The yeast cell cycle.

This review focuses on the G1 regulation of the p34cdc2/CDC28 kinase by cyclin-like proteins, which has substantially altered our understanding of cell cycle control. We discuss advances in elucidating the molecular composition of the mitotic apparatus, an essential step in understanding its cell-cycle-dependent assembly and functions.

The yeast cell cycle gene CDC34 encodes a ubiquitin-conjugating enzyme.

Mutants in the gene CDC34 of the yeast Saccharomyces cerevisiae are defective in the transition from G1 to the S phase of the cell cycle. This gene was cloned and shown to encode a 295-residue protein that has substantial sequence similarity to the product of the yeast RAD6 gene. The RAD6 gene is required for a variety of cellular functions including DNA repair and was recently shown to encode a ubiquitin-conjugating enzyme. When produced in Escherichia coli, the CDC34 gene product catalyzed the covalent attachment of ubiquitin to histones H2A and H2B in vitro, demonstrating that the CDC34 protein is another distinct member of the family of ubiquitin-conjugating enzymes. The cell cycle function of CDC34 is thus likely to be mediated by the ubiquitin-conjugating activity of its product.

The Ubc3 (Cdc34) ubiquitin-conjugating enzyme is ubiquitinated and phosphorylated in vivo.

The transition from G1 to S phase of the cell cycle in Saccharomyces cerevisiae requires the activity of the Ubc3 (Cdc34) ubiquitin-conjugating enzyme. S. cerevisiae cells lacking a functional UBC3 (CDC34) gene are able to execute the Start function that initiates the cell cycle but fail to form a mitotic spindle or enter S phase. The Ubc3 (Cdc34) enzyme has previously been shown to catalyze the attachment of multiple ubiquitin molecules to model substrates, suggesting that the role of this enzyme in cell cycle progression depends on its targeting an endogenous protein(s) for degradation. In this report, we demonstrate that the Ubc3 (Cdc34) protein is itself a substrate for both ubiquitination and phosphorylation. Immunochemical localization of the gene product to the nucleus renders it likely that the relevant substrates similarly reside within the nucleus.

Intragenic suppression among CDC34 (UBC3) mutations defines a class of ubiquitin-conjugating catalytic domains.

Ubiquitin-conjugating (E2) enzymes contain several regions within their catalytic domains that are highly conserved. However, within some of these conserved regions are several residues that may be used to define different classes of catalytic domains for the E2 enzymes. One class can be defined by the Ubc1 protein, which contains K-65, D-90, and D-120, while the corresponding positions within the Cdc34 (Ubc3) protein, which defines a second class of enzymes, contain S-73, S-97, and S-139, respectively. The presence of these differences within otherwise highly conserved regions of this family suggests that these residues may be critical for the specificity of Cdc34 function or regulation. Therefore, we have constructed a series of cdc34 alleles encoding mutant proteins in which these serine residues have been changed to other amino acid residues, including alanine and aspartic acid. In vivo complementation studies showed that S-97, which lies near the active site C-95, is essential for Cdc34 function. The addition of a second mutation in CDC34, which now encoded both the S97D and S73K changes, restored partial function to the Cdc34 enzyme. Moreover, the deletion of residues 103 to 114 within Cdc34, which are not present in the Ubc1-like E2s, allowed the S73K/S97D mutant to function as efficiently as wild-type Cdc34 protein. Finally, the cloning and sequencing of the temperature-sensitive alleles of CDC34 indicated that A-62 is also unique to the Cdc34 class of E2 enzymes and that mutations at this position can be detrimental to Cdc34 function. Our results suggest that several key residues within conserved regions of the E2 enzyme family genetically interact with each other and define a class of E2 catalytic domains.

The abundance of Met30p limits SCF(Met30p) complex activity and is regulated by methionine availability.

Ubiquitin-mediated degradation plays a crucial role in many fundamental biological pathways, including the mediation of cellular responses to changes in environmental conditions. A family of ubiquitin ligase complexes, called SCF complexes, found throughout eukaryotes, is involved in a variety of biological pathways. In Saccharomyces cerevisiae, an SCF complex contains a common set of components, namely, Cdc53p, Skp1p, and Hrt1p. Substrate specificity is defined by a variable component called an F-box protein. The F- box is a approximately 40-amino-acid motif that allows the F-box protein to bind Skp1p. Each SCF complex recognizes different substrates according to which F-box protein is associated with the complex. In yeasts, three SCF complexes have been demonstrated to associate with the ubiquitin-conjugating enzyme Cdc34p and have ubiquitin ligase activity. F-box proteins are not abundant and are unstable. As part of the SCF(Met30p) complex, the F-box protein Met30p represses methionine biosynthetic gene expression when availability of L-methionine is high. Here we demonstrate that in vivo SCF(Met30p) complex activity can be regulated by the abundance of Met30p. Furthermore, we provide evidence that Met30p abundance is regulated by the availability of L-methionine. We propose that the cellular responses mediated by an SCF complex are directly regulated by environmental conditions through the control of F-box protein stability.

Mouse receptor interacting protein 3 does not contain a caspase-recruiting or a death domain but induces apoptosis and activates NF-kappaB.

The death domain-containing receptor superfamily and their respective downstream mediators control whether or not cells initiate apoptosis or activate NF-kappaB, events critical for proper immune system function. A screen for upstream activators of NF-kappaB identified a novel serine-threonine kinase capable of activating NF-kappaB and inducing apoptosis. Based upon domain organization and sequence similarity, this novel kinase, named mRIP3 (mouse receptor interacting protein 3), appears to be a new RIP family member. RIP, RIP2, and mRIP3 contain an N-terminal kinase domain that share 30 to 40% homology. In contrast to the C-terminal death domain found in RIP or the C-terminal caspase-recruiting domain found in RIP2, the C-terminal tail of mRIP3 contains neither motif and is unique. Despite this feature, overexpression of the mRIP3 C terminus is sufficient to induce apoptosis, suggesting that mRIP3 uses a novel mechanism to induce death. mRIP3 also induced NF-kappaB activity which was inhibited by overexpression of either dominant-negative NIK or dominant-negative TRAF2. In vitro kinase assays demonstrate that mRIP3 is catalytically active and has autophosphorylation site(s) in the C-terminal domain, but the mRIP3 catalytic activity is not required for mRIP3 induced apoptosis and NF-kappaB activation. Unlike RIP and RIP2, mRIP3 mRNA is expressed in a subset of adult tissues and is thus likely to be a tissue-specific regulator of apoptosis and NF-kappaB activity. While the lack of a dominant-negative mutant precludes linking mRIP3 to a known upstream regulator, characterizing the expression pattern and the in vitro functions of mRIP3 provides insight into the mechanism(s) by which cells modulate the balance between survival and death in a cell-type-specific manner.

Phosphorylation of nuclear MyoD is required for its rapid degradation.

MyoD is a basic helix-loop-helix transcription factor involved in the activation of genes encoding skeletal muscle-specific proteins. Independent of its ability to transactivate muscle-specific genes, MyoD can also act as a cell cycle inhibitor. MyoD activity is regulated by transcriptional and posttranscriptional mechanisms. While MyoD can be found phosphorylated, the functional significance of this posttranslation modification has not been established. MyoD contains several consensus cyclin-dependent kinase (CDK) phosphorylation sites. In these studies, we examined whether a link could be established between MyoD activity and phosphorylation at putative CDK sites. Site-directed mutagenesis of potential CDK phosphorylation sites in MyoD revealed that S200 is required for MyoD hyperphosphorylation as well as the normally short half-life of the MyoD protein. Additionally, we determined that turnover of the MyoD protein requires the proteasome and Cdc34 ubiquitin-conjugating enzyme activity. Results of these studies demonstrate that hyperphosphorylated MyoD is targeted for rapid degradation by the ubiquitin pathway. The targeted degradation of MyoD following CDK phosphorylation identifies a mechanism through which MyoD activity can be regulated coordinately with the cell cycle machinery (CDK2 and CDK4) and/or coordinately with the cellular transcriptional machinery (CDK7, CDK8, and CDK9).

Requirement of the self-glucosylating initiator proteins Glg1p and Glg2p for glycogen accumulation in Saccharomyces cerevisiae.

Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous nutritional control in many cells. We report the identification of two Saccharomyces cerevisiae genes, GLG1 and GLG2, whose products are implicated in the biogenesis of glycogen. These genes encode self-glucosylating proteins that in vitro can act as primers for the elongation reaction catalyzed by glycogen synthase. Over a region of 258 residues, the Glg proteins have 55% sequence identify to each other and approximately 33% identity to glycogenin, a mammalian protein postulated to have a role in the initiation of glycogen biosynthesis. Yeast cells defective in either GLG1 or GLG2 are similar to the wild type in their ability to accumulate glycogen. Disruption of both genes results in the inability of the cells to synthesize glycogen despite normal levels of glycogen synthase. These results suggest that a self-glucosylating protein is required for glycogen biosynthesis in a eukaryotic cell. The activation state of glycogen synthase in glg1 glg2 cells is suppressed, suggesting that the Glg proteins may additionally influence the phosphorylation state of glycogen synthase.

Cdc53p acts in concert with Cdc4p and Cdc34p to control the G1-to-S-phase transition and identifies a conserved family of proteins.

Regulation of cell cycle progression occurs in part through the targeted degradation of both activating and inhibitory subunits of the cyclin-dependent kinases. During G1, CDC4, encoding a WD-40 repeat protein, and CDC34, encoding a ubiquitin-conjugating enzyme, are involved in the destruction of these regulators. Here we describe evidence indicating that CDC53 also is involved in this process. Mutations in CDC53 cause a phenotype indistinguishable from those of cdc4 and cdc34 mutations, numerous genetic interactions are seen between these genes, and the encoded proteins are found physically associated in vivo. Cdc53p defines a large family of proteins found in yeasts, nematodes, and humans whose molecular functions are uncharacterized. These results suggest a role for this family of proteins in regulating cell cycle proliferation through protein degradation.

The identification of transposon-tagged mutations in essential genes that affect cell morphology in Saccharomyces cerevisiae.

The yeast Saccharomyces cerevisiae reproduces by budding, and many genes are required for proper bud development. Mutations in some of these genes cause cells to die with an unusual terminal morphology-elongated or otherwise aberrantly shaped buds. To gain insight into bud development, we set out to identify novel genes that encode proteins required for proper bud morphogenesis. Previous studies screened collections of conditional mutations to identify genes required for essential functions, including bud formation. However, genes that are not susceptible to the generation of mutations that cause a conditional phenotype will not be identified in such screens. To identify a more comprehensive collection of mutants, we used transposon mutagenesis to generate a large collection of lethal disruption mutations. This collection was used to identify 209 mutants with disruptions that cause an aberrant terminal bud morphology. The disruption mutations in 33 of these mutants identify three previously uncharacterized genes as essential, and the mutant phenotypes suggest roles for their products in bud morphogenesis.

Genetic interactions between REG1/HEX2 and GLC7, the gene encoding the protein phosphatase type 1 catalytic subunit in Saccharomyces cerevisiae.

Mutations in GLC7, the gene encoding the type 1 protein phosphatase catalytic subunit, cause a variety of abberrant phenotypes in yeast, such as impaired glycogen synthesis and relief of glucose repression of the expression of some genes. Loss of function of the REG1/HEX2 gene, necessary for glucose repression of several genes, was found to suppress the glycogen-deficient phenotype of the glc7-1 allele. Deletion of REG1 in a wild-type background led to overaccumulation of glycogen as well as slow growth and an enlarged cell size. However, loss of REG1 did not suppress other phenotypes associated with GLC7 mutations, such as inability to sporulate or, in cells bearing the glc7Y-170 allele, lack of growth at 14 degrees. The effect of REG1 deletion on glycogen accumulation is not simply due to derepression of glucose-repressed genes, although it does require the presence of SNF1, which encodes a protein kinase essential for expression of glucose-repressed genes and for glycogen accumulation. We propose that REG1 has a role in controlling glycogen accumulation.

Most of the yeast genomic sequences are not essential for cell growth and division.

To determine the fraction of the yeast Saccharomyces cerevisiae genome that is required for normal cell growth and division, we constructed diploid strains that were heterozygous for random single disruptions. We monitored the effects of approximately 200 independent disruptions by sporulating the diploids and examining the phenotype of the resulting haploid strains. We found that only 12% of the disruptions were haploid-lethal, 14% resulted in slow growth, and an additional 4% were associated with some other new phenotype (such as an auxotrophic requirement). No obvious new phenotype was detected for 70% of the disruptions.

Mutational analysis of Cak1p, an essential protein kinase that regulates cell cycle progression.

In Saccharomyces cerevisiae, entry into S phase requires the activation of the protein kinase Cdc28p through binding with cyclin Clb5p or Clb6p, as well as the destruction of the cyclin-dependent kinase inhibitor Sic1p. Mutants that are defective in this activation event arrest after START, with unreplicated DNA and multiple, elongated buds. These mutants include cells defective in CDC4, CDC34 or CDC53, as well as cells that have lost all CLB function. Here we describe mutations in another gene, CAK1, that lead to a similar arrest. Cells that are defective in CAK1 are inviable and arrest with a single nucleus and multiple, elongated buds. CAK1 encodes a protein kinase most closely related to the Cdc2p family of protein kinases. Mutations that lead to the production of an inactive kinase that can neither autophosphorylate, nor phosphorylate Cdc28p in vitro are also incapable of rescuing a cell with a deletion of CAK1. These results underscore the importance of the Cak1p protein kinase activity in cell cycle progression.

An essential domain within Cdc34p is required for binding to a complex containing Cdc4p and Cdc53p in Saccharomyces cerevisiae.

The CDC34 gene of the yeast Saccharomyces cerevisiae encodes a ubiquitin-conjugating protein that transfers ubiquitin onto substrates to signal rapid degradation via the proteasome. Cdc34p has been implicated in signaling the destruction of a variety of substrates including the cyclin-dependent kinase inhibitor, Sic1p, which must be degraded for cells to enter S-phase. Mutants lacking CDC34 activity fail to degrade Sic1p and fail to enter S-phase, a phenotype that is also shared with cells lacking CDC4 and CDC53 activity. Here we demonstrate that Cdc4p, Cdc34p, and Cdc53p interact in vivo. We have mapped a Cdc4p/Cdc53p-binding region on Cdc34p; this region is essential for S-phase entry and thus the association of these three proteins is required for Sic1p degradation. All three proteins migrate in gel filtration to sizes that greatly exceed their actual size suggesting that they form stable associations with other proteins and we observe Cdc4p, Cdc34p, and Cdc53p fractionating into overlapping families of high molecular weight complexes. Finally, we demonstrate that Cdc4p, Cdc34p, and Cdc53p are stable throughout the cell cycle and that Cdc34p permanently resides as part of a complex throughout the cell cycle. This suggests that all Cdc34p substrates are ubiquitinated by a similar high molecular weight complex.

Ubiquitin-dependent proteolysis and cell cycle control in yeast.

Genetic and biochemical data indicate that ubiquitin-mediated proteolysis is involved in the regulated turnover of proteins required for controlling cell cycle progression. In general, mutations in some genes that encode proteins involved in the ubiquitin pathway cause cell cycle defects and affect the turnover of cell cycle regulatory proteins. Furthermore, some cell cycle regulatory proteins are short-lived, ubiquitinated, and degraded by the ubiquitin pathway. This review will examine how the ubiquitin pathway plays a role in regulating progression from the G1 to the S phase of the cell cycle, as well as the G2 to M phase transition.

Two glycogen synthase isoforms in Saccharomyces cerevisiae are coded by distinct genes that are differentially controlled.

In previous work, we identified a Saccharomyces cerevisiae glycogen synthase gene, GSY1, which codes for an 85-kDa polypeptide present in purified yeast glycogen synthase (Farkas, I., Hardy, T.A., DePaoli-Roach, A.A., and Roach, P.J. (1990) J. Biol. Chem. 265, 20879-20886). We have now cloned another gene, GSY2, which encodes a second S. cerevisiae glycogen synthase. The GSY2 sequence predicts a protein of 704 residues, molecular weight 79,963, with 80% identity to the protein encoded by GSY1. Amino acid sequences obtained from a second polypeptide of 77 kDa present in yeast glycogen synthase preparations matched those predicted by GSY2. GSY1 resides on chromosome VI, and GSY2 is located on chromosome XII. Disruption of the GSY1 gene produced a strain retaining about 85% of wild type glycogen synthase activity at stationary phase, while disruption of the GSY2 gene yielded a strain with only about 10% of wild type enzyme activity. The level of glycogen synthase activity in yeast cells disrupted for GSY1 increased in stationary phase, whereas the activity remained at a constant low level in cells disrupted for GSY2. Disruption of both genes resulted in a viable haploid that totally lacked glycogen synthase activity and was defective in glycogen deposition. In conclusion, yeast expresses two forms of glycogen synthase with activity levels that behave differently in the growth cycle. The GSY2 gene product appears to be the predominant glycogen synthase with activity linked to nutrient depletion.

Rapid amplification of uncharacterized transposon-tagged DNA sequences from genomic DNA.

Although the entire DNA sequence of the yeast genome has been determined, the functions of nearly a third of the identified genes are unknown. Recently, we described a collection of mutants, each with a transposon-tagged disruption in an essential gene in Saccharomyces cerevisiae. Identification of these essential genes and characterization of their mutant phenotypes should help assign functions to these thousands of novel genes, and since each mutation in our collection is physically marked by the uniform, unique DNA sequence of the transposable element, it should be possible to use the polymerase chain reaction (PCR) to amplify the DNA adjacent to the transposon. However, existing PCR methods include steps that make their use on a large scale cumbersome. In this report, we describe a semi-random, two-step PCR protocol, ST-PCR. This method is simpler and more specific than current methods, requiring only genomic DNA and two pairs of PCR primers, and involving two successive PCR reactions. Using this method, we have rapidly and easily identified the essential genes identified by several of our mutants.

Developmental and tissue-specific expression of mouse pelle-like protein kinase.

The NF-kappaB/c-Rel proteins are a family of evolutionarily conserved transcription factors activated during development that in the adult, mediate many processes including the immune response. A high degree of sequence similarity is shared between the NF-kappaB/c-Rel family of transcription factors and the Drosophila Dorsal protein as well as between its cytoplasmic inhibitor, IkappaBalpha, and the Drosophila Cactus protein. Genetic analyses of Dorsal have defined components of a signaling pathway for Dorsal activation, including a serine/threonine kinase, Pelle, placed upstream of Dorsal and Cactus. We demonstrate that this pathway is likely to be conserved in mammals by the isolation of a cDNA that encodes a novel mouse protein highly related to Pelle, mPLK (mouse Pelle-like protein kinase). Expression of mPLK mRNA is developmentally regulated in the mouse and in adult tissue mPLK expression is greatest in the liver, a tissue that expresses a high level of NF-kappaB. Recombinant mPLK produced in bacteria is a protein kinase capable of autophosphorylating and phosphorylating IkappaBalpha.

Modulation of tumor necrosis factor and interleukin-1-dependent NF-kappaB activity by mPLK/IRAK.

The innate immune response is an important defense against pathogenic agents. A component of this response is the NF-kappaB-dependent activation of genes encoding inflammatory cytokines such as interleukin-8 (IL-8) and cell adhesion molecules like E-selectin. Members of the serine/threonine innate immune kinase family of proteins have been proposed to mediate the innate immune response. One serine/threonine innate immune kinase family member, the mouse Pelle-like kinase/human interleukin-1 receptor-associated kinase (mPLK/IRAK), has been proposed to play an obligate role in promoting IL-1-mediated inflammation. However, it is currently unknown whether mPLK/IRAK catalytic activity is required for IL-1-dependent NF-kappaB activation. The present study demonstrates that mPLK/IRAK catalytic activity is not required for IL-1-mediated activation of an NF-kappaB-dependent signal. Intriguingly, catalytically inactive mPLK/IRAK inhibits type 1 tumor necrosis factor (TNF) receptor-dependent NF-kappaB activation. The pathway through which mPLK/IRAK mediates this TNF response is TRADD- and TRAF2-independent. Our data suggest that in addition to its role in IL-1 signaling, mPLK/IRAK is a component of a novel signal transduction pathway through which TNF R1 activates NF-kappaB-dependent gene expression.