Haploinsufficiency of the lysosomal sialidase NEU1 results in a model of pleomorphic rhabdomyosarcoma in mice

Rhabdomyosarcoma, the most common pediatric sarcoma, has no effective treatment for the pleomorphic subtype. Still, what triggers transformation into this aggressive phenotype remains poorly understood. Here we used Ptch1+/-/ETV7TG/+/- mice with enhanced incidence of rhabdomyosarcoma to generate a model of pleomorphic rhabdomyosarcoma driven by haploinsufficiency of the lysosomal sialidase neuraminidase 1. These tumors share mostly features of embryonal and some of alveolar rhabdomyosarcoma. Mechanistically, we show that the transforming pathway is increased lysosomal exocytosis downstream of reduced neuraminidase 1, exemplified by the redistribution of the lysosomal associated membrane protein 1 at the plasma membrane of tumor and stromal cells. Here we exploit this unique feature for single cell analysis and define heterogeneous populations of exocytic, only partially differentiated cells that force tumors to pleomorphism and promote a fibrotic microenvironment. These data together with the identification of an adipogenic signature shared by human rhabdomyosarcoma, and likely fueling the tumor's metabolism, make this model of pleomorphic rhabdomyosarcoma ideal for diagnostic and therapeutic studies.

Abstract 1901: VisComm: A cloud-based platform for accessing and creating scientific visualization for the pediatric cancer research community

Scientific visualization is important to cancer research as different graphs enable researchers to spot trends and detect outliers in large-scale, high-dimensional data. For example, mutation landscape maps integrate mutational and clinical data, revealing which somatic alterations are most closely associated with diagnoses, age, ethnicity, and outcome; t-SNE plots allow dissection of cancer subtypes based on gene expression or methylation profiling. These graphs are often created by researchers after extensive data curation and optimization of the layout using software tools designed for data visualization and are commonly presented as static figures in published literature. To permit seamless access to the genomic and epigenomic visualization tools created at St. Jude, we developed Visualization Community (https://viz.stjude.cloud/, VisComm), which allows researchers to explore published graphs, develop custom visualization for their private data, and also serve as a public repository for completed graphs, as part of the St. Jude Cloud data sharing ecosystem. Sixty-five (65) interactive graphs from 31 publications in 231 pediatric subtypes are currently available, including t-SNE plots of RNA-seq and methylation data, mutation landscape maps, variant visualization on protein or genomic axes, epigenome states, and chromatin contact interactions. The graphs are searchable by cancer diagnosis, molecular profiling, research interest, publication, and research organization with features designed to enable user customization. To date, 5,014 users, 25% of whom are recurrent, have accessed VisComm. On average, each public visualization has over 4,000 views while the most accessed graph has >13,000 views. We have developed the ability to work in teams with in VisComm to empower collaborative development of new visualizations and controlled release by a research team. This feature enables the professional quality visualizations serve as resources for focused scientific communities such as the Audacious Goals Initiative (AGI) - Retina), a multi-institutional cross-disciplinary research team whose goal is restoring vision to patients by regenerating retinas. Videos and textual tutorials are being prepared using data generated from programs such as the Genome for Kids (G4K), a clinical genomic profiling program, that will help guide users to apply the underlying visualization software tools and to integrate public and private data of all types. VisComm is the first platform dedicated to enabling the development and sharing of interactive high-quality data-rich visualizations, which we expect will enhance data exploration and hypothesis generation for cancer research.

Citation Format: David B. Finkelstein, James Madson, Lucian Vacaroiu, Alexander M. Gout, Leonard Hirja, Stephanie Sandor, Andrew Thrasher, Colleen Reilly, Jian Wang, Xiaolong Chen, Xin Zhou, Jinghui Zhang, Clay McLeod. VisComm: A cloud-based platform for accessing and creating scientific visualization for the pediatric cancer research community [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1901.

Data-driven approaches to advance research and clinical care for pediatric cancer

Pediatric cancer is a rare disease with a distinct etiology and mutational landscape compared with adult cancer. Multi-omics profiling of retrospective and prospective cohorts coupled with innovative computational analysis have been instrumental in uncovering mechanisms of tumorigenesis and drug resistance that are now informing pediatric cancer clinical therapy. In this review we present the major data resources of pediatric cancer and actionable insights into pediatric cancer etiology stemming from the identification of oncogenic gene fusions, mutational signature analysis, systems biology, cancer predisposition and survivorship studies - that have led to improved clinical diagnosis, discovery of new drug-targets, pharmacological therapy, and screening for genetic predisposition. Ultimately, integration of large-scale omics datasets generated through international collaboration is required to maximize the power of data-driven approaches to advance pediatric cancer research informing clinical therapy.

Tissue-Specific Regulation of the Wnt/β-Catenin Pathway by PAGE4 Inhibition of Tankyrase

Spatiotemporal control of Wnt/β-catenin signaling is critical for organism development and homeostasis. The poly-(ADP)-ribose polymerase Tankyrase (TNKS1) promotes Wnt/β-catenin signaling through PARylation-mediated degradation of AXIN1, a component of the β-catenin destruction complex. Although Wnt/β-catenin is a niche-restricted signaling program, tissue-specific factors that regulate TNKS1 are not known. Here, we report prostate-associated gene 4 (PAGE4) as a tissue-specific TNKS1 inhibitor that robustly represses canonical Wnt/β-catenin signaling in human cells, zebrafish, and mice. Structural and biochemical studies reveal that PAGE4 acts as an optimal substrate decoy that potently hijacks substrate binding sites on TNKS1 to prevent AXIN1 PARylation and degradation. Consistently, transgenic expression of PAGE4 in mice phenocopies TNKS1 knockout. Physiologically, PAGE4 is selectively expressed in stromal prostate fibroblasts and functions to establish a proper Wnt/β-catenin signaling niche through suppression of autocrine signaling. Our findings reveal a non-canonical mechanism for TNKS1 inhibition that functions to establish tissue-specific control of the Wnt/β-catenin pathway.

ATRX In-Frame Fusion Neuroblastoma Is Sensitive to EZH2 Inhibition via Modulation of Neuronal Gene Signatures

ATRX alterations occur at high frequency in neuroblastoma of adolescents and young adults. Particularly intriguing are the large N-terminal deletions of ATRX (Alpha Thalassemia/Mental Retardation, X-linked) that generate in-frame fusion (IFF) proteins devoid of key chromatin interaction domains, while retaining the SWI/SNF-like helicase region. We demonstrate that ATRX IFF proteins are redistributed from H3K9me3-enriched chromatin to promoters of active genes and identify REST as an ATRX IFF target whose activation promotes silencing of neuronal differentiation genes. We further show that ATRX IFF cells display sensitivity to EZH2 inhibitors, due to derepression of neurogenesis genes, including a subset of REST targets. Taken together, we demonstrate that ATRX structural alterations are not loss-of-function and put forward EZH2 inhibitors as a potential therapy for ATRX IFF neuroblastoma.

Myc-induced nuclear antigen constrains a latent intestinal epithelial cell-intrinsic anthelmintic pathway

Expulsion of parasitic gastrointestinal nematodes requires diverse effector mechanisms coordinated by a Th2-type response. The evolutionarily conserved JmjC protein; Myc Induced Nuclear Antigen (Mina) has been shown to repress IL4, a key Th2 cytokine, suggesting Mina may negatively regulate nematode expulsion. Here we report that expulsion of the parasitic nematode Trichuris muris was indeed accelerated in Mina deficient mice. Unexpectedly, this was associated not with an elevated Th2- but rather an impaired Th1-type response. Further reciprocal bone marrow chimera and conditional KO experiments demonstrated that retarded parasite expulsion and a normal Th1-type response both required Mina in intestinal epithelial cells (IECs). Transcriptional profiling experiments in IECs revealed anti-microbial α-defensin peptides to be the major target of Mina-dependent retention of worms in infected mice. In vitro exposure to recombinant α-defensin peptides caused cytotoxic damage to whipworms. These results identify a latent IEC-intrinsic anthelmintic pathway actively constrained by Mina and point to α-defensins as important effectors that together with Mina may be attractive therapeutic targets for the control of nematode infection.

Identification of OAT1/OAT3 as Contributors to Cisplatin Toxicity

Cisplatin is among the most widely used anticancer drugs and known to cause a dose‐limiting nephrotoxicity, which is partially dependent on the renal uptake carrier OCT2. We here report a previously unrecognized, OCT2‐independent pathway of cisplatin‐induced renal injury that is mediated by the organic anion transporters OAT1 and OAT3. Using transporter‐deficient mouse models, we found that this mechanism regulates renal uptake of a mercapturic acid metabolite of cisplatin that acts as a precursor of a potent nephrotoxin. The function of these two transport systems can be simultaneously inhibited by the tyrosine kinase inhibitor nilotinib through noncompetitive mechanisms, without compromising the anticancer properties of cisplatin. Collectively, our findings reveal a novel pathway that explains the fundamental basis of cisplatin‐induced nephrotoxicity, with potential implications for its therapeutic management.

Prox1-Heterozygosis Sensitizes the Pancreas to Oncogenic Kras-Induced Neoplastic Transformation

The current paradigm of pancreatic neoplastic transformation proposes an initial step whereby acinar cells convert into acinar-to-ductal metaplasias, followed by progression of these lesions into neoplasias under sustained oncogenic activity and inflammation. Understanding the molecular mechanisms driving these processes is crucial to the early diagnostic and prevention of pancreatic cancer. Emerging evidence indicates that transcription factors that control exocrine pancreatic development could have either, protective or facilitating roles in the formation of preneoplasias and neoplasias in the pancreas. We previously identified that the homeodomain transcription factor Prox1 is a novel regulator of mouse exocrine pancreas development. Here we investigated whether Prox1 function participates in early neoplastic transformation using in vivo, in vitro and in silico approaches. We found that Prox1 expression is transiently re-activated in acinar cells undergoing dedifferentiation and acinar-to-ductal metaplastic conversion. In contrast, Prox1 expression is largely absent in neoplasias and tumors in the pancreas of mice and humans. We also uncovered that Prox1-heterozygosis markedly increases the formation of acinar-to-ductal-metaplasias and early neoplasias, and enhances features associated with inflammation, in mouse pancreatic tissues expressing oncogenic Kras. Furthermore, we discovered that Prox1-heterozygosis increases tissue damage and delays recovery from inflammation in pancreata of mice injected with caerulein. These results are the first demonstration that Prox1 activity protects pancreatic cells from acute tissue damage and early neoplastic transformation. Additional data in our study indicate that this novel role of Prox1 involves suppression of pathways associated with inflammatory responses and cell invasiveness.

Structural Basis for the Interaction between Pyk2-FAT Domain and Leupaxin LD Repeats

Proline-rich tyrosine kinase 2 (Pyk2) is a nonreceptor tyrosine kinase and belongs to the focal adhesion kinase (FAK) family. Like FAK, the C-terminal focal adhesion-targeting (FAT) domain of Pyk2 binds to paxillin, a scaffold protein in focal adhesions; however, the interaction between the FAT domain of Pyk2 and paxillin is dynamic and unstable. Leupaxin is another member in the paxillin family and was suggested to be the native binding partner of Pyk2; Pyk2 gene expression is strongly correlated with that of leupaxin in many tissues including primary breast cancer. Here, we report that leupaxin interacts with Pyk2-FAT. Leupaxin has four leucine–aspartate (LD) motifs. The first and third LD motifs of leupaxin preferably target the two LD-binding sites on the Pyk2-FAT domain, respectively. Moreover, the full-length leupaxin binds to Pyk2-FAT as a stable one-to-one complex. Together, we propose that there is an underlying selectivity between leupaxin and paxillin for Pyk2, which may influence the differing behavior of the two proteins at focal adhesion sites.

Distinct epigenetic signatures delineate transcriptional programs during virus-specific CD8(+) T cell differentiation

The molecular mechanisms that regulate the rapid transcriptional changes that occur during cytotoxic T lymphocyte (CTL) proliferation and differentiation in response to infection are poorly understood. We have utilized ChIP-seq to assess histone H3 methylation dynamics within naive, effector, and memory virus-specific T cells isolated directly ex vivo after influenza A virus infection. Our results show that within naive T cells, codeposition of the permissive H3K4me3 and repressive H3K27me3 modifications is a signature of gene loci associated with gene transcription, replication, and cellular differentiation. Upon differentiation into effector and/or memory CTLs, the majority of these gene loci lose repressive H3K27me3 while retaining the permissive H3K4me3 modification. In contrast, immune-related effector gene promoters within naive T cells lacked the permissive H3K4me3 modification, with acquisition of this modification occurring upon differentiation into effector/memory CTLs. Thus, coordinate transcriptional regulation of CTL genes with related functions is achieved via distinct epigenetic mechanisms.

Persistent host markers in pandemic and H5N1 influenza viruses

Avian influenza viruses have adapted to human hosts, causing pandemics in humans. The key host-specific amino acid mutations required for an avian influenza virus to function in humans are unknown. Through multiple-sequence alignment and statistical testing of each aligned amino acid, we identified markers that discriminate human influenza viruses from avian influenza viruses. We applied strict thresholds to select only markers which are highly preserved in human influenza virus isolates over time. We found that a subset of these persistent host markers exist in all human pandemic influenza virus sequences from 1918, 1957, and 1968, while others are acquired as the virus becomes a seasonal influenza virus. We also show that human H5N1 influenza viruses are significantly more likely to contain the amino acid predominant in human strains for a few persistent host markers than avian H5N1 influenza viruses. This sporadic enrichment of amino acids present in human-hosted viruses may indicate that some H5N1 viruses have made modest adaptations to their new hosts in the recent past. The markers reported here should be useful in monitoring potential pandemic influenza viruses.

Large-scale sequence analysis of avian influenza isolates

The spread of H5N1 avian influenza viruses (AIVs) from China to Europe has raised global concern about their potential to infect humans and cause a pandemic. In spite of their substantial threat to human health, remarkably little AIV whole-genome information is available. We report here a preliminary analysis of the first large-scale sequencing of AIVs, including 2196 AIV genes and 169 complete genomes. We combine this new information with public AIV data to identify new gene alleles, persistent genotypes, compensatory mutations, and a potential virulence determinant.

Trends in the quality of data from 5168 oligonucleotide microarrays from a single facility

Improvements in the quality of gene expression data were investigated based on a database consisting of 5168 oligonucleotide microarrays collected over 3 years. The database includes diverse treatments of human and mouse samples collected from multiple laboratories. The array designs and algorithms used to capture the data have also changed over the 3 years of data collection. All hybridizations and labeling were conducted in the Hartwell Center for Bioinformatics and Biotechnology at St. Jude's Children's Research Hospital. Quality metrics for each human and mouse array were collected and analyzed. Statistical tests, such as ANOVA and linear regression, were applied to test for the effects of array design, algorithm, and time. The quality metrics tested were average background, actin 3'/5' ratio, Bio B signal, percent present, and scale factor. ANOVA results indicate that both recent algorithms and chip designs significantly correlate with improvements in Bio B, scale factor, percent present, and average background. Significant quality improvements correlated with new chip designs, algorithms, and their interaction. In addition, within one chip type analyzed by the same algorithm significant improving trends were still observed. Scale factor, percent present, and average background significantly improved over time for U133A arrays analyzed by the Affymetrix MicroArray Suite 5.0 algorithm according to linear regression. Proportionally fewer outlier arrays (those with less than 25% present calls) were seen over time. Also, high throughput periods did not increase the proportion of outliers, indicating that laboratory monitoring of quality is successfully preventing failures.

The Stanford Microarray Database: data access and quality assessment tools

The Stanford Microarray Database (SMD; http://genome-www.stanford.edu/microarray/) serves as a microarray research database for Stanford investigators and their collaborators. In addition, SMD functions as a resource for the entire scientific community, by making freely available all of its source code and providing full public access to data published by SMD users, along with many tools to explore and analyze those data. SMD currently provides public access to data from 3500 microarrays, including data from 85 publications, and this total is increasing rapidly. In this article, we describe some of SMD's newer tools for accessing public data, assessing data quality and for data analysis.

hsp82 is an essential protein that is required in higher concentrations for growth of cells at higher temperatures

hsp82 is one of the most highly conserved and abundantly synthesized heat shock proteins of eucaryotic cells. The yeast Saccharomyces cerevisiae contains two closely related genes in the HSP82 gene family. HSC82 was expressed constitutively at a very high level and was moderately induced by high temperatures. HSP82 was expressed constitutively at a much lower level and was more strongly induced by heat. Site-directed disruption mutations were produced in both genes. Cells homozygous for both mutations did not grow at any temperature. Cells carrying other combinations of the HSP82 and HSC82 mutations grew well at 25 degrees C, but their ability to grow at higher temperatures varied with gene copy number. Thus, HSP82 and HSC82 constitute an essential gene family in yeast cells. Although the two proteins had different patterns of expression, they appeared to have equivalent functions; growth at higher temperatures required higher concentrations of either protein. Biochemical analysis of hsp82 from vertebrate cells suggests that the protein binds to a variety of other cellular proteins, keeping them inactive until they have reached their proper intracellular location or have received the proper activation signal. We speculate that the reason cells require higher concentrations of hsp82 or hsc82 for growth at higher temperatures is to maintain proper levels of complex formation with these other proteins.

Sharp boundaries demarcate the chromatin structure of a yeast heat-shock gene

In both induced and basally transcribed states, the chromatin structure of the yeast HSP82 heat-shock locus exhibits a remarkable degree of organization with respect to DNA sequence. The promoter region contains a constitutive DNase I hypersensitive site. The transcription unit is markedly sensitive to DNase I, and exhibits a sharp transition from a phased half- to a whole nucleosomal cleavage periodicity at the 3' end. Distant upstream and downstream regions are also organized into distinct arrays of phased nucleosomes. Each array is demarcated by DNase I hypersensitive sites that display internal protected regions, suggesting the presence of DNA binding proteins. In addition, since these sites are of mononucleosomal DNA length, they may acquire a nucleosomal structure under certain environmental conditions without disrupting flanking nucleosomal phasing frames. Thus, the HSP82 locus is organized into specific, phased, chromatin structures that appear to function in transcriptional initiation, RNA polymerase passage, transcriptional termination, and the establishment of chromatin-domain microenvironments.

Complete sequence of the heat shock-inducible HSP90 gene of Saccharomyces cerevisiae

We have sequenced the yeast HSP90 gene, which encodes the (apparent) Mr = 90,000 heat shock-inducible protein of this organism. All sequences required for the heat shock-regulated expression of this gene reside on a segment of DNA containing no more than 273 nucleotides 5' to the transcription origin. Transcript mapping reveals that the mature hsp90 mRNA contains a 59-nucleotide 5' untranslated segment, a coding segment of 2130 nucleotides (sufficient to encode a protein of Mr = 81,419), and a 3' untranslated segment of 128 nucleotides. Although the sequences surrounding the coding region of the HSP90 gene are quite similar to those surrounding other sequenced yeast genes, we find as well a limited homology between sequences located 5' to this gene and the putative heat shock-regulatory sequences located 5' to the heat shock-inducible genes of Drosophila.

Heat shock-regulated production of Escherichia coli beta-galactosidase in Saccharomyces cerevisiae

The HSP90 gene of the yeast Saccharomyces cerevisiae encodes a heat shock-inducible protein with an Mr of 90,000 (hsp90) and unknown function. We fused DNA fragments of a known sequence (namely, either end of a 1.4-kilobase EcoRI fragment which contains the S. cerevisiae TRP1 gene) to an EcoRI site within the coding sequence of the HSP90 gene. When these fusions are introduced into S. cerevisiae they direct the synthesis of unique truncated hsp90 proteins. By determining the size and charge of these proteins we were able to deduce the translational reading frame at the (EcoRI) fusion site. This information allowed us to design and construct a well-defined in-frame fusion between the S. cerevisiae HSP90 gene and the Escherichia coli lacZ gene. When this fused gene is introduced into S. cerevisiae on a multicopy plasmid vector, it directs the heat shock-inducible synthesis of a fused protein, which is an enzymatically active beta-galactosidase. Thus, for the first time, it is possible to quantitate the heat shock response in a eucaryotic organism with a simple enzyme assay.

Identification and expression of a cloned yeast heat shock gene

We have isolated the yeast HSP90 gene which encodes the Mr = 90,000 heat shock-inducible protein of this organism. When this gene is introduced into yeast on a multicopy plasmid vector, a dramatic increase is observed in the level of synthesis of the Mr = 90,000 heat shock-inducible protein. This protein overproduction is due to expression of the plasmid-borne HSP90 gene, which is under the same heat shock regulation as its chromosomal counterpart. The presence of an increased dosage of the HSP90 gene has no effect on the synthesis of the other major heat shock-inducible proteins and does not alter the heat shock-associated phenotype of thermal tolerance.

Alterations of transcription during heat shock of Saccharomyces cerevisiae

The pattern of polyadenylated RNA of the yeast Saccharomyces cerevisiae changes dramatically upon heat shock. By in vitro translation, we have demonstrated that a 2.9-kilobase heat shock RNA encodes the Mr = 90,000 yeast heat shock protein. Heat shock-responsive genes have been isolated by differential plaque filter hybridization of a recombinant library of yeast DNA inserted in the vector lambda Charon 4. The putative yeast gene products of a number of these recombinants molecules has been determined by in vitro translation of hybrid-selected RNA. We have used one of these hybrid phages (lambda Yhsil) to demonstrate that the heat shock-induced alteration in the level of the 2.9-kilobase polyadenylated RNA which encodes the Mr = 90,000 yeast heat shock protein is regulated at the level of transcription.

alpha-Factor-mediatd modification of a 32P-labeled protein by MATa cells of Saccharomyces cerevisiae

Addition of the polypeptide mating pheromone alpha-factor to haploid MATa cells of Saccharomyces cerevisiae results in the modification of a 32P-labeled protein (P17) with an apparent Mr of 17,000 to a form having an apparent Mr of 17,500 (P17). 32P associated with both P17 and P17 exhibits an unusually rapid rate of turnover. The conversion of P17 to P17 precedes the appearance of morphologically abnormal cells and, in contrast to other responses elicited by this pheromone, this change in apparent molecular weight does not require protein synthesis. Upon removal of alpha-factor, the P17/P17 ratio returns to pretreatment levels.

Alterations in translatable ribonucleic acid after heat shock of Saccharomyces cerevisiae

Changes in populations of translatable messenger ribonucleic acids (mRNA's) after heat shock of Saccharomyces cerevisiae were examined and found to correlate very closely with transient alterations in patterns of in vivo protein synthesis. Initial changes included an increase in translatable species coding for polypeptides synthesized during heat shock; this increase was found to be dependent on transcription but did not require ongoing protein synthesis. A decrease was observed in the level of translatable mRNA's coding for polypeptides whose synthesis was repressed after heat shock. This decrease was much more rapid than can be explained solely by termination of transcription. Requirements for this rapid loss of RNA from the translatable pool included both transcription and an active rna1 gene product but not protein synthesis. After the initial changes in translatable RNA induced by heat shock, the patterns of both in vivo and in vitro translation products began to revert to the preshock levels. This recovery period, unlike the earlier changes, was dependent upon a requisite period of protein synthesis.

Metabolism of alpha-factor by a mating type cells of Saccharomyces cerevisiae

When a mating type cells of Saccharomyces cerevisiae are exposed to the mating pheromone alpha-factor in liquid cultures, there is a time-dependent loss of alpha-factor activity from the culture fluid. This loss of biological activity can be directly correlated with the proteolysis of the pheromone by a mating type cells. The metabolism of alpha-factor by a mating type cells may be measured by using either in vitro 125I-labeled or in vivo 35S-labeled pheromone. Addition of chloroquine to growing cultures of a mating type cells at concentrations which cause no detectable alterations in cell growth produces a potentiation of alpha-factor mediated cell cycle arrest. This potentiation of alpha-factor activity is directly correlated with the inhibition of alpha-factor proteolysis. Thus, while proteolytic digestion of alpha-factor appears to be related to the mechanism whereby a mating type cells "detoxify" alpha-factor and recover from cell cycle arrest, proteolysis of the mating factor is not necessary for alpha-factor mediated cell cycle arrest.

Reaction order of Saccharomyces cerevisiae alpha-factor-mediated cell cycle arrest and mating inhibition

Alpha-factor-mediated cell cycle arrest and mating inhibition of a mating-type cells of Saccharomyces cerevisiae have been examined in liquid cultures. Cell cycle arrest may be monitored unambiguously by the appearance of morphologically abnormal cells after administration of alpha factor, whereas mating inhibition is determined by comparing the mating efficiency in the absence or presence of added alpha factor. For both cell cycle arrest and mating inhibition, a dose-dependent response may be observed at limiting concentrations of the pheromone. If cell cycle arrest and mating inhibition require a small number of alpha-factor molecules, one might expect that responsive/nonresponsive cells = K(alpha factor)(N) where N is the order of dependence of cell cycle arrest (or mating inhibition) on alpha-factor concentration. The value of N has been determined to be 0.98 +/- 0.18 (standard error of the mean) for cell cycle arrest and 1.08 +/- 0.32 for mating inhibition. These results support the notion that saturation of a single site by alpha factor is sufficient to cause cell cycle arrest or mating inhibition of a mating-type cells.