Adenovirus E1A is associated with a serine/threonine protein kinase

Multiple domains in the 50 kDa form of E4F1 regulate promoter-specific repression and E1A trans-activation

The 50 kDa N-terminal product of the cellular transcription factor E4F1 (p50E4F1) mediates E1A289R trans-activation of the adenovirus E4 gene, and suppresses E1A-mediated transformation by sensitizing cells to cell death. This report shows that while both E1A289R and E1A243R stimulate p50E4F1 DNA binding activity, E1A289R trans-activation, as measured using GAL-p50E4F1 fusion proteins, involves a p50E4F1 transcription regulatory (TR) region that must be promoter-bound and is dependent upon E1A CR3, CR1 and N-terminal domains. Trans-activation is promoter-specific, as GAL-p50E4F1 did not stimulate commonly used artificial promoters and was strongly repressive when competing against GAL-VP16. p50E4F1 and E1A289R stably associate in vivo using the p50E4F1 TR region and E1A CR3, although their association in vitro is indirect and paradoxically disrupted by MAP kinase phosphorylation of E1A289R, which stimulates E4 trans-activation in vivo. Multiple cellular proteins, including TBP, bind the p50E4F1 TR region in vitro. The mechanistic implications for p50E4F1 function are discussed.

Regulation of nucleocytoplasmic trafficking of viral proteins: an integral role in pathogenesis?

Signal-dependent targeting of proteins into and out of the nucleus is mediated by members of the importin (IMP) family of transport receptors, which recognise targeting signals within a cargo protein and mediate passage through the nuclear envelope-embedded nuclear pore complexes. Regulation of this process is paramount to processes such as cell division and differentiation, but is also critically important for viral replication and pathogenesis; phosphorylation appears to play a major role in regulating viral protein nucleocytoplasmic trafficking, along with other posttranslational modifications. This review focuses on viral proteins that utilise the host cell IMP machinery in order to traffic into/out of the nucleus, and in particular those where trafficking is critical to viral replication and/or pathogenesis, such as simian virus SV40 large tumour antigen (T-ag), human papilloma virus E1 protein, human cytomegalovirus processivity factor ppUL44, and various gene products from RNA viruses such as Rabies. Understanding of the mechanisms regulating viral protein nucleocytoplasmic trafficking is paramount to the future development of urgently needed specific and effective anti-viral therapeutics. This article was originally intended for the special issue "Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import". The Publisher apologizes for any inconvenience caused.

Binding of p110 retinoblastoma protein inhibits nuclear import of simian virus SV40 large tumor antigen

Nuclear import of the simian virus 40 large tumor antigen (T-ag) is dependent on its nuclear localization signal (NLS) within amino acids 126-132 that is recognized by the importin alpha/beta1 heterodimer, as well as a protein kinase CK2 site at serine 112 upstream of the NLS, which enhances the interaction approximately 50-fold. Here we show for the first time that T-ag nuclear import is negatively regulated by N-terminal sequences (amino acids 102-110), which represent the binding site (BS) for the retinoblastoma (Rb) tumor suppressor protein (p110(Rb)). Quantitative confocal laser scanning microscopic analysis of the transport properties of T-ag constructs with or without Rb binding site mutations in living transfected cells or in a reconstituted nuclear transport system indicates that the presence of the RbBS significantly reduces nuclear accumulation of T-ag. A number of approaches, including the analysis of T-ag nuclear import in an isogenic cell pair with and without functional p110(Rb) implicate p110(Rb) binding as being responsible for the reduced nuclear accumulation, with the Ser(106) phosphorylation site within the RbBS appearing to enhance the inhibitory effect. Immunoprecipitation experiments confirmed association of T-ag and p110(Rb) and dependence thereof on negative charge at Ser(106). The involvement of p110(Rb) in modulating T-ag nuclear transport has implications for the regulation of nuclear import of other proteins from viruses of medical significance that interact with p110(Rb), and how this may relate to transformation.

How the Rb tumor suppressor structure and function was revealed by the study of Adenovirus and SV40

The review recounts the history of how the study of the DNA tumor viruses including polyoma, SV40 and Adenovirus brought key insights into the structure and function of the Retinoblastoma protein (Rb). Knudsen's model of the two-hit hypothesis to explain patterns of hereditary and sporadic retinoblastoma provided the foundation for the tumor suppressor hypothesis that ultimately led to the cloning of the Rb gene. The discovery that SV40 and Adenovirus could cause tumors when inoculated into animals was startling not only because SV40 had contaminated the poliovirus vaccine and Adenovirus was a common cause of viral induced pneumonia but also because they provided an opportunity to study the genetics and biochemistry of cancer. Studies of mutant forms of these viruses led to the identification of the E1A and Large T antigen (LT) oncogenes and their small transforming elements including the Adenovirus Conserved Regions (CR), the SV40 J domain and the LxCxE motif. The immunoprecipitation studies that initially revealed the size and ultimately the identity of cellular proteins that could bind to these transforming elements were enabled by the widespread development of highly specific monoclonal antibodies against E1A and LT. The identification of Rb as an E1A and LT interacting protein quickly led to the cloning of p107, p130, p300, CBP, p400 and TRRAP and the concept that viral transformation was due, at least in part, to the perturbation of the function of normal cellular proteins. In addition, studies on the ability of E1A to transactivate the Adenovirus E2 promoter led to the cloning of the heterodimeric E2F and DP transcription factor and recognition that Rb repressed transcription of cellular genes required for cell cycle entry and progression. More recent studies have revealed how E1A and LT combine the activity of Rb and the other cellular associated proteins to perturb expression of many genes during viral infection and tumor formation.

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

RB, the most investigated tumor suppressor gene, is the founder of the RB family of growth/tumor suppressors, which comprises also p107 (RBL1) and Rb2/p130 (RBL2). The protein products of these genes, pRb, p107 and pRb2/p130, respectively, are also known as 'pocket proteins', because they share a 'pocket' domain responsible for most of the functional interactions characterizing the activity of this family of cellular factors. The interest in these genes and proteins springs essentially from their ability to regulate negatively cell cycle processes and for their ability to slow down or abrogate neoplastic growth. The pocket domain of the RB family proteins is dramatically hampered in its functions by the interference of a number of proteins produced by the small DNA viruses. In the last two decades, the 'viral hypothesis' of cancer has received a considerable renewed impulse from the notion that small DNA viruses, such as Adenovirus, Human papillomavirus (HPV) and Polyomavirus, produce factors that can physically interact with major cellular regulators and alter their function. These viral proteins (oncoproteins) act as multifaceted molecular devices that have evolved to perform very specific tasks. Owing to these features, viral oncoproteins have been widely employed as invaluable experimental tools for the identification of several key families of regulators, particularly of the cell cycle homeostasis. Adenovirus early-region 1A (E1A) is the most widely investigated small DNA tumor virus oncoprotein, but relevant interest in human oncology is raised by the E1A-related E7 protein from transforming HPV strains and by Polyomavirus oncoproteins, particularly large and small T antigens from Simian virus 40, JC virus and BK virus.

A kinase activity associated with simian virus 40 large T antigen phosphorylates upstream binding factor (UBF) and promotes formation of a stable initiation complex between UBF and SL1

Simian virus 40 large T antigen is a multifunctional protein which has been shown to modulate the expression of genes transcribed by RNA polymerase I (Pol I), II, and III. In all three transcription systems, a key step in the activation process is the recruitment of large T antigen to the promoter by direct protein-protein interaction with the TATA binding protein (TBP)-TAF complexes, namely, SL1, TFIID, and TFIIIB. However, our previous studies on large T antigen stimulation of Pol I transcription also revealed that the binding to the TBP-TAFI complex SL1 is not sufficient to activate transcription. To further define the molecular mechanism involved in large T antigen-mediated Pol I activation, we examined whether the high-mobility group box-containing upstream binding factor (UBF) plays any role in this process. Here, using cell labeling experiments, we showed that large T antigen expression induces an increase in UBF phosphorylation. Further biochemical analysis demonstrated that UBF is phosphorylated by a kinase activity that is strongly associated with large T antigen, and that the carboxy-terminal activation domain of UBF is required for the phosphorylation to occur. Using in vitro reconstituted transcription assays, we demonstrated that the inability of alkaline phosphatase treated UBF to efficiently activate transcription can be rescued by large T antigen. Moreover, we showed that large T antigen-induced UBF phosphorylation promotes the formation of a stable UBF-SL1 complex. Together, these results provide strong evidence for an important role for the large T antigen-associated kinase in mediating the stimulation of RNA Pol I transcription.

Reduced phosphorylation of p50 is responsible for diminished NF-kappaB binding to the major histocompatibility complex class I enhancer in adenovirus type 12-transformed cells

Reduced cell surface levels of major histocompatibility complex class I antigens enable adenovirus type 12 (Ad12)-transformed cells to escape immunosurveillance by cytotoxic T lymphocytes (CTL), contributing to their tumorigenic potential. In contrast, nontumorigenic Ad5-transformed cells harbor significant cell surface levels of class I antigens and are susceptible to CTL lysis. Ad12 E1A mediates down-regulation of class I transcription by increasing COUP-TF repressor binding and decreasing NF-kappaB activator binding to the class I enhancer. The mechanism underlying the decreased binding of nuclear NF-kappaB in Ad12-transformed cells was investigated. Electrophoretic mobility shift assay analysis of hybrid NF-kappaB dimers reconstituted from denatured and renatured p50 and p65 subunits from Ad12- and Ad5-transformed cell nuclear extracts demonstrated that p50, and not p65, is responsible for the decreased ability of NF-kappaB to bind to DNA in Ad12-transformed cells. Hypophosphorylation of p50 was found to correlate with restricted binding of NF-kappaB to DNA in Ad12-transformed cells. The importance of phosphorylation of p50 for NF-kappaB binding was further demonstrated by showing that an NF-kappaB dimer composed of p65 and alkaline phosphatase-treated p50 from Ad5-transformed cell nuclear extracts could not bind to DNA. These results suggest that phosphorylation of p50 is a key step in the nuclear regulation of NF-kappaB in adenovirus-transformed cells.

Tat-associated kinase, TAK, activity is regulated by distinct mechanisms in peripheral blood lymphocytes and promonocytic cell lines

TAK, a multisubunit cellular protein kinase that specifically associates with the human immunodeficiency virus Tat proteins and hyperphosphorylates the carboxyl-terminal domain of RNA polymerase II, is a cofactor for Tat and mediates its transactivation function. The catalytic subunit of TAK has been identified as cyclin-dependent kinase Cdk9, and its regulatory partner has been identified as cyclin T1; these proteins are also components of positive transcription elongation factor P-TEFb. TAK activity is up-regulated upon activation of peripheral blood lymphocytes and following macrophage differentiation of promonocytic cell lines. We have found that activation of peripheral blood lymphocytes results in increased mRNA and protein levels of both Cdk9 and cyclin T1. Cdk9 and cyclin T1 induction occurred in purified CD4(+) primary T cells activated by a variety of stimuli. In contrast, phorbol ester-induced differentiation of promonocytic cell lines into macrophage-like cells produced a large induction of cyclin T1 protein expression from nearly undetectable levels, while Cdk9 protein levels remained at a constant high level. Measurements of cyclin T1 mRNA levels in a promonocytic cell line suggested that regulation of cyclin T1 occurs at a posttranscriptional level. These results suggest that cyclin T1 and TAK function may be required in differentiated monocytes and further show that TAK activity can be regulated by distinct mechanisms in different cell types.

E1A 12S and 13S of the transformation-defective adenovirus type 12 strain CS-1 inactivate proteins of the RB family, permitting transactivation of the E2F-dependent promoter

The transformation-defective Vero cell host range mutant CS-1 of the highly oncogenic adenovirus type 12 (Ad12) (Ad12-CS-1) has a 69-bp deletion in the early region 1A (E1A) gene that removes the carboxy-terminal half of conserved region 2 and the amino-terminal half of the Ad12-specific so-called spacer that seems to play a pivotal role in the oncogenicity of the virus. Despite its deficiency in immortalizing and transforming primary rodent cells, we found that the E1A 13S protein of Ad12-CS-1 retains the ability to bind p105-RB, p107, and p130 in nuclear extract binding assays with glutathione S-transferase-E1A fusion proteins and Western blot analysis. Like wild-type E1A, the mutant protein was able to dissociate E2F from retinoblastoma-related protein-containing complexes, as judged from gel shift experiments with purified 12S and 13S proteins from transfection experiments with an E1A expression vector or from infection with the respective virus. Moreover, in transient expression assays, the 12S and 13S products of wild-type Ad12 and Ad12-CS-1 were shown to transactivate the Ad12 E1A promoter containing E2F-1 and E2F-5-motifs, respectively, in a comparable manner. The same results were obtained from transfection assays with the E2F motif-dependent E2 promoter of adenovirus type 5 or the human dihydrofolate reductase promoter. These data suggest that efficient infection by Ad12 and the correlated virus-induced reprogramming of the infected cells, including the induction of cell cycle-relevant mechanisms (e.g. E2F activation), can be uncoupled from the transformation properties of the virus.

Hepatitis C virus NS5A protein is phosphorylated in vitro by a stably bound protein kinase from HeLa cells and by cAMP-dependent protein kinase A-alpha catalytic subunit

Hepatitis C virus (HCV) has a positive-strand RNA genome that codes for a polyprotein precursor, which is processed co- and post-translationally by cellular and viral proteinases into three structural and at least six non-structural (NS) proteins. The NS5A protein, expressed in mammalian cells, exists in two phosphorylated forms of 56-kDa and 58-kDa. In this study, we provide evidence for a stable association between NS5A and a protein kinase from HeLa cells and hepatocellular carcinoma (HepG2) cells by co-immunoprecipitation and by affinity to immobilized glutathione-S-transferase (GST)-NS5A fusion protein produced in E. coli. This protein kinase could phosphorylate in vitro the native NS5A on serine residues, (GST)-NS5A, histone H1, and casein as substrates. In addition, the GST-NS5A was also phosphorylated in vitro by the cAMP-dependent protein kinase A-alpha catalytic subunit.

Cyclin-dependent kinase regulation during G1 phase and cell cycle regulation by TGF-beta

The aim of this review is to provide insight into the molecular mechanisms by which transforming growth factor-beta (TGF-beta) modulates cell cycle progression in different cell types. Particular attention is focused on the differences between these mechanisms in cells of epithelial origin and in mesenchymally derived cells. This is important because many transformed epithelial cells lose responsiveness to the growth-inhibitory effects of TGF-beta, thus generating a more fibroblast-like phenotype. Loss of negative growth control, including a lack of response to growth-inhibitory factors, is a common feature of many tumor cells. G1 phase cyclin-dependent kinases (cdks) and their inhibitors (ckis) are central to the pathways that regulate commitment to cellular division in response to positive as well as negative growth effectors. Many checkpoints are deregulated in oncogenesis, and this is often due to alterations in cyclin-cdk complexes. The loss of R-point regulation, in particular, can allow cell growth and division to proceed autonomously of external signals. This may occur due to either the aberrant expression of positive regulators, such as the cyclins and cdks, or the loss of negative regulators, such as the ckis. Beginning with a survey of the role of the cdks in the mammalian cell cycle, the review examines how cdk activity is modulated by cyclin binding, phosphorylation, and ckis, including the Ink4 proteins and the closely related inhibitors p21Cip1 and p27Kip1. Particular attention is paid to the role of p27Kip1 and p21Cip1 in the mechanisms of TGF-beta-induced suppression or stimulation of the cell cycle and how these mechanisms contrast between epithelial cells and cells of mesenchymal origin. Other aspects of TGF-beta signal transduction are discussed, including its effects on cyclin and cdk expression in various cell types, and the downstream targets of cdks and their modulation by TGF-beta and other growth factors are also discussed. These include proteins of the retinoblastoma family, and the related modulation of the transcriptional activity of the E2F family members. Finally, the role of cell cycle regulatory proteins in oncogenesis is review in view of the findings described here.

Association of p300 and CBP with simian virus 40 large T antigen

p300 and the CREB-binding protein CBP are two large nuclear phosphoproteins that are structurally highly related. Both function, in part, as transcriptional adapters and are targeted by the adenovirus E1A oncoprotein. We show here that p300 and CBP interact with another transforming protein, the simian virus 40 large T antigen (T). This interaction depends on the integrity of a region of T which is critical for its transforming and mitogenic properties and includes its LXCXE Rb-binding motif. T interferes with normal p300 and CBP function on at least two different levels. The presence of T alters the phosphorylation states of both proteins and inhibits their transcriptional activities on certain promoters. Although E1A and T show little sequence similarity, they interact with the same domain of p300 and CBP, suggesting that this region exhibits considerable flexibility in accommodating diverse protein ligands.

Viral transactivators specifically target distinct cellular protein kinases that phosphorylate the RNA polymerase II C-terminal domain

Phosphorylation of the carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II has been implicated as an important step in transcriptional regulation. Previously, we reported that a cellular CTD kinase, TAK, is targeted by the human immunodeficiency virus transactivator Tat. In the present study, we analyzed several other transactivators for the ability to interact with CTD kinases in vitro. The adenovirus E1A and herpes simplex virus VP16 proteins, but not other transactivators tested, were found to associate with a cellular kinase activity that hyperphosphorylates the CTD. The interaction is dependent upon a functional activation domain of E1A or VP16, suggesting that the interaction with a CTD kinase is relevant for the transactivation function of these proteins. The CTD kinase activities that interact with E1A and VP16 are related to each other but distinct from TAK. The Tat-, E1A- and VP16-associated CTD kinase activities detected in our assay also appear unrelated to MO15, the catalytic component of the CTD kinase activity of the general transcription factor TFIIH. Thus, this study has identified a novel interaction between viral transactivators and a cellular CTD kinase and suggests that at least two CTD kinases may mediate responses to viral transactivators.

Suppression of mutations in two Saccharomyces cerevisiae genes by the adenovirus E1A protein

The protein products of the adenoviral E1A gene are implicated in a variety of transcriptional and cell cycle events, involving interactions with several proteins present in human cells, including parts of the transcriptional machinery and negative regulators of cell division such as the Rb gene product and p107. To determine if there are functional homologs of E1A in Saccharomyces cerevisiae, we have developed a genetic screen for mutants that depend on E1A for growth. The screen is based on a colony color sectoring assay which allows the identification of mutants dependent on the maintenance and expression of an E1A-containing plasmid. Using this screen, we have isolated five mutants that depend on expression of the 12S or 13S cDNA of E1A for growth. A plasmid shuffle assay confirms that the plasmid-dependent phenotype is due to the presence of either the 12S or the 13S E1A cDNA and that both forms of E1A rescue growth of all mutants equally well. The five mutants fall into two classes that were named web1 and web2 (for "wants E1A badly"). Plasmid shuffle assays with mutant forms of E1A show that conserved region 1 (CR1) is required for rescue of the growth of the web1 and web2 E1A-dependent yeast mutants, while the N-terminal 22 amino acids are only partially required; conserved region 2 (CR2) and the C terminus are dispensable. The phenotypes of mutants in both the web1 and the web2 groups are due to a single gene defect, and the yeast genes that fully complement the mutant phenotypes of both groups were cloned. The WEB1 gene sequence encodes a 1,273-amino-acid protein that is identical to SEC31, a protein involved in the budding of transport vesicles from the endoplasmic reticulum. The WEB2 gene encodes a 1,522-amino-acid protein with homology to nucleic acid-dependent ATPases. Deletion of either WEB1 or WEB2 is lethal. Expression of E1A is not able to rescue the lethality of either the web1 or the web2 null allele, implying allele-specific mutations that lead to E1A dependence.

Binding of an interferon-inducible protein (p202) to the retinoblastoma protein

Many of the antimicrobial, immunomodulatory, and cell growth regulatory activities of the interferons are mediated by interferon-inducible proteins. One family of such murine proteins is encoded by six or more adjacent and structurally related genes (gene 200 cluster). Two homologous human genes have also been reported. p202, encoded by the Ifi202 gene in the gene 200 cluster, is a 52-kDa nuclear phosphoprotein. Constitutive overexpression of p202 in transfected cells is growth-inhibitory. We report here that p202 binds the cell growth regulatory retinoblastoma protein (pRb) in vitro and in vivo. The binding is due to direct interaction between the two proteins. p202 has two nonoverlapping segments for binding pRb, and pRb has two nonoverlapping segments (one of them including the pocket region) for binding p202. The hypophosphorylated form of pRb binds to p202, p202 is the first interferon-inducible protein found to bind pRb.

Simian virus 40 large T antigen affects the Saccharomyces cerevisiae cell cycle and interacts with p34CDC28

Simian virus 40 tumor (T) antigen, an established viral oncoprotein, causes alterations in cell growth control through interacting with, and altering the function of, cellular proteins. To examine the effects of T antigen on cell growth control, and to identify the cellular proteins with which it may functionally interact, T antigen was expressed in the budding yeast Saccharomyces cerevisiae. The yeast cells expressing T antigen showed morphological alterations as well as growth inhibition attributable, at least in part, to a lag in progression from G1 to S. This point in the cell cycle is also known to be affected by T antigen in mammalian cells. Both p34CDC28 and p34CDC2Hs were shown to bind to a chimeric T antigen-glutathione S-transferase fusion protein, indicating that T antigen interacts directly with cell cycle proteins which control the G1 to S transition. This interaction was confirmed by in vivo cross-linking experiments, in which T antigen and p34CDC28 were coimmunoprecipitated from extracts of T-antigen-expressing yeast cells. These immunoprecipitated complexes could phosphorylate histone H1, indicating that kinase activity was retained. In addition, in autophosphorylation reactions, the complexes phosphorylated a novel 60-kDa protein which appeared to be underphosphorylated (or underrepresented) in p34CDC28-containing complexes from cells which did not express T antigen. These results suggest that T antigen interacts with p34CDC28 and alters the kinase function of p34CDC28-containing complexes. These events correlate with alterations in the yeast cell cycle at the G1 to S transition.

Nerve growth factor regulates the expression and activity of p33cdk2 and p34cdc2 kinases in PC12 pheochromocytoma cells

In the absence of serum, nerve growth factor (NGF) promotes the survival and differentiation of the PC12 pheochromocytoma cell line. In the presence of serum, NGF acts primarily as a differentiation factor and negative regulator of cell cycling. To investigate NGF control of cell cycling, we have analyzed the regulation of cyclin dependent kinases during PC12 cell differentiation. NGF treatment leads to a reduction in the steady-state protein levels of p33cdk2 and p34cdc2, two key regulators of cell cycle progression. The decrease in p33cdk2 and p34cdc2 coincides with a decrease in the enzymatic activity of cyclinA-p34cdc2, cyclinB-p34cdc2, cyclinE-p33cdk2, and cyclinA-p33cdk2 kinases. The decline in p33cdk2 and p34cdc2 kinase activity in response to NGF is accelerated in cells that over-express the p140trk NGF receptor, suggesting that the timing of the down- regulation is dependent on the level of p140trk and the strength of the NGF signal. The level of cyclin A, a regulatory subunit of p33cdk2 and p34cdc2, is relatively constant during PC12 differentiation. Nevertheless, the DNA binding activity of the cyclinA-associated transcription factor E2F/DP decreases. Thus, NGF down-regulates the activity of cyclin dependent kinases and cyclin-transcription factor complexes during PC12 differentiation.

p35 is a neural-specific regulatory subunit of cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) was originally isolated through its structural homology to human Cdc2, a key regulator of cell-cycle progression. In tissue samples from adult mice, Cdk5 protein is found at the highest level in brain, at an intermediate level in testis, and at low or undetectable levels in all other tissues, but brain is the only tissue that shows Cdk5 histone H1 kinase activity. No equivalent kinase activity has been found in tissue culture cell lines despite high levels of Cdk5. This raised the possibility that a Cdk5 regulatory subunit was responsible for the activation of Cdk5 in brain. Here we describe the cloning and characterization of a regulatory subunit for Cdk5 known as p35. p35 displays a neuronal cell-specific pattern of expression, it associates physically with Cdk5 in vivo and activates the Cdk5 kinase. p35 differs from the mammalian cyclins and thus represents a new type of regulatory subunit for cyclin-dependent kinase activity.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

Human immunodeficiency virus type 1 Nef associates with a cellular serine kinase in T lymphocytes

With T-cell lines constitutively expressing Nef from the SF2 strain of human immunodeficiency virus type 1 (HIV-1SF2) in the form of a hybrid CD8-Nef fusion protein or T-cell lines chronically infected with HIV-1SF2, a cellular serine kinase was found that specifically associates with Nef. Proteins of 62 kDa and 72 kDa, which coimmunoprecipitated with Nef, were phosphorylated in in vitro kinase assays. This Nef-associated serine kinase activity was not blocked by inhibitors of protein kinase C or protein kinase A and was lost when Nef was truncated at amino acid 94 or 99. These findings present evidence that a serine kinase activity is associated with Nef expressed in human T lymphocytes.

Expression and biochemical properties of a protein serine/threonine phosphatase encoded by bacteriophage lambda

The predicted amino acid sequence encoded by the open reading frame 221 (orf221) of bacteriophage lambda exhibited a high degree of similarity to the catalytic subunits of a variety of protein serine/threonine phosphatases belonging to PP1, PP2A, and PP2B groups. Cloning and expression of the orf221 gene in Escherichia coli provided direct evidence that the gene codes for a protein serine/threonine phosphatase. The single-subunit recombinant enzyme was purified in soluble form and shown to possess a unique repertoire of biochemical properties--e.g., an absolute requirement for Mn2+, resistance to okadaic acid, inhibitors 1 and 2, and ability to dephosphorylate casein, adenovirus E1A proteins, and the alpha subunit of phosphorylase kinase. No phosphotyrosine phosphatase activity was observed. Mutational and biochemical analyses identified the conserved residues 73-77 and Cys138 to be important for activity. The name PP-lambda is proposed for this unusual prokaryotic enzyme.

Simian virus 40 large T antigen associates with cyclin A and p33cdk2

In this paper we provide evidence that a fraction of large T antigen of simian virus 40 (SV40) interacts with cyclin A and p33cdk2 in both virus-infected and stably transformed cells. Immunoprecipitates of SV40 large T antigen from SV40-infected or SV40 large-T-antigen-transformed cells contain cyclin A, p33cdk2, and histone H1 kinase activity. Conversely, immunoprecipitates of cyclin A from these cells contain SV40 large T antigen. In this respect, SV40 large T antigen has properties similar to those of the E1A oncogene of adenoviruses and the E7 oncogene of human papillomaviruses.

A- and B-type cyclins differentially modulate substrate specificity of cyclin-cdk complexes

Both cyclins A and B associate with and thereby activate cyclin-dependent protein kinases (cdks). We have investigated which component in the cyclin-cdk complex determines its substrate specificity. The A- and B-type cyclin-cdk complexes phosphorylated histone H1 and their cyclin subunits in an indistinguishable manner, irrespective of the catalytic subunit, p33cdk2 or p34cdc2. In contrast, only the cyclin A-cdk complexes phosphorylated the Rb-related p107 protein in vitro. Likewise, binding studies revealed that cyclin A-cdk complexes bound stably to p107 in vitro, whereas cyclin B-cdk complexes did not detectably associate with p107, under identical assay conditions. Binding to p107 required both cyclin A and a cdk as neither subunit alone bound to p107. These results demonstrate that although the kinase subunit provides a necessary component for binding, it is the cyclin subunit that plays the critical role in targeting the complex to p107. Finally, we show that the cyclin A-p33cdk2 complex phosphorylated p107 in vitro at most of its sites that are also phosphorylated in human cells, suggesting that the cyclin A-p33cdk2 complex is a major kinase for p107 in vivo.

The adenovirus E1A-associated kinase consists of cyclin E-p33cdk2 and cyclin A-p33cdk2

The adenovirus E1A oncoproteins form stable complexes with several cellular proteins. Association of E1A with these proteins has been shown to be important for the oncogenic potential of E1A. Several of these proteins have been identified and include the product of the retinoblastoma gene and a key cell cycle regulatory protein, cyclin A. E1A also associates with a potent histone H1 kinase. The two major components of this activity are the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes. Both the cyclin E-p33cdk2 and cyclin A-p33cdk2 complexes have been implicated in regulatory events controlling entry into or passage through DNA synthesis. Although the architecture of such interactions remains unclear, it is likely that by targeting such complexes, adenovirus is affecting some aspect of cell cycle control.

Human papillomavirus type 16 E7 associates with a histone H1 kinase and with p107 through sequences necessary for transformation

The transforming function of human papillomavirus type 16 (HPV16) E7 has been shown to depend on activities additional to the ability to bind RB. In this paper we describe two further properties of E7 which may also contribute to transformation, an association with a histone H1 kinase at the G2/M phase of the cell cycle and an ability to bind the RB-related protein p107. The region of E7 identified previously as important for RB binding was found to be involved in the association with the kinase and complex formation with p107, although analysis of E7 point mutants within this region revealed a difference in the precise sequence requirement for RB and p107 binding. Association with the kinase activity correlated with the ability to bind RB, but the restriction of the kinase association to the G2/M phase of the cell cycle implies that this activity might not be directly mediated by RB binding. Since kinase-binding-deficient E7 mutants are also transformation defective, this may represent an independent function of E7 which plays a role in the G2/M phase of the cell cycle.

Adenovirus-E1A proteins transform cells by sequestering regulatory proteins

Cell transformation by adenovirus-E1A proteins is mediated by binding to cellular proteins whose functions are thereby inactivated or altered. The various properties of the E1A proteins are reviewed in relation to their binding to cellular proteins. A number of the cellular proteins which associate to E1A have been identified: the retinoblastoma-susceptibility protein (Rb), the p107 protein, cyclin A and the p33cdk2 kinase. Recent data have shown that those proteins are also able to bind to transcription factor E2F. Binding of Rb to E2F represses the transcription-activating potential of E2F. E1A can sequester the regulatory proteins, like Rb, and thereby release free, active E2F. The domains in E1A that are essential for this transcriptional regulation are also required for the transforming properties of E1A.

Regulation of the cell cycle by the cdk2 protein kinase in cultured human fibroblasts

In mammalian cells inhibition of the cdc2 function results in arrest in the G2-phase of the cell cycle. Several cdc2-related gene products have been identified recently and it has been hypothesized that they control earlier cell cycle events. Here we have studied the relationship between activation of one of these cdc2 homologs, the cdk2 protein kinase, and the progression through the cell cycle in cultured human fibroblasts. We found that cdk2 was activated and specifically localized to the nucleus during S phase and G2. Microinjection of affinity-purified anti-cdk2 antibodies but not of affinity-purified anti-cdc2 antibodies, during G1, inhibited entry into S phase. The specificity of these effects was demonstrated by the fact that a plasmid-driven cdk2 overexpression counteracted the inhibition. These results demonstrate that the cdk2 protein kinase is involved in the activation of DNA synthesis.

Cyclin E/cdk2 and cyclin A/cdk2 kinases associate with p107 and E2F in a temporally distinct manner

Cyclin E is classified as a putative G1 cyclin on the basis of its cyclic pattern of mRNA expression, with maximal levels being detected near the G1/S boundary. We report here that cyclin E is found associated with the transcription factor E2F in a temporally regulated fashion. E2F is known to be a critical transcription factor for the expression of some S phase-specific proteins and is thought to be important for a series of others. Antisera specific for cyclin E were raised and used to demonstrate an association between cyclin E and E2F. This cyclin E/E2F complex was seen in a variety of human cell lines from various tissues, but its appearance was detected primarily during the G1 phase of the cell cycle. The cyclin E/E2F association decreased as cells entered S phase, just as the association of E2F with cyclin A became detectable. We characterized the cyclin E-E2F complex further to show that both the cyclin-dependent kinase-2 (cdk2) and p107 were present. Therefore, the p107/E2F complex is associated with two different cdk2 kinase complexes--one containing cyclin A and the other containing cyclin E--and the appearance of these complexes is temporally regulated during the cell cycle. The presence of cyclin E/E2F complexes in the G1 phase suggests a role for cyclin E in the control of genes required for the G1-to-S transition.

Induction of AP-1 DNA-binding activity and c-fos mRNA by the adenovirus 243R E1A protein and cyclic AMP requires domains necessary for transformation

The 243R E1A protein can act in synergy with cyclic AMP to induce AP-1 DNA-binding activity and c-fos mRNA in mouse S49 cells. A series of deletion mutants was used to identify two domains of the 243R protein that were required for these effects. Interestingly, these domains correlated precisely with regions known to be necessary for E1A-mediated transformation. One domain was located at the N terminus of E1A. The other domain spanned residues 36 to 81, corresponding to conserved region 1 of E1A. S49 cellular proteins that associate with E1A were coimmunoprecipitated with anti-E1A antibody. These included the previously identified proteins p300, p130, p107, p105Rb, and cyclin A. In addition, proteins of 90 kDa and a series of proteins in the 120- to 170-kDa range were identified. Binding of p300, p90, and the 120- to 170-kDa proteins was abolished in cells expressing mutants of E1A that were unable to induce AP-1 DNA-binding activity and c-fos mRNA. These data strongly suggest that specific cellular E1A-binding proteins are involved in the induction of AP-1 DNA-binding activity and c-fos mRNA by the synergistic action of the 243R E1A protein and cyclic AMP and that these transcriptional events are related to the transformation process.

Phosphorylation of adenovirus E1A proteins by the p34cdc2 protein kinase

Adenovirus early region 1A (E1A) products are phosphorylated nuclear oncoproteins which appear to derive transforming activity largely through interactions with cellular proteins including the tumor suppressor p105/Rb-1 and cyclin A (p60cycA), a regulatory subunit associated with p34cdc2 and the related protein kinase p33cdk2. We have identified several sites of phosphorylation on E1A proteins previously and showed that phosphorylation at Ser-89 alters electrophoretic mobility significantly and affects E1A-mediated transforming activity to some extent. We now report that both Ser-89 and Ser-219, the major E1A phosphorylation site, were phosphorylated in vitro by p34cdc2 purified from HeLa cells. We also found that E1A proteins seemed to be phosphorylated at the highest levels in vivo in mitotic cells which express maximal levels of p34cdc2 kinase activity. Thus, in addition to forming complexes with p60cycA, a regulator of p34cdc2 and related kinases, and p105/Rb-1 which exhibits cell cycle-dependent phosphorylation, E1A proteins seem to be substrates for p34cdc2. These data suggested that a link could exist between phosphorylation, cell cycle progression, and the regulation of transforming activity of E1A proteins.

The retinoblastoma protein physically associates with the human cdc2 kinase

The protein product (pRB) of the retinoblastoma susceptibility gene functions as a negative regulator of cell proliferation, and its activity appears to be modulated by phosphorylation. Using a new panel of anti-human pRB monoclonal antibodies, we have investigated the biochemical properties of this protein. These antibodies have allowed us to detect a pRB-associated kinase that has been identified as the cell cycle-regulating kinase p34cdc2 or a closely related enzyme. Since this associated kinase phosphorylates pRB at most of the sites used in vivo, these results suggest that this kinase is one of the major regulators of pRB. The associated kinase activity follows the pattern of phosphorylation seen for pRB in vivo. The associated kinase activity is not seen in the G1 phase but appears in the S phase, and the levels continue to increase throughout the remainder of the cell cycle.

The retinoblastoma protein physically associates with the human cdc2 kinase

The protein product (pRB) of the retinoblastoma susceptibility gene functions as a negative regulator of cell proliferation, and its activity appears to be modulated by phosphorylation. Using a new panel of anti-human pRB monoclonal antibodies, we have investigated the biochemical properties of this protein. These antibodies have allowed us to detect a pRB-associated kinase that has been identified as the cell cycle-regulating kinase p34cdc2 or a closely related enzyme. Since this associated kinase phosphorylates pRB at most of the sites used in vivo, these results suggest that this kinase is one of the major regulators of pRB. The associated kinase activity follows the pattern of phosphorylation seen for pRB in vivo. The associated kinase activity is not seen in the G1 phase but appears in the S phase, and the levels continue to increase throughout the remainder of the cell cycle.

A cyclin A-protein kinase complex possesses sequence-specific DNA binding activity: p33cdk2 is a component of the E2F-cyclin A complex

The E2F transcription factor has been found in association with the cyclin A protein, and this complex accumulates during the S phase of the cell cycle, suggesting that E2F may play a role in cell cycle control. In independent studies, cyclin A has been shown to be associated with two other proteins, the Rb-related p107 protein and the cdc2-related p33 cdk2 protein kinase. Through an analysis of the E2F-cyclin A complex, we now find that both the p107 protein and the cdc2-related p33cdk2 kinase are components of the previously described complex. Moreover, the complex possesses H1 kinase activity. These results thus define a cyclin A-cdk2 kinase complex that possesses sequence-specific DNA binding activity. This suggests that the cdk2 kinase may phosphorylate other DNA-bound substrates, and that one role of the E2F factor may be to localize this protein kinase to the DNA.

Interaction between human cyclin A and adenovirus E1A-associated p107 protein

The products of the adenovirus early region 1A (E1A) gene are potent oncoproteins when tested in standard transformation and immortalization assays. Many of the changes induced by E1A may be due to its interaction with cellular proteins. Four of these cellular proteins are the retinoblastoma protein (pRB), p107, cyclin A, and p33cdk2. The pRB and p107 proteins are structurally related and have several characteristics in common, including that they both bind to the SV40 large T oncoprotein as well as to E1A. Cyclin A and p33cdk2 are thought to function in the control of the cell cycle. They bind to one another, forming a kinase that closely resembles the cell cycle-regulating complexes containing p34cdc2. Cyclin A is now shown to bind to p107 in the absence of E1A. The association of p107 with cyclin A suggests a direct link between cell cycle control and the function of p107.