cdc2 phosphorylation of threonine 124 activates the origin-unwinding functions of simian virus 40 T antigen

Phosphorylation of simian virus 40 (SV40) T antigen on threonine 124 activates viral DNA replication in vivo and in vitro. We have manipulated the modification of T-antigen residue 124 both genetically and biochemically and have investigated individual replication functions of T antigen under conditions suitable for in vitro DNA replication. We find that the hexamer assembly, helicase, DNA polymerase alpha-binding, and transcriptional-autoregulation functions are independent of phosphorylation of threonine 124. In contrast, neither T antigen with an alanine mutation of threonine 124 made in human cells nor unphosphorylated T antigen made in Escherichia coli binds the SV40 replication origin as stably as phosphorylated wild-type T antigen does. Furthermore, modification of threonine 124 is essential for complete unwinding of the SV40 replication origin. We conclude that phosphorylation of threonine 124 enhances specific interactions of T antigen with SV40 origin DNA. Our findings do not exclude the possibility that phosphorylation of threonine 124 may affect additional undefined steps in DNA replication. We also show that DNase footprinting and KMnO4 modification assays are not as stringent as immunoprecipitation and origin-dependent strand displacement assays for detecting defects in the origin-binding and -unwinding functions of T antigen. Differences in the assays may explain discrepancies in previous reports on the role of T-antigen phosphorylation in DNA binding.

Dimerization and DNA binding alter phosphorylation of Fos and Jun

Fos and Jun form dimeric complexes that bind to activator protein 1 (AP-1) DNA sequences and regulate gene expression. The levels of expression and activities of these proteins are regulated by a variety of extracellular stimuli. They are thought to function in nuclear signal transduction processes in many different cell types. The role of Fos and Jun in gene transcription is complex and may be regulated in several ways including association with different dimerization partners, interactions with other transcriptional factors, effects on DNA topology, and reduction/oxidation of a conserved cysteine residue in the DNA-binding domain. In addition, phosphorylation has been suggested to control the activity of Fos and Jun. Here we show that phosphorylation of Fos and Jun by several protein kinases is affected by dimerization and binding to DNA. Jun homodimers are phosphorylated efficiently by casein kinase II, whereas Fos-Jun heterodimers are not. DNA binding also reduces phosphorylation of Jun by casein kinase II, p34cdc2 (cdc2) kinase, and protein kinase C. Phosphorylation of Fos by cAMP-dependent protein kinase and cdc2 is relatively insensitive to dimerization and DNA binding, whereas phosphorylation of Fos and Jun by DNA-dependent protein kinase is dramatically stimulated by binding to the AP-1 site. These results imply that different protein kinases can distinguish among Fos and Jun proteins in the form of monomers, homodimers, and heterodimers and between DNA-bound and non-DNA-bound proteins. Thus, potentially, these different states of Fos and Jun can be recognized and regulated independently by phosphorylation.

Dimethyl sulfoxide-mediated primer Tm reduction: a method for analyzing the role of renaturation temperature in the polymerase chain reaction

We report a method for optimizing the specificity of product formation in the polymerase chain reaction (PCR). This technique is based on the use of dimethyl sulfoxide (DMSO) and takes into account primer Tm. The reduction in Tm by DMSO is directly correlated with renaturation temperature such that a DMSO gradient reflects a temperature gradient. We use this relationship to show that optimum product formation usually occurs at or within several degrees of the midpoint Tm of a given primer pair. We illustrate these correlations using three examples deriving PCR products from a human cDNA library, representing the casein kinase II alpha and beta subunits as well as the 5' untranslated region for the beta subunit. By following product formation as a function of renaturation temperature, we postulate rules for cycle design based on primer Tm. Implications for the use of degenerate primers are discussed.

Identification of a nerve growth factor- and epidermal growth factor-regulated protein kinase that phosphorylates the protooncogene product c-Fos

Nerve growth factor (NGF) treatment of rat pheochromocytoma (PC12) cells induces the synthesis of the transcription factor c-Fos, which becomes highly phosphorylated relative to that produced as a result of depolarization of the cell. A peptide derived from the carboxyl terminus of c-Fos (residues 359-370, RKGSSSNEPSSD) containing putative phosphorylation sites was used to detect a NGF-stimulated Fos kinase. NGF treatment of PC12 cells resulted in a rapid activation of a protein kinase which phosphorylated both the c-Fos peptide and authentic c-Fos at its carboxyl terminus. The kinase was selectively activated by NGF and epidermal growth factor but was not induced by depolarization or other agents. The c-Fos peptide was phosphorylated at a serine corresponding to Ser362, a site critically implicated in the capacity of c-Fos to exhibit transrepressive activity [Ofir, R., Dwarki, V. J., Rashid, D. & Verma, I. M. (1990) Nature (London) 348, 80-82)]. The NGF-stimulated Fos kinase may play an important role in regulating the expression and transforming potential of c-Fos.

Casein kinase II phosphorylates p34cdc2 kinase in G1 phase of the HeLa cell division cycle

The activity of p34cdc2 kinase is regulated in the phases of vertebrate cell cycle by mechanisms of phosphorylation and dephosphorylation. In this paper, we demonstrate that casein kinase II (CKII) phosphorylates p34cdc2 in vivo and in vitro at Ser39 during the G1 phase of HeLa cell division cycle. Human p34cdc2 shows a typical phosphorylation sequence motif site for CKII at Ser39 (ES39EEE). In our experiments, either p34cdc2 expressed and purified from bacteria or p34cdc2 immunoprecipitated from HeLa cells enriched in G1 by elutriation were substrates for in vitro phosphorylation by CKII. Phosphoamino acid analysis, N-chlorosuccinimide mapping, and two-dimensional tryptic mapping of p34cdc2 phosphorylated in vitro were performed to determine the phosphorylation site. A synthetic peptide spanning residues 33-50 of human p34cdc2, including the CKII site, was used to map the site. In addition, phosphorylation at Ser39 also occurs in vivo, since p34cdc2 is phosphorylated during G1 on serine, and its two-dimensional tryptic map shows two phosphopeptides that comigrate exactly with the synthetic peptides used as standard.

Jun is phosphorylated by several protein kinases at the same sites that are modified in serum-stimulated fibroblasts

c-jun is a member of the family of immediate-early genes whose expression is induced by factors such as serum stimulation, phorbol ester, and differentiation signals. Here we show that increased Jun synthesis after serum stimulation is accompanied by a concomitant increase in phosphorylation. Several serine-threonine kinases were evaluated for their ability to phosphorylate Jun in vitro. p34cdc2, protein kinase C, casein kinase II, and pp44mapk phosphorylated Jun efficiently, whereas cyclic AMP-dependent protein kinase and glycogen synthase kinase III did not. The sites phosphorylated by p34cdc2 were similar to those phosphorylated in vivo after serum induction. The major sites of phosphorylation were mapped to serines 63, 73, and 246. Phosphorylation of full-length Jun with several kinases did not affect the DNA-binding activity of Jun homodimers or Fos-Jun heterodimers. Comparison of the DNA binding and in vitro transcription properties of wild-type and mutated proteins containing either alanine or aspartic acid residues in place of Ser-63, -73, and -246 revealed only minor differences among homodimeric complexes and no differences among Fos-Jun heterodimers. Thus, phosphorylation of Jun did not produce a significant change in dimerization, DNA-binding, or in vitro transcription activity. The regulatory role of phosphorylation in the modulation of Jun function is likely to be considerably more complex than previously suggested.

Purification and characterization of a novel antimicrobial peptide from maize (Zea mays L.) kernels

Several small, acid-soluble, basic peptides with anti-microbial properties have been isolated from maize (inbred B73) kernels. One of these peptides (MBP-1) has been purified to homogeneity and characterized. The peptide has a molecular weight of 4127.08 as determined by plasma desorption mass spectroscopy, has no free cysteines, and is predominantly alpha-helical as determined by circular dichroism. The primary sequence of the peptide (33 residues) has been determined by Edman degradation and shows no homology to the thionins, a group of cysteine-rich peptides found in some cereals including wheat, barley, and sorghum, as well as several dicot species. Like the thionins, however, MBP-1 has been found to have antimicrobial properties in vitro. MBP-1 inhibits spore germination or hyphal elongation of several plant pathogenic fungi, including two seed pathogens of maize (Fusarium moniliforme Sheld. and Fusarium graminearum (Gibberella zeae (Schw.) Petsch)), and several bacteria, including a bacterial pathogen of maize (Clavibacter michiganense ssp. nebraskense). A synthetic MBP-1 peptide, air-oxidized and purified by reverse phase chromatography, was equally antifungal as compared with the naturally occurring peptide.

Antipeptide antibodies against the 5-HT1A receptor

The availability of the primary amino acid sequence for a large number of molecules provides a fruitful opportunity for their cellular localization by utilizing the procedure of antipeptide antibody formation. This procedure permits a synthetic peptide sequence to be attached to a carrier molecule for the purpose of inoculating an animal to raise specific antibodies against the selected protein sequence. In this report we describe a number of steps that can be taken to increase the likelihood that the selected peptide sequence will be specific and antigenic. In addition, we describe how the peptides are synthesized, purified and coupled to keyhold limpet hemocyanin. The preparation of the antibody and its characterization are also presented in this method report. The immunocytochemical staining at both the light and ultrastructural level with serotonin (5-HT1A) receptor antipeptide antibodies is discussed. The advantages and disadvantages of this procedure are summarized.

Molecular characterization of the Schwann cell myelin protein, SMP: structural similarities within the immunoglobulin superfamily

The Schwann cell myelin protein (SMP), previously defined in quail and chick by a monoclonal antibody, is in vivo exclusively expressed by myelinating and nonmyelinating Schwann cells and oligodendrocytes. The isolation of the complete nucleotide sequence of SMP is reported here. The predicted polypeptide chain reveals that SMP is a transmembrane molecule of the immunoglobulin superfamily showing sequence similarities with several surface glycoproteins expressed in the nervous and immune systems. In spite of a 43.5% overall sequence identity between rat myelin-associated glycoprotein (MAG) and quail SMP, SMP does not seem to be the avian homolog of MAG, since their expression, regulation, and functions are significantly different. Unusual sequence arrangements shared by SMP, MAG, and two lymphoid antigens suggest the existence of a particular subgroup in the immunoglobulin superfamily.

Increased S100β neurotrophic activity in Alzheimer's disease temporal lobe

The confirming diagnosis of Alzheimer's disease includes an assessment of the concentration of neuritic plaques in the temporal lobe of the brain. The presence of abnormal levels of neurotrophic factors in Alzheimer's disease is one possible explanation for the increased concentration of aggregates of overgrown neurites in the neuritic plaques of Alzheimer's disease. The protein S100 beta, a neurotrophic factor produced by astroglia in the brain, induces neurite outgrowth in cerebral cortical neurons. The generation of specific S100 beta antibodies, the cloning of a full-length cDNA encoding the S100 beta mRNA, and the development of a neurite extension assay system for S100 beta allowed testing of the hypothesis that Alzheimer's disease S100 beta expression is elevated in brain temporal lobe where neuritic plaques are concentrated. The levels of S100 beta protein, mRNA, and specific neurotrophic activity were elevated 10-20-fold in extracts of temporal lobe from autopsy samples of Alzheimer's disease patients compared to those of aged control patients. The cells containing the increased S100 beta were reactive astrocytes; the neuritic plaques were surrounded by S100 beta-containing astrocytes. The elevated levels of S100 beta provides a link between the prominent reactive gliosis and neuritic plaque formation in this common disease of the elderly and raises the possibility that S100 beta contributes to Alzheimer's disease neuropathology.

Structural and functional characterization of full-length heparin-binding growth associated molecule

Heparin-binding growth-associated molecule (HB-GAM) was purified from adult bovine brain and chicken heart. The yield of HB-GAM is increased by 5- to 10-fold when 250 mM NaCl is added to the homogenization buffer, indicating that HB-GAM may exist as a complex with an insoluble component of the tissue. The complete amino acid sequence of the brain-derived HB-GAM was established by automated Edman degradation of the intact protein and chemically or enzymatically derived fragments. The mass of bovine HB-GAM as determined by plasma desorption time-of-flight mass spectrometry is 15,291 mass units, which compares favorably with the calculated mass of 15,289 based on the amino acid sequence. Therefore, HB-GAM has not undergone any major post-translational modifications other than cleavage of the signal peptide. These results indicate that previous amino acid sequence analysis of this protein was carried out using truncated HB-GAM. Full-length HB-GAM is not a mitogen for Balb/3T3 clone A31, Balb MK, NRK, or human umbilical vein endothelial cells. HB-GAM does, however, have adhesive properties and neurite extension activity for chick embryo cerebral cortical derived neurons when presented to these cells as a substrate. HB-GAM had little neurite extension activity when presented as a soluble factor.

Fos is phosphorylated by p34cdc2, cAMP-dependent protein kinase and protein kinase C at multiple sites clustered within regulatory regions

The proto-oncogene c-fos encodes a nuclear protein (Fos) that functions in transcriptional regulation in response to extracellular signals. Fos is extensively modified in the nucleus by serine and threonine phosphorylation. It has been suggested that phosphorylation may play an important role in regulating Fos function in normal and transformed cells. As a first step in addressing this issue, we have used purified Fos as a substrate for several serine-threonine protein kinases, including cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and p34cdc2. Each of these kinases phosphorylated Fos at several unique sites. These sites were located within two regions that were previously shown to reduce the transcriptional activity of Fos in vitro. Several of the sites modified in vitro were also shown to be phosphorylated in serum-stimulated fibroblasts. These findings demonstrate that Fos is a target for several protein kinases involved in signal transduction and suggest that phosphorylation could regulate the transcriptional properties of Fos.

The retinoblastoma protein is phosphorylated on multiple sites by human cdc2

The retinoblastoma gene product (pRB) is a nuclear phosphoprotein that is thought to play a key role in the negative regulation of cellular proliferation. pRB is phosphorylated in a cell cycle dependent manner, and studies in both actively dividing and differentiated cells suggest that this modification may be essential for cells to progress through the cell cycle. Using tryptic phosphopeptide mapping we have shown that pRB is phosphorylated on multiple serine and threonine residues in vivo and that many of these phosphorylation events can be mimicked in vitro using purified p34cdc2. Using synthetic peptides corresponding to potential cdc2 phosphorylation sites, we have developed a strategy which has allowed the identification of five sites. S249, T252, T373, S807 and S811 are phosphorylated in vivo, and in each case these sites correspond closely to the consensus sequence for phosphorylation by p34cdc2. This and the observation that pRB forms a specific complex with p34cdc2 in vivo suggests that p34cdc2 or a p34cdc2-related protein is a major pRB kinase.

Immunocytochemical localization of casein kinase II during interphase and mitosis

We have developed specific antibodies to synthetic peptide antigens that react with the individual subunits of casein kinase II (CKII). Using these antibodies, we studied the localization of CKII in asynchronous HeLa cells by immunofluorescence and immunoelectron microscopy. Further studies were done on HeLa cells arrested at the G1/S transition by hydroxyurea treatment. Our results indicate that the CKII alpha and beta subunits are localized in the cytoplasm during interphase and are distributed throughout the cell during mitosis. Further electron microscopic investigation revealed that CKII alpha subunit is associated with spindle fibers during metaphase and anaphase. In contrast, the CKII alpha' subunit is localized in the nucleus during G1 and in the cytoplasm during S. Taken together, our results suggest that CKII may play significant roles in cell division control by shifting its localization between the cytoplasm and nucleus.

Characterization of synthetic peptide substrates for p34cdc2 protein kinase

Synthetic peptide substrates for the cell division cycle regulated protein kinase, p34cdc2, have been developed and characterized. These peptides are based on the sequences of two known substrates of the enzyme, Simian Virus 40 Large T antigen and the human cellular recessive oncogene product, p53. The peptide sequences are H-A-D-A-Q-H-A-T-P-P-K-K-K-R-K-V-E-D-P-K-D-F-OH (T antigen) and H-K-R-A-L-P-N-N-T-S-S-S-P-Q-P-K-K-K-P-L-D-G-E-Y-NH2 (p53), and they have been employed in a rapid assay of phosphorylation in vitro. Both peptides show linear kinetics and an apparent Km of 74 and 120 microM, respectively, for the purified human enzyme. The T antigen peptide is specifically phosphorylated by p34cdc2 and not by seven other protein serine/threonine kinases, chosen because they represent major classes of such enzymes. The peptides have been used in whole cell lysates to detect protein kinase activity, and the cell cycle variation of this activity is comparable to that measured with specific immune and affinity complexes of p34cdc2. In addition, the peptide phosphorylation detected in mitotic cells is depleted by affinity adsorption of p34cdc2 using either antibodies to p34cdc2 or by immobilized p13, a p34cdc2-binding protein. Purification of peptide kinase activity from mitotic HeLa cells yields an enzyme indistinguishable from p34cdc2. These peptides should be useful in the investigation of p34cdc2 protein kinase and their regulation throughout the cell division cycle.

Sequence-specific antirepression of histone H1-mediated inhibition of basal RNA polymerase II transcription

To understand the principles of control and selectivity in gene expression, the biochemical mechanisms by which promoter- and enhancer-binding factors regulate transcription by RNA polymerase II were analyzed. A general observed repressor of transcription was purified and identified as histone H1. Since many aspects of H1 binding to naked DNA resemble its interaction with chromatin, purified H1 bound to naked DNA was used as a model for the repressed state of the DNA template. Three sequence-specific transcription factors, Sp1, GAL4-VP16, and GAGA factor, were shown to counteract H1-mediated repression (antirepression). In addition, Sp1 and GAL4-VP16, but not the GAGA factor, activated transcription in the absence of H1. Therefore, true activation and antirepression appear to be distinct activities of sequence-specific factors. Furthermore, transcription antirepression by GAL4-VP16 was sustained for several rounds of transcription. These findings, together with previous studies on H1, suggest that H1 participates in repression of the genome in the ground state and that sequence-specific transcription factors induce selected genes by a combination of true activation and release of basal repression that is mediated at least in part by H1.

Synthetic peptide substrates for casein kinase II

Synthetic peptide substrates for CKII are useful reagents in the analysis of phosphorylation sites when used in conjunction with biochemical and genetic analysis of the protein substrates for the enzyme. A multidisciplinary approach should be applied to the characterization of the synthetic peptide products, including amino acid analysis, sequencing, and mass spectrometry. Synthetic procedures for CKII substrate peptides often result in anisole adducts and dehydrated forms. Mass spectrometry is invaluable in identifying these contaminants, and preparative HPLC can be used to separate them from the desired product. Quantitative analysis of the CKII phosphorylation of peptides can utilize phosphocellulose paper if the peptide has a basic sequence, or thin-layer chromatography, if the peptide has no basic portion. Qualitative analysis using electrophoresis and mass spectrometry help to establish the stoichiometry of phosphorylation. Sequence analysis of phosphoserine after beta elimination and derivatization is useful in quantifying adjacent phosphorylation sites. Overall, application of a variety of techniques permits detailed analysis of CKII phosphorylation sites on synthetic peptides that are model substrates.

Cell cycle control of DNA replication by a homologue from human cells of the p34cdc2 protein kinase

The regulation of DNA replication during the eukaryotic cell cycle was studied in a system where cell free replication of simian virus 40 (SV40) DNA was used as a model for chromosome replication. A factor, RF-S, was partially purified from human S phase cells based on its ability to activate DNA replication in extracts from G1 cells. RF-S contained a human homologue of the Schizosaccharomyces pombe p34cdc2 kinase, and this kinase was necessary for RF-S activity. The limiting step in activation of the p34 kinase at the G1 to S transition may be its association with a cyclin since addition of cyclin A to a G1 extract was sufficient to start DNA replication. These observations suggest that the role of p34cdc2 in controlling the start of DNA synthesis has been conserved in evolution.

Structural analysis of bovine pancreatic thread protein

Pancreatic thread protein (PTP) forms double helical threads in the neutral pH range after purification, undergoing freely reversible, pH-dependent globule-fibril transformation. The purified bovine PTP consists on SDS gels of two carbohydrate-free polypeptide chains (Gross et al., 1985). Plasma desorption mass spectrometry and amino acid sequence analysis now confirm that bovine PTP contains two disulfide-bonded polypeptides, an A chain of 101 amino acid residues with a molecular weight of 11,073 and a B chain of 35 residues with a molecular weight of 3970. The intact protein exhibits a molecular weight of 15,036, agreeing greater than 99.9% with the molecular weight calculated from the sequence. The B chain sequence was determined by gas-phase Edman degradation of the intact polypeptide. The A chain sequence was determined from overlapping peptides generated by cleavage at lysyl, tryptophanyl, and aspartyl-prolyl residues. Based upon the bovine PTP cDNA structure, the two chains of the protein result from cleavage of a single polypeptide with removal of a dipeptide between the NH2-terminal A chain and COOH-terminal B chain. Comparison of bovine PTP with other proteins reveals significant structural relatedness with the single-chain homologues from human and rat pancreas and with the motif associated with Ca2(+)-dependent carbohydrate recognition domains. The physiological role of PTP has not yet been resolved. The protein is present in very high concentration in pancreatic secretion and it has been detected in brain lesions in Alzheimer's disease and Down syndrome and in regenerating rat pancreatic islets. The present results provide a firm protein base for ongoing molecular, physical-chemical, and structure-function studies of this unusual protein.

Immunocytochemical localization of the neuron-specific form of the c-src gene product, pp60c-src(+), in rat brain

Neurons express high levels of a variant form of the c-src gene product, denoted pp60c-src(+), which contains a 6 amino acid insert in the amino-terminal half of the c-src protein. We have determined the localization of pp60c-src(+) in neurons using an affinity-purified anti-peptide antibody, referred to as affi-SB12, that exclusively recognizes this neuron-specific form of the c-src gene product. Using affi-SB12, we examined the distribution of pp60c-src(+) by immunoperoxidase staining of sections through adult rat brains, pp60c-src(+) was widely distributed in rat brain and appeared to be differentially expressed in subpopulations of neurons. The majority of immunoreactive neurons was found in the mesencephalon, cerebellum, pons, and medulla. Telencephalic structures that contained substantial populations of pp60c-src(+)-immunoreactive neurons included layer V of the cerebral cortex and the ventral pallidum. Within individual neurons, pp60c-src(+) immunoreactivity was localized to the cell soma and dendritic processes, while labeling of axons and nerve terminals (puncta) was not as readily detected. Dense accumulations of immunoreactive axons were rare, being most prominent in portions of the inferior and superior olive, and in the spinal trigeminal nucleus. While the regional distribution of pp60c-src(+) immunoreactivity does not correlate with any specific neuronal cell type or first messenger system, this unique pattern of expression of pp60c-src(+) suggests the existence of a previously uncharacterized functional organization within the brain. Furthermore, the localization of this neuron-specific tyrosine kinase in functionally important areas of the nerve cell, namely, dendritic processes, axons, and nerve terminals, suggests that pp60c-src(+) may regulate pleiotropic functions in specific classes of neurons in the adult central nervous system.

Human p53 is phosphorylated by p60-cdc2 and cyclin B-cdc2

The human anti-oncoprotein p53 is shown to be a substrate of cdc2. The primary site of phosphorylation is serine-315. Serine-315 is phosphorylated by both p60-cdc2 and cyclin B-cdc2 enzymes. The phosphorylation of p53 is cell cycle-dependent. The abundance of p53 also oscillates during the cell cycle. The protein is largely absent from cells that have just completed division but accumulates in cells during G1 phase. Phosphorylation by cdc2 might regulate the antiproliferative activity of p53.

Purification and characterization of an insulin-like growth factor II variant from human plasma

An insulin-like growth factor II variant (IGF-II variant) was purified from Cohn fraction IV1 of human plasma by ion exchange, gel filtration, and reversed-phase high pressure liquid chromatography. The amino-terminal sequence of the first 35 amino acid residues showed a replacement of Ser-29 of IGF-II with the tetrapeptide Arg-Leu-Pro-Gly of IGF-II variant. Peptides isolated and sequenced after digestion with endoproteinase Asp-N and endoproteinase Glu-C disclosed no differences with the sequence predicted from an IGF-II variant cDNA clone isolated by Jansen, M., van Shaik, F. M. A., van Tol, H., Van den Brande, J. L., and Sussenbach, J. S. (1985) FEBS Lett., 179, 243-246. The molecular ion of intact IGF-II variant was 7809.4 mass units, as measured by plasma desorption mass spectrometry. This is in close agreement with the molecular ion of 7812.8 mass units calculated from the determined sequence and indicates the entire amino acid sequence had been accounted for. Binding of IGF-II variant to purified insulin-like growth factor I (IGF-I) receptors demonstrated a 2-3-fold lower affinity for this receptor compared with IGF-I or IGF-II. The dissociation constants for IGF-I, IGF-II, and IGF-II variant are 0.23, 0.38, and 0.80 nM, respectively. In a growth assay, the concentration of IGF-II and IGF-II variant required to stimulate the half-maximal growth of MCF-7 cells was 4 and 13 nM, respectively. Finally, the amount of IGF-II variant that can be purified by this method constitutes approximately 25% of the total IGF-II isolated from Cohn fraction IV1 of human plasma.

Phosphorylation of large tumour antigen by cdc2 stimulates SV40 DNA replication

Simian virus 40 large tumour antigen (T) is a replication origin binding protein required for viral DNA synthesis. Unphosphorylated T antigen is deficient in promoting DNA replication in vitro but can be activated by phosphorylation at residue threonine 124 by the cdc2 protein kinase. This observation demonstrates that T is regulated by phosphorylation and provides a model for cdc2 function in the control of DNA replication.

Serum-stimulated cell growth causes oscillations in casein kinase II activity

We have tested the effects of serum-stimulated growth of quiescent WI38 human lung fibroblasts on cellular casein kinase II (CK-II) activity. Using the casein kinase II synthetic substrate RRREEETEEE we find a transient 6-fold elevation in CK-II activity in cell homogenates within 30 min following serum stimulation. Additional cycles of CK-II activation and inactivation are seen at 12 and 24 h after stimulation. The oscillations in CK-II activity are largely independent of de novo protein synthesis, and, thus, are likely to reflect cycles of post-translational activation and inhibition of the cellular kinase pool. In contrast to the activity profile of CK-II, we find that cyclic AMP-dependent protein kinase is rapidly inhibited upon serum-stimulation of WI38 cells. These results demonstrate that CK-II activity is subject to unique cellular regulation during proliferation and are consistent with the postulate that CK-II plays an important role in cell growth.