, in the rrp1-1 mutant, 66S pre-ribosomal particles accumulate that contain 27SA(3) and 27SB(L) pre-ribosomal RNAs.

Proteomic analysis of human Nop56p-associated pre-ribosomal ribonucleoprotein complexes. Possible link between Nop56p and the nucleolar protein treacle responsible for Treacher Collins syndrome

Nop56p is a component of the box C/D small nucleolar ribonucleoprotein complexes that direct 2'-O-methylation of pre-rRNA during its maturation. Genetic analyses in yeast have shown that Nop56p plays important roles in the early steps of pre-rRNA processing. However, its precise function remains elusive, especially in higher eukaryotes. Here we describe the proteomic characterization of human Nop56p (hNop56p)-associated pre-ribosomal ribonucleoprotein complexes. Mass spectrometric analysis of purified pre-ribosomal ribonucleoprotein complexes identified 61 ribosomal proteins, 16 trans-acting factors probably involved in ribosome biogenesis, and 29 proteins whose function in ribosome biogenesis is unknown. Identification of pre-rRNA species within hNop56p-associated pre-ribosomal ribonucleoprotein complexes, coupled with the known functions of yeast orthologs of the probable trans-acting factors identified in human, demonstrated that hNop56p functions in the early to middle stages of 60 S subunit synthesis in human cells. Interestingly, the nucleolar phosphoprotein treacle, which is responsible for the craniofacial disorder associated with Treacher Collins syndrome, was found to be a constituent of hNop56p-associated pre-rRNP complexes. The association of hNop56p and treacle within the complexes was independent of rRNA integrity, indicating a direct interaction. In addition, the protein compositions of the treacle-associated and hNop56p-associated pre-ribosomal ribonucleoprotein complexes were very similar, suggesting functional similarities between these two complexes with respect to ribosome biogenesis in human cells.

Ssf1p prevents premature processing of an early pre-60S ribosomal particle

Ssf1p and Ssf2p are two nearly identical and functionally redundant nucleolar proteins. In the absence of Ssf1p and Ssf2p, the 27SA(2) pre-rRNA was prematurely cleaved, inhibiting synthesis of the 27SB and 7S pre-rRNAs and the 5.8S and 25S rRNA components of the large ribosomal subunit. On sucrose gradients, Ssf1p sedimented with pre-60S ribosomal particles. The 27SA(2), 27SA(3), and 27SB pre-rRNAs were copurified with tagged Ssf1p, as were 23 large subunit ribosomal proteins and 21 other proteins implicated in ribosome biogenesis. These included four Brix family proteins, Ssf1p, Rpf1p, Rpf2p, and Brx1p, indicating that the entire family functions in ribosome synthesis. This complex is distinct from recently reported pre-60S complexes in RNA and protein composition. We describe a multistep pathway of 60S preribosome maturation.

The sigma(70)-like motif: a eukaryotic RNA binding domain unique to a superfamily of proteins required for ribosome biogenesis

Little is understood about the role of nucleolar RNA binding proteins in ribosome biogenesis, although there is a clear need for them based on the strict folding requirements of the pre-rRNA. We have identified a superfamily of RNA binding proteins whose members are required for different stages of ribosome biogenesis. The Imp4 superfamily is composed of five individual families (Imp4, Rpf1, Rpf2, Brx1, and Ssf) that all possess the sigma(70)-like motif, a eukaryotic RNA binding domain with prokaryotic origins. The Imp4 superfamily members associate with RNAs that are consistent with their distinct roles in ribosome biogenesis and suggest the mechanisms by which they function.

The Brix domain protein family -- a key to the ribosomal biogenesis pathway?

Six (one archaean and five eukaryotic) protein families have similar domain architecture that includes a central globular Brix domain, and optional N- and obligatory C-terminal segments, both with charged low-complexity regions. Biological data for some proteins in this superfamily suggest a role in ribosome biogenesis and rRNA binding.

Cotranscription and intergenic splicing of human P2Y11 and SSF1 genes

The P2Y(11) receptor is an ATP receptor positively coupled to the cAMP and phosphoinositide pathways. Ssf1 is a Saccharomyces cerevisiae nuclear protein, which plays an important role in mating. The gene encoding the human orthologue of SSF1 is adjacent to the P2Y(11) gene on chromosome 19. During the screening of placenta cDNA libraries, we isolated a chimeric clone resulting from the intergenic splicing between the P2Y(11) and SSF1 genes. The fusion protein was stably expressed in CHO-K1 cells where it generated a cAMP response to ATP qualitatively indistinguishable from that of the P2Y(11) receptor. According to both Western blotting and cAMP response, the expression of the fusion protein in the transfected cells was clearly lower than that of the P2Y(11) receptor. Both P2Y(11) and SSF1 probes detected a 5.6-kb messenger RNA with a similar pattern of intensity in each of 11 human tissues. The ubiquitous presence of chimeric transcripts and their up-regulation during granulocytic differentiation indicate that the transgenic splicing between the P2Y(11) and the SSF1 genes is a common and regulated phenomenon. There are very few examples of intergenic splicing in mammalian cells, and this is the first case involving a G-protein-coupled receptor.

The archaeal homolog of the Imp4 protein, a eukaryotic U3 snoRNP component

Homologs of the Imp4 protein, a component specific to the eukaryotic U3 snoRNP complex, have been found in all archaeal genomes. The archaeal and eukaryotic Imp4 proteins that are related to four other protein families, the Imp4-like, the SSF1 homologs and two sets of hypothetical proteins, are characterized by the Imp4 signature pattern. These findings, together with the presence of other snoRNPs homologs in Archaea, provide evidence for similar RNA processing and folding in Eukarya and Archaea.

The black-pearl gene of Drosophila defines a novel conserved protein family and is required for larval growth and survival

Using a transposon insertion line of the Drosophila Genome Project we have cloned the black-pearl gene (blp), analyzed cDNA clones, generated various mutants, and characterized their phenotypes. The blp gene codes for a protein of 15.7 kDa calculated molecular weight that has been conserved from yeast to plants and mammals with high homology. A domain of these new proteins shows distant similarity to DnaJ domains indicating a functionally relevant interaction with other proteins. The P element insertion in line P1539 lies within the 5' untranslated leader of the black-pearl gene. Flies homozygous for this insertion are semi-lethal, escapers produce very few offspring and show melanotic inclusions in the hemocoel ('black pearls') similar to various melanotic 'tumor' mutants. Two small deletions confined to the blp gene and two EMS-induced mutations are homozygous lethal. These null mutants appear normal up to a prolonged first instar larval stage but fail to grow and die. Thus in Drosophila the blp gene is specifically required for larval growth. The evolutionary conservation in both unicellular and multicellular organisms suggests for the new protein family described here a fundamental role in cell growth.

Identification and validation of a gene involved in anchorage-independent cell growth control using a library of randomized hairpin ribozymes

We have developed a library of hairpin ribozyme genes that can be delivered and expressed in mammalian cells with the purpose of identifying genes involved in a specific phenotype. By applying the appropriate phenotypic selection criteria in tissue culture, we can enrich for ribozymes that knock down expression of an unknown gene or genes in a particular pathway. Once specific ribozymes are selected, their target binding sequence is used to identify and clone the target gene. We have applied this technology to identify a putative tumor suppressor gene that has been activated in HF cells, a nontransformed revertant of HeLa cells. Using soft agar growth as the selection criteria for gain of transformation, we have isolated ribozymes capable of triggering anchorage-independent growth. Isolation of one of these ribozymes, Rz 568, led to the identification and cloning of the human homologue of the Drosophila gene ppan, a gene involved in DNA replication, cell proliferation, and larval development. This novel human gene, PPAN, was verified as the biologically relevant target of Rz 568 by creating five additional "target validation" ribozymes directed against additional sites in the PPAN mRNA. Rz 568 and all of the target validation ribozymes reduced the level of PPAN mRNA in cells and promoted anchorage-independent growth. Exogenous expression of PPAN in HeLa and A549 tumor cells reduced their ability to grow in soft agar, underscoring its role in regulating anchorage-dependent growth. This study describes a novel method for gene discovery where the intracellular application of hairpin ribozyme libraries was used to identify a novel gene based solely on a phenotype.

Cloning and characterization of peter pan, a novel Drosophila gene required for larval growth

We identified a new Drosophila gene, peter pan (ppan), in a screen for larval growth-defective mutants. ppan mutant larvae do not grow and show minimal DNA replication but can survive until well after their heterozygotic siblings have pupariated. We cloned the ppan gene by P-element plasmid rescue. ppan belongs to a highly conserved gene family that includes Saccharomyces cerevisiae SSF1 and SSF2, as well as Schizosaccharomyces pombe, Arabidopsis, Caenorhabditis elegans, mouse, and human homologues. Deletion of both SSF1 and SSF2 in yeast is lethal, and depletion of the gene products causes cell division arrest. Mosaic analysis of ppan mutant clones in Drosophila imaginal disks and ovaries demonstrates that ppan is cell autonomous and required for normal mitotic growth but is not absolutely required for general biosynthesis or DNA replication. Overexpression of the wild-type gene causes cell death and disrupts the normal development of adult structures. The ppan gene family appears to have an essential and evolutionarily conserved role in cell growth.

Identification of SSF1, CNS1, and HCH1 as multicopy suppressors of a Saccharomyces cerevisiae Hsp90 loss-of-function mutation

Hsp90 functions in a multicomponent chaperone system to promote the maturation and maintenance of a diverse, but specific, set of target proteins that play key roles in the regulation of cell growth and development. To identify additional components of the Hsp90 chaperone system and its targets, we searched for multicopy suppressors of various temperature-sensitive mutations in the yeast Hsp90 gene, HSP82. Three suppressors were isolated for one Hsp90 mutant (glutamate --> lysine at amino acid 381). Each exhibited a unique, allele-specific pattern of suppression with other Hsp90 mutants and had unique structural and biological properties. SSF1 is a member of an essential gene family and functions in the response to mating pheromones. CNS1 is an essential gene that encodes a component of the Hsp90 chaperone machinery. The role of HCH1 is unknown; its sequence has no strong homology to any protein of known function. SSF1 and CNS1 were weak suppressors, whereas HCH1 restored wild-type growth rates at all temperatures tested to cells expressing the E381K mutant. Overexpression of CNS1 or HCH1, but not SSF1, enhanced the maturation of a heterologous Hsp90 target protein, p60(v-src). These results suggest that like Cns1p, Hch1p is a general modulator of Hsp90 chaperone functions, whereas Ssf1p likely is either an Hsp90 target protein or functions in the same pathway as an Hsp90 target protein.

A nucleolar protein that affects mating efficiency in Saccharomyces cerevisiae by altering the morphological response to pheromone

SSF1 and SSF2 are redundant essential yeast genes that, when overexpressed, increase the mating efficiency of cells containing a defective Ste4p Gbeta subunit. To identify the precise function of these genes in mating, different responses to pheromone were assayed in cells that either lacked or overexpressed SSF gene products. Cells containing null alleles of both SSF1 and SSF2 displayed the normal transcriptional induction response to pheromone but were unable to form mating projections. Overexpression of SSF1 conferred the ability to form mating projections on cells containing a temperature-sensitive STE4 allele, but had only a small effect on transcriptional induction. SSF1 overexpression preferentially increased the mating efficiency of a strain containing a null allele of SPA2, a gene that functions specifically in cell morphology. To investigate whether Ssf1p plays a direct physical role in mating projection formation, its subcellular location was determined. An Ssf1p-GFP fusion was found to localize to the nucleolus, implying that the role of SSF gene products in projection formation is indirect. The region of Ssf1p-GFP localization in cells undergoing projection formation was larger and more diffuse, and was often present in a specific orientation with respect to the projection. Although the function of Ssf1p appears to originate in the nucleus, it is likely that it ultimately acts on one or more of the proteins that is directly involved in the morphological response to pheromone. Because many of the proteins required for projection formation during mating are also required for bud formation during vegetative growth, regulation of the activity or amount of one or more of these proteins by Ssf1p could explain its role in both mating and dividing cells.