The retinoblastoma protein is phosphorylated on multiple sites by human cdc2

Collaboration of G1 cyclins in the functional inactivation of the retinoblastoma protein

The retinoblastoma gene product (pRB) constrains cell proliferation by preventing cell-cycle progression from the G1 to S phase. Its growth-inhibitory effects appear to be reversed by hyperphosphorylation occurring during G1. This process is thought to involve G1 cyclins and cyclin-dependent kinases (cdks). Here we report that the cell cycle-dependent phosphorylation of mammalian pRB is faithfully reproduced when it is expressed in Saccharomyces cerevisiae. As is the case in mammalian cells, this phosphorylation requires an intact oncoprotein-binding domain and is inhibited by a negative growth factor, in this case a mating pheromone. Expression of pRB in cln (-) mutants indicates that specific combinations of endogenous G1 cyclins, Cln3 and either Cln1 or Cln2 are required for pRB hyperphosphorylation in yeast. Moreover, expression of mammalian G1 cyclins in cln (-) yeast cells indicates that the functions of Cln2 and Cln3 in pRB hyperphosphorylation can be complemented by human cyclin E and cyclin D1, respectively. These observations suggest a functional heterogeneity among G1 cyclin-cdk complexes and indicate a need for the involvement of multiple G1 cyclins in promoting pRB hyperphosphorylation and resulting cell-cycle progression.

Differential regulation of E2F transactivation by cyclin/cdk2 complexes

The mammalian transcription factor E2F plays a critical role in the expression of genes required for cellular proliferation. To understand how E2F is regulated, we have developed a reconstituted in vitro transcription assay. Using this E2F-responsive assay, we can demonstrate that E2F-mediated transcription can be directly repressed by the tumor suppressor protein pRB. This inhibition is abolished by phosphorylation of pRB with either cyclin A/cdk2 or cyclin E/cdk2. However, these cyclin/kinase complexes exhibit differences in the ability to phosphorylate E2F. Only cyclin A/cdk2 can phosphorylate E2F effectively, and this phosphorylation abolishes its ability to bind DNA and mediate trans-activation. Thus, this in vitro transcriptional assay allows activation and inactivation of E2F transcription, and our findings demonstrate how transcriptional regulation of E2F can be linked to cell cycle-dependent activation of kinases.

Changes of cell cycle-regulating genes in interferon-treated Daudi cells

Interferon (IFN) modulates the expression of several genes and some of them are considered to be responsible for the inhibition of cellular growth. However, the alterations of cell cycle-regulating genes produced by IFN still remain unclear. Accordingly, we studied the expression of cell cycle-regulating genes during IFN-induced growth arrest. Cell cycle synchronized and unsynchronized Daudi Burkitt lymphoma cells were treated with IFN. Both the cell cycle distribution and the expression of cell cycle-regulating genes (cdk2, cdc2, cyclins A, B, C, D3, cdc25, and wee 1) were studied by flow cytometry and by Northern blot hybridization or the reverse-transcription polymerase chain reaction, respectively. Treated cells passed through the first G1 phase and gradually accumulated in the following G1 phase. Expression of cyclins A, B, and D3 oscillated along with the cell cycle progression in control cells, and the alterations of cyclin B expression were especially prominent. Both cdc2 and cdk2 also showed changes, but these were not so distinct as observed with cyclin B. Expression of cdc25 and wee1 was little affected by cell cycle progression. In IFN-treated cells, expression of cyclins A and B were down-regulated, while that of cyclin C was not. Cyclin D3 expression was also down-regulated at 48 h, followed by an increase at 72 h. Expression of both cdc2 and cdk2 was down-regulated, especially that of the later. Wee1 expression was down-regulated by IFN but, the expression of cdc25 remained stable. These findings suggest that the modulation of cell cycle-regulating genes, particular by cyclin A and cdk2, plays an important role in IFN-induced cellular growth arrest.

The helix-loop-helix protein Id-2 enhances cell proliferation and binds to the retinoblastoma protein

Cell growth and differentiation are usually antagonistic. Proteins of the basic helix-loop-helix (bHLH) family bind DNA and play important roles in the differentiation of specific cell types. Id proteins heterodimerize with bHLH transcription factors, blocking their activation of lineage-specific gene expression and thereby inhibiting cellular differentiation. To examine the effect of Id-2 on cell proliferation, we overexpressed Id-2 in the human osteosarcoma cell line U2OS. Id-2 expression in U2OS reduced the serum requirement for growth and stimulated cellular proliferation by shortening the doubling time and increasing the percentage of cells in S phase. We demonstrated that Id-2 expression was able to reverse the inhibition of cellular proliferation and the block in cell cycle progression mediated by the product of the retinoblastoma tumor suppressor gene pRB. This effect was not associated with changes in the state of pRb phosphorylation in transfected cells. In vitro, unphosphorylated pRb from cell lysates specifically bound Id-2 but was not able to bind a mutated form of Id-2 lacking the HLH domain that also did not antagonize the growth arrest by pRb. In vitro-synthesized pRb containing mutations within the E1A/large T-binding pocket did not bind Id-2. However, wild-type pRb was able to bind to a region of Id-2 corresponding to only the HLH domain. In vivo, a physical association between Id-2 and pRb was seen in cross-linked extracts from SAOS-2 cells transfected with Id-2 and pRb. Our data identify a role for Id-2 in the regulation of cellular proliferation and suggest that the interaction between Id-2 and pRB is a molecular pathway over which synchronous changes in growth and differentiation are mediated in vivo.

Characterization of a chicken cDNA encoding the retinoblastoma gene product

We have isolated a chicken cDNA that encodes the retinoblastoma susceptibility gene product (RB). The predicted amino acid sequence of the chicken RB protein is highly similar to that of the mouse, human and Xenopus RB proteins in regions of known functions; however, chicken RB has distinct species-specific differences, including a shorter N-terminal region as compared to the mouse and human RB proteins. In vitro-translated chicken RB co-migrates on SDS-polyacrylamide gels with endogenous RB synthesized in transformed chicken spleen cells. Finally, chicken RB is located in the nucleus of chicken embryo fibroblasts when overexpressed from a retroviral vector.

Changes of G1 cyclins, cdk2, and cyclin A during the differentiation of HL60 cells induced by TPA

Differentiation induction by 12-o-tetradecanoyl 13-acetate (TPA) results in the growth arrest of HL60 cells in the G1 phase. However, little is known about the changes of cell cycle-regulating genes during this differentiation process. We investigated the changes of mRNA for various cyclins (A, C, D1, D2, D3 and E) and cdk2. Synchronized HL60 cells began to proliferate immediately after release from cell cycle block and cell cycle synchrony was obvious until the second S phase. TPA-treated cells accumulated in G1 phase within 24 h and most of the cells were arrested in this phase at 36 h. The expression of cyclins and cdk2 was studied by Northern blot hybridization of the reverse-transcription polymerase chain reaction (RT-PCR). TPA treatment altered the expression of all genes studied. The expression of cdk2 and cyclin A mRNA was markedly down-regulated. Cyclin E mRNA expression was also prominently down-regulated from 12 h to 36 h, at which time a second increase of its expression was observed in control cells. In contrast, the expression of cyclin D1 mRNA was induced by TPA, while its expression in control cells was undetectable by Northern blot hybridization throughout the cell cycle. Cyclin C expression was faint and fluctuated irrelevant of cell cycle, but its expression in both control and TPA-treated cells was higher than at baseline. Cyclin D2 expression remained stable in control cells and TPA treatment resulted in slight down-regulation at 12 h, but no difference was observed after 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of oncoprotein binding by a single Cys706-to-Tyr substitution in the retinoblastoma protein

We previously found a new single amino acid substitution at codon 706 (Cys-to-Tyr) of the retinoblastoma (RB) gene in a sporadic retinoblastoma patient. The glutathione S-transferase-RB fused protein containing this mutation was here tested for binding to SV40 large T antigen and adenovirus E1A protein, and was shown to have lost its binding affinity. Thus, Tyr, as well as Phe, residues substituted for Cys706 were found to abolish the RB protein 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.

Tumor suppressor genes and their roles in breast cancer

Tumor suppressor genes have been identified by the occurrence of mutations in many families with hereditary forms of cancer, exposed during development of the tumor by loss of heterozygosity. They have a number of diverse functions. For example, both the RB gene of retinoblastoma and the p53 gene, which is commonly mutated in breast and colon cancer among others, produce proteins involved in distinct steps of cell cycle control, while the nm23 product prevents metastasis. Here we review the data developed until now on the possible presence and role of mutations in these and other tumor suppressor genes in breast cancer. A more complete understanding of the tumor suppressor genes could not only provide diagnostic information, but could lead to specific gene therapy to replace suppressor functions lost in individual tumors.

Cell cycle controls: potential targets for chemical carcinogens?

The progression of the cell cycle is controlled by the action of both positive and negative growth regulators. The key players in this activity include a family of cyclins and cyclin-dependent kinases, which are themselves regulated by other kinases and phosphatases. Maintenance of balanced cell cycle controls may be directly linked to genomic stability. Loss of the check-points involved in cell cycle control may result in unrepaired DNA damage during DNA synthesis or mitosis leading to genetic mutations and contributing to carcinogenesis.

Cell cycle activity and expression of prohibitin mRNA

Prohibitin, a novel intracellular antiproliferative protein, blocks entry into the S phase of the cell division cycle when its mRNA is microinjected into normal fibroblasts or HeLa cells. To learn more about the interaction between prohibitin and the cell cycle, we studied the effect of microinjecting prohibitin mRNA at different points during the transition from G0 to S phase and analyzed prohibitin mRNA and protein levels in different parts of the cell cycle. The antiproliferative activity of microinjected prohibitin mRNA is high in G0/G1 and falls as cells approach S phase. Prohibitin mRNA and protein levels are high in G1, fall with S phase, rise again in G2, and fall in M. Together, these findings suggest that endogenous prohibitin contributes to the control of the G1 to S transition in cycling cells in a complex manner, which involves both a transcriptional and posttranslational mechanism.

A Drosophila G1-specific cyclin E homolog exhibits different modes of expression during embryogenesis

We have isolated a Drosophila homolog of the human G1-specific cyclin E gene. Cyclin E proteins thus constitute an evolutionarily conserved subfamily of metazoan cyclins. The Drosophila cyclin E gene, DmcycE, encodes two proteins with a common C-terminal region and unique N-terminal regions. Unlike other Drosophila cyclins, DmcycE exhibits a dynamic pattern of expression during development. DmcycE is supplied maternally, but at the completion of the cleavage divisions and prior to mitosis 14, the maternal transcripts are rapidly degraded in all cells except the pole (germ) cells. Two modes of DmcycE expression are observed in the subsequent divisions. During cycles 14, 15 and 16 in non-neural cells, DmcycE mRNA levels show no cell-cycle-associated variation. DmcycE expression in these cells is therefore independent of the cell cycle phase. In contrast, expression in proliferating embryonic peripheral nervous system cells occurs during interphase as a brief pulse that initiates before and overlaps with S phase, demonstrating the presence of a G1 phase in these embryonic neural cell cycles. DmcycE appears not to be expressed in cells that undergo endoreplication cycles during polytenization. The structural homology to human cyclin E, the ability of DmcycE to rescue a G1 cyclin-deficient yeast strain, the presence of multiple PEST sequences characteristic of G1-specific cyclins and expression during G1 phase in proliferating peripheral nervous system cells all argue that Drosophila cyclin E is a G1 cyclin. Constitutive DmcycE expression in embryonic cycles lacking a G1 phase, in contrast to expression during the G1-S phase transition in cycles exhibiting a G1 phase, implicates DmcycE expression in the regulation of the G1 to S phase transition during Drosophila embryogenesis.

Cellular substrates of p34cdc2 and its companion cyclin-dependent kinases

Cyclin-dependent kinase (cdks) are key components of the engine that drives the cell proliferation cycle in all eukaryotes. These kinases are related to p34(cdc2) and associate with regulatory subunits belonging to the cyclin family. To understand how cdks promote cell cycle progression, it will be important to identify their physiological substrates and to determine how phosphorylation influences the functions of these substrates. This article discusses recent progress as well as some of the problems related to the quest for cdk substrates.

Expression of a retinoblastoma transgene results in dwarf mice

Introduction of the normal retinoblastoma gene (RB) into different tumor cells possessing inactivated RB genes suppresses their tumorigenicity in nude mice. These results suggest that RB replacement is a potential strategy for developing future clinical treatments of cancer. In a transgenic mouse model, we found that the quantity of RB protein in a given cell may play an important role in dictating its effect. Four founder mice containing 1-7 copies of a human RB cDNA transgene under the transcriptional control of the human RB promoter were generated. Most of the transgenic mice were smaller than nontransgenic littermates. This effect was found as early as embryonic day 15. The degree of dwarfism correlated roughly with the copy number of the transgene and the corresponding level of RB protein. The expression pattern of the transgene products was similar to that of the endogenous mouse RB gene with regard to tissue and temporal distribution. Transferring the transgene to RB deficient mice, which are nonviable, resulted in the development of normal, healthy mice, indicating that the human RB gene can functionally complement the mouse homolog. These studies demonstrate that the effect of RB on overall mouse development is closely dependent upon its dosage.

Transforming growth factor beta 1-mediated inhibition of smooth muscle cell proliferation is associated with a late G1 cell cycle arrest

The effect of transforming growth factor beta 1 (TGF beta 1) on the proliferative response of aortic smooth muscle cells (SMC) in vitro was investigated. TGF beta 1 substantially inhibited the growth of human and bovine SMC. Rapidly growing SMC and quiescent serum-stimulated SMC were inhibited by TGF beta 1 with an ID50 of approximately 0.5 ng/ml and maximal inhibition was observed at 10 ng/ml TGF beta 1. In the presence of TGF beta 1, quiescent serum-stimulated SMC progress into the G1 phase of the cell cycle, but become reversibly arrested at a point temporally located 1-2 hours from S phase. Release from this late G1 TGF beta 1 arrest point results in S phase entry within 2 hours. Associated with this inhibitory effect is a decrease in the histone H1 kinase activity of p34cdc2 protein kinase while TGF beta 1 has no effect on the transcription or translation of p34cdc2. Under these growth inhibitory conditions, TGF beta 1 is still capable of upregulating the expression of fibronectin mRNA. These results suggest that TGF beta 1 growth inhibition in SMC is associated with the regulation of p34cdc2 activity in late G1.

The human papillomavirus E7 protein as a transforming and transactivating factor

The HPV proteins encoded by the early viral genes, including E6 and E7, are thought to subvert the normal regulatory pathways of infected cells to accommodate viral replication. Mechanistically some of this is accomplished by protein-protein interactions between viral proteins and a number of key cellular regulatory proteins that include tumor suppressor gene products. By undermining cellular regulatory pathways the HPV oncogenes cause hyperproliferation and the perturbation of normal cellular differentiation pathways. Although expression of the high-risk HPV-encoded E6 and E7 oncoproteins may be important prerequisites for cellular transformation, it is very likely that additional cellular changes are necessary for carcinogenic progression. The elucidation of the role of the early HPV genes in the initiation and/or maintenance of carcinogenic progression will continue to be a fascinating area of investigation and may reveal new opportunities for antiviral therapy and antitumor intervention.

Molecular characterization of the retinoblastoma susceptibility gene

Retinoblastoma is recognized as a hereditary cancer. Genetic and epidemiological analysis of the disease has been incorporated into a two-hit mutational inactivation hypothesis of the origin of retinoblastoma. The molecular cloning and characterization of the retinoblastoma gene and gene product has allowed a critical testing of this two-hit hypothesis. All the predications of the model have been born out by experiment so far. These include inheritance of one mutated RB allele as the origin of hereditary retinoblastoma, subsequent loss of the remaining allele upon tumorigenesis, the involvement of the same RB gene in both sporadic and hereditary retinoblastoma, the somatic mutation of both RB alleles in sporadic retinoblastoma, the lack of RB expression in any retinoblastoma yet examined, and the recessiveness of mutated RB alleles. The RB gene exhibits functional properties consistent with its role as a suppressor of tumor formation. For example, re-expression of RB in tumor cells lacking endogenous RB leads to a loss of tumorigenic properties. RB protein can also inhibit progression through the cell division cycle, and it physically and/or functionally interacts with important cell cycle regulatory molecules. Although confirmation of the two-hit hypothesis seems complete, we can not rule out the possibility that other genes are involved in the genesis of this tumor. For example, there seems to be variable resistance to tumor development even in patients inheriting retinoblastoma susceptibility. Further, heterozygous RB null mice do not develop retinoblastoma, but develop a characteristic brain tumor instead. The molecular isolation of the RB gene is an important achievement in research on cancer. For the first time, it has become possible to examine, at the molecular level, genes that inhibit the growth of tumor cells. The precise mechanism of action of RB is unknown, but a broad outline is beginning to emerge. RB seems to negatively influence tumor cell growth by participating in regulation of the cell division cycle. RB has also been implicated in differentiation; its effect on the cell division cycle and its effects on differentiation may be different manifestations of the same function. Since RB is involved in oncogenesis, gene regulation, and cellular differentiation, it is obviously an attractive gene for intense study; understanding the function and mechanism of action of RB will impact the understanding of many, important cell processes.

Physical interaction of the retinoblastoma protein with human D cyclins

The retinoblastoma protein (pRb) functions as a regulator of cell proliferation and in turn is regulated by cyclin-dependent kinases. Cyclins D1 and D3 can form complexes with pRb that resemble those formed by several viral oncoproteins and are disrupted by the adenovirus E1A oncoprotein and derived peptides. These cyclins contain a sequence motif similar to the pRb-binding conserved region II motif of the viral oncoproteins. Alteration of this motif in cyclin D1 prevents formation of cyclin D1-pRb complexes while enhancing the biological activity of cyclin D1 assayed in vivo. We conclude that cyclins D1 and D3 interact with pRb in a fashion distinct from cyclins A and E, which can induce pRb hyperphosphorylation, and that cyclin D1 activity may be regulated by its association with pRb.

Functional interactions of the retinoblastoma protein with mammalian D-type cyclins

The retinoblastoma gene product (Rb) can interact efficiently with two of three D-type G1 cyclins (D2 and D3) in vitro. Binding depended upon the minimal regions of Rb necessary for its growth-suppressive activity, as well as upon the D-type cyclin sequence motif shared with Rb-binding DNA tumor virus oncoproteins. Coexpression of the three D-type cyclins with the cyclin-dependent kinase (cdk4) in insect cells generated Rb kinase activity. By contrast, cyclins D2 and D3, but not D1, activated another such kinase, cdk2. Introduction of cyclin D2 and Rb into the Rb-deficient cell line SAOS-2 led to overt Rb hyperphosphorylation, whereas Rb, expressed alone or together with cyclin D1, remained unphosphorylated. Cyclin D2-dependent phosphorylation inhibited its binding to the transcription factor E2F and reversed the Rb G1 exit block in the cell cycle. Thus, all D-type cyclins do not function equivalently, and one of them plays a major role in reversing the cycle-blocking function of a known tumor suppressor.

Human papillomaviruses, tumour suppressor genes and cervical cancer

There is now a considerable body of evidence that links HPV infection with anogenital squamous carcinoma, particularly for specific 'high risk' HPV types (HPV16 and 18) and invasive carcinoma of the cervix. Recent advances in the molecular study of these viruses have elucidated some potential mechanisms by which they may contribute to the development of these diseases. In this review we concentrate on the interactions of 2 of the HPV encoded proteins, E6 and E7, with cellular tumour suppressor gene products. We provide a model of how these interactions may be important in tumourigenesis and draw together current knowledge of this exciting and rapidly evolving field.

Targets of cyclin-dependent protein kinases

Our current understanding of the eukaryotic cell cycle attributes a key regulatory role to cyclin-dependent protein kinases. It is important, therefore, to identify the physiological substrates of these kinases, and to understand how the phosphorylation of such proteins promotes cell cycle progression.

Integration of cell cycle control with transcriptional regulation by the retinoblastoma protein

Rapid progress in several areas of molecular biology has led to the realization that the retinoblastoma protein may play a pivotal role in the coordination between cell cycle control and regulation of gene expression. This role is a subtle one, and is important in only certain mammalian cell types. Exploring the details of these connections, and why only some cells rely on them, is already beginning to shed light on the regulation of cell multiplication.

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.

The retinoblastoma protein associates with the protein phosphatase type 1 catalytic subunit

The retinoblastoma protein (p110RB) interacts with many cellular proteins in complexes potentially important for its growth-suppressing function. We have developed and used an improved version of the yeast two-hybrid system to isolate human cDNAs encoding proteins able to bind p110RB. One clone encodes a novel type 1 protein phosphatase catalytic subunit (PP-1 alpha 2), which differs from the originally defined PP-1 alpha by an amino-terminal 11-amino-acid insert. In vitro-binding assays demonstrated that PP-1 alpha isoforms preferentially bind the hypophosphorylated form of p110RB. Moreover, similar p110RB sequences are required for binding PP-1 alpha 2 and SV40 large T antigen. Cell cycle synchrony experiments revealed that this association occurs from mitosis to early G1. The implications of these findings on the regulation of both proteins are discussed.

Interaction of myogenic factors and the retinoblastoma protein mediates muscle cell commitment and differentiation

The experiments reported here document that the tumor suppressor retinoblastoma protein (pRB) plays an important role in the production and maintenance of the terminally differentiated phenotype of muscle cells. We show that pRB inactivation, through either phosphorylation, binding to T antigen, or genetic alteration, inhibits myogenesis. Moreover, inactivation of pRB in terminally differentiated cells allows them to reenter the cell cycle. In addition to its involvement in the myogenic activities of MyoD, pRB is also required for the cell growth-inhibitory activity of this myogenic factor. We also show that pRB and MyoD directly bind to each other, both in vivo and in vitro, through a region that involves the pocket and the basic-helix-loop-helix domains, respectively. All the results obtained are consistent with the proposal that the effects of MyoD on the cell cycle and of pRB on the myogenic pathway result from the direct binding of the two molecules.

Retinoblastoma protein and the cell cycle

Deregulation of the cell cycle may contribute one of the primary mechanisms through which cancer arises. Eukaryotic cell division has been found to be a strictly controlled process, involving response to both positive and negative external signals and assessment of the cell's internal state. Several recent discoveries have strengthened and refined the theory that the retinoblastoma protein is involved in the decision between cell division and differentiation, and have begun to provide an outline of the nature of this involvement.

Cloning and characterization of a human cDNA encoding a novel putative cytoplasmic protein-tyrosine-phosphatase

We have cloned and characterized a human cDNA encoding a new member of the family of cytosolic type protein-tyrosine-phosphatase (PTP), designated as PTPG1, from an adult colon tissue cDNA library by using the PCR product as probe. We obtained 5 cDNA clones, which cover the predicted open reading frame encoding a 88-kDa protein composed of 780 amino acids, and it had no apparent signal or transmembrane sequences, suggesting that it is a cytosolic protein. The N-terminal region had a PTP catalytic domain that is 30-40% identical to previously reported human PTPs. This revealed that the enzyme composes an additional family of human PTPs. PTPG1 was characterized by a long non-enzymatic domain located at the C-terminus, including PEST sequences which are characteristic for short half-life proteins in eukaryotes. Northern blot analysis of PTPG1 mRNA showed a 4.6-kb transcript that was detected in a wide variety of cell lines to suggest its extensive expression.

The synthesis of p58cyclin A and the phosphorylation of p34cdc2 are inhibited in human lymphoid cells arrested in G1 by alpha-interferon

Daudi cells arrest in G1 in the presence of alpha-interferon. Such cells have little p58cyclin A, probably due to inhibition of p58cyclin A synthesis. The phosphorylation-associated migration shift of p34cdc2 is not seen in alpha-interferon-arrested cells. Cells arrested in late G1 by aphidicolin have abundant p58cyclin A and phosphorylated p34cdc2. Cell sorting showed that p58cyclin A increases in proliferating cells in late G1 and coincides with phosphorylation of p34cdc2.

Mice deficient for Rb are nonviable and show defects in neurogenesis and haematopoiesis

The retinoblastoma gene, a prototypic tumour-suppressor gene, encodes a nuclear phosphoprotein (Rb). To understand better the role of Rb in development and in tumorigenesis, mice with an insertional mutation in exon 20 of the Rb-1 locus were generated. Homozygous mutants die before the 16th embryonic day with multiple defects. The haematopoietic system is abnormal; there is a significant increase in the number of immature nucleated erythrocytes. In the nervous system, ectopic mitoses and massive cell death are found, particularly in the hindbrain. All spinal ganglion cells die, but the neural retina is unaffected. Transfer of the human retinoblastoma (RB) mini-transgene into the mutant mice corrects the developmental defects. Thus, Rb is essential for normal mouse development.

Regulation of retinoblastoma protein functions by ectopic expression of human cyclins

The retinoblastoma susceptibility gene (RB) product, the retinoblastoma protein (pRb), functions as a regulator of cell proliferation. Introduction of the RB gene into SAOS-2 osteosarcoma cells, which lack functional pRb, prevents cell cycle progression. Such growth-suppressive functions can be modulated by phosphorylation of pRb, which occurs via cell cycle-regulated kinases. We show that constitutively expressed cyclins A and E can overcome pRb-mediated suppression of proliferation. pRb becomes hyperphosphorylated in cells overexpressing these cyclins, and this phosphorylation is essential for cyclin A- and cyclin E-mediated rescue of pRb-blocked cells. This suggests that G1 and S phase cyclins can act as regulators of pRb function in the cell cycle by promoting pRb phosphorylation.

The retinoblastoma protein and the regulation of cell cycling

Increasing attention has been focused on how the retinoblastoma (RB) protein regulates cell growth. Recent evidence indicates that it is a substrate for phosphorylation by cyclin-dependent kinase-cyclin complexes and suggests that this phosphorylation modulates the ability of this protein to regulate transit through the cell cycle, perhaps in its G1 phase.

A cDNA encoding a pRB-binding protein with properties of the transcription factor E2F

The retinoblastoma protein (pRB) plays an important role in the control of cell proliferation, apparently by binding to and regulating cellular transcription factors such as E2F. Here we describe the characterization of a cDNA clone that encodes a protein with properties of E2F. This clone, RBP3, was identified by the ability of its gene product to interact with pRB. RBP3 bound to pRB both in vitro and in vivo, and this binding was competed by viral proteins known to disrupt pRB-E2F association. RBP3 bound to E2F recognition sequences in a sequence-specific manner. Furthermore, transient expression of RBP3 caused a 10-fold transactivation of the adenovirus E2 promoter, and this transactivation was dependent on the E2F recognition sequences. These properties suggest that RBP3 encodes E2F, or an E2F-like protein.

Adenovirus E1a prevents the retinoblastoma gene product from repressing the activity of a cellular transcription factor

The retinoblastoma (Rb) gene product forms a complex with the cellular transcription factor DRTF1, a property assumed to be important for mediating negative growth control because certain viral oncogenes, such as adenovirus E1a, prevent this interaction and mutant Rb alleles, which have lost the capacity to regulate growth, encode proteins that fail to associate with DRTF1. In this study, we show that the wild-type Rb protein can specifically repress transcription from promoters driven by DRTF1 whereas a naturally occurring mutant Rb protein cannot. Furthermore, Rb-mediated transcriptional repression can be overridden by adenovirus E1a; this requires regions in E1a necessary for cellular transformation. The Rb protein therefore acts in trans to repress the transcriptional activity of DRTF1 whereas adenovirus E1a prevents this interaction and thus maintains DRTF1 in a constitutively active state. The Rb protein and adenovirus E1a therefore have opposite effects on the activity of a common molecular target. Transcriptional repression mediated by the Rb protein and inactivation of repression by the E1a protein are likely to play an important role in mediating their biological effects.