Reversal of Rous sarcoma-specific immunoglobulin phosphorylation on tyrosine (ADP as phosphate acceptor) catalyzed by the src gene kinase

The emerging role of cellular post-translational modifications in modulating growth and productivity of recombinant Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are one of the most commonly used host cell lines used for the production human therapeutic proteins. Much research over the past two decades has focussed on improving the growth, titre and cell specific productivity of CHO cells and in turn lowering the costs associated with production of recombinant proteins. CHO cell engineering has become of particular interest in recent years following the publication of the CHO cell genome and the availability of data relating to the proteome, transcriptome and metabolome of CHO cells. However, data relating to the cellular post-translational modification (PTMs) which can affect the functionality of CHO cellular proteins has only begun to be presented in recent years. PTMs are important to many cellular processes and can further alter proteins by increasing the complexity of proteins and their interactions. In this review, we describe the research presented from CHO cells to date related on three of the most important PTMs; glycosylation, phosphorylation and ubiquitination.

Phosphorylation and Dephosphorylation of Tau Protein by the Catalytic Subunit of PKA, as Probed by Electrophoretic Mobility Retard

Background:

Tau is a microtubule associated protein that regulates the stability of microtubules and the microtubule-dependent axonal transport. Its hyperphosphorylated form is one of the hallmarks of Alzheimer’s disease and other tauopathies and the major component of the paired helical filaments that form the abnormal proteinaceous tangles found in these neurodegenerative diseases. It is generally accepted that the phosphorylation extent of tau is the result of an equilibrium in the activity of protein kinases and phosphatases. Disruption of the balance between both types of enzyme activities has been assumed to be at the origin of tau hyperphosphorylation and the subsequent toxicity and progress of the disease.

Objective:

We explore the possibility that, beside the phosphatase action on phosphorylated tau, the catalytic subunit of PKA catalyzes both tau phosphorylation and also tau dephosphorylation, depending on the ATP/ADP ratio.

Methods:

We use the shift in the relative electrophoretic mobility suffered by different phosphorylated forms of tau, as a sensor of the catalytic action of the enzyme.

Results:

The results are in agreement with the long-known thermodynamic reversibility of the phosphorylation reaction (ATP + Protein = ADP+Phospho-Protein) catalyzed by PKA and many other protein kinases.

Conclusion:

The results contribute to put the compartmentalized energy state of the neuron and the mitochondrial-functions disruption upstream of tau-related pathologies.

Exploring protein phosphorylation by combining computational approaches and biochemical methods

Post-translational modifications of proteins expand their functional diversity, regulating the response of cells to a variety of stimuli. Among these modifications, phosphorylation is the most ubiquitous and plays a prominent role in cell signaling. The addition of a phosphate often affects the function of a protein by altering its structure and dynamics. However, these alterations are often difficult to study and the functional and structural implications remain unresolved. New approaches are emerging to overcome common obstacles related to the production and manipulation of these samples. Here, we summarize the available methods for phosphoprotein purification and phosphomimetic engineering, highlighting the advantages and disadvantages of each. We propose a general workflow for protein phosphorylation analysis combining computational and biochemical approaches, building on recent advances that enable user-friendly and easy-to-access Molecular Dynamics simulations. We hope this innovative workflow will inform the best experimental approach to explore such post-translational modifications. We have applied this workflow to two different human protein models: the hemeprotein cytochrome c and the RNA binding protein HuR. Our results illustrate the usefulness of Molecular Dynamics as a decision-making tool to design the most appropriate phosphomimetic variant.

Heat Oscillations Driven by the Embryonic Cell Cycle Reveal the Energetic Costs of Signaling

All living systems function out of equilibrium and exchange energy in the form of heat with their environment. Thus, heat flow can inform on the energetic costs of cellular processes, which are largely unknown. Here, we have repurposed an isothermal calorimeter to measure heat flow between developing zebrafish embryos and the surrounding medium. Heat flow increased over time with cell number. Unexpectedly, a prominent oscillatory component of the heat flow, with periods matching the synchronous early reductive cleavage divisions, persisted even when DNA synthesis and mitosis were blocked by inhibitors. Instead, the heat flow oscillations were driven by the phosphorylation and dephosphorylation reactions catalyzed by the cell-cycle oscillator, the biochemical network controlling mitotic entry and exit. We propose that the high energetic cost of cell-cycle signaling reflects the significant thermodynamic burden of imposing accurate and robust timing on cell proliferation during development.

The crucial role of protein phosphorylation in cell signaling and its use as targeted therapy (Review)

Protein phosphorylation is an important cellular regulatory mechanism as many enzymes and receptors are activated/deactivated by phosphorylation and dephosphorylation events, by means of kinases and phosphatases. In particular, the protein kinases are responsible for cellular transduction signaling and their hyperactivity, malfunction or overexpression can be found in several diseases, mostly tumors. Therefore, it is evident that the use of kinase inhibitors can be valuable for the treatment of cancer. In this review, we discuss the mechanism of action of phosphorylation, with particular attention to the importance of phosphorylation under physiological and pathological conditions. We also discuss the possibility of using kinase inhibitors in the treatment of tumors.

Why nature chose phosphate to modify proteins

The advantageous chemical properties of the phosphate ester linkage were exploited early in evolution to generate the phosphate diester linkages that join neighbouring bases in RNA and DNA (Westheimer 1987 Science 235, 1173-1178). Following the fixation of the genetic code, another use for phosphate ester modification was found, namely reversible phosphorylation of the three hydroxyamino acids, serine, threonine and tyrosine, in proteins. During the course of evolution, phosphorylation emerged as one of the most prominent types of post-translational modification, because of its versatility and ready reversibility. Phosphoamino acids generated by protein phosphorylation act as new chemical entities that do not resemble any natural amino acid, and thereby provide a means of diversifying the chemical nature of protein surfaces. A protein-linked phosphate group can form hydrogen bonds or salt bridges either intra- or intermolecularly, creating stronger hydrogen bonds with arginine than either aspartate or glutamate. The unique size of the ionic shell and charge properties of covalently attached phosphate allow specific and inducible recognition of phosphoproteins by phosphospecific-binding domains in other proteins, thus promoting inducible protein-protein interaction. In this manner, phosphorylation serves as a switch that allows signal transduction networks to transmit signals in response to extracellular stimuli.

Why nature chose phosphate to modify proteins

The advantageous chemical properties of the phosphate ester linkage were exploited early in evolution to generate the phosphate diester linkages that join neighbouring bases in RNA and DNA (Westheimer 1987 Science 235, 1173-1178). Following the fixation of the genetic code, another use for phosphate ester modification was found, namely reversible phosphorylation of the three hydroxyamino acids, serine, threonine and tyrosine, in proteins. During the course of evolution, phosphorylation emerged as one of the most prominent types of post-translational modification, because of its versatility and ready reversibility. Phosphoamino acids generated by protein phosphorylation act as new chemical entities that do not resemble any natural amino acid, and thereby provide a means of diversifying the chemical nature of protein surfaces. A protein-linked phosphate group can form hydrogen bonds or salt bridges either intra- or intermolecularly, creating stronger hydrogen bonds with arginine than either aspartate or glutamate. The unique size of the ionic shell and charge properties of covalently attached phosphate allow specific and inducible recognition of phosphoproteins by phosphospecific-binding domains in other proteins, thus promoting inducible protein-protein interaction. In this manner, phosphorylation serves as a switch that allows signal transduction networks to transmit signals in response to extracellular stimuli.

Deregulation of mitogen-activated protein kinase at low pH due to a structural rearrangement of activation segment

Autophosphorylation of recombinant mitogen-activated protein kinase (MAPK) on Tyr was found to be several-fold stimulated at weakly acidic pH (5.5-6.0), whereas the phosphorylation of a protein substrate, myelin basic protein, was greatly inhibited at pH below 6. 0. In contrast to phosphorylation at pH 8.0, both MAPK autophosphorylation and MAPK phosphorylation with upstream MAPK kinase at low pH failed to stimulate essentially its kinase activity towards the exogenous protein substrate. Immunoprecipitation and ELISA with an activation segment-specific antibody, kinetic analysis, and reversible phosphorylation assay revealed a difference in the folding of MAPK activation segment at pH 5.5 and 8.0. The data suggest that a rearrangement of the activation segment at low pH promotes a stable low-activity conformation of the enzyme which is favorable for intramolecular autophosphorylation. In this conformation, the phosphorylation of the exogenous protein substrate is inhibited due to persistent blocking of the enzyme catalytic center by the activation segment.

Phosphoregulatory tyrosine of Xenopus mitogen-activated protein kinase is out of the reach of the enzyme catalytic center after autophosphorylation. Biochemical evidence for conformational changes upon phosphorylation

Autophosphorylation of the recombinant mitogen-activated protein kinase (MAPK) from Xenopus laevis has been studied to detect the conformational changes in the region of regulatory phosphorylation upon enzyme activation. Slow autophosphorylation of Xenopus MAPK occurred predominantly on tyrosine, the major phosphoregulatory site of MAPKs, through an intramolecular mechanism and was accompanied by a low magnitude stimulation of the catalytic activity towards an exogenous substrate, myelin basic protein. Autophosphorylated but not unphosphorylated enzyme was shown to interact with the protein substrate. In contrast to the previously reported reversibility of many tyrosine kinase reactions, the tyrosine phosphorylation of Xenopus MAPK was found to be irreversible in the presence of high ADP concentrations, although ADP could competitively inhibit both autophosphorylation and myelin basic protein phosphorylation. We concluded, therefore, that the phosphoregulatory tyrosine is no more accessible to an intramolecular phosphotransferase reaction and is out of the reach of the enzyme catalytic center after phosphorylation. The conformational changes in the region of regulatory phosphorylation resulted in a reduced immunoprecipitation of autophosphorylated and MAPK-kinase-phosphorylated forms of the enzyme by a polyclonal antibody raised against a synthetic peptide corresponding to residues 173-197 of Xenopus MAPK which includes the sites of regulatory phosphorylation. The reduced recognition was not due to the phosphorylation itself, since the antibody efficiently immunoprecipitated SDS-denatured forms of the phosphorylated enzyme. The antibody was not a neutralizing antibody, allowing unphosphorylated MAPK to undergo autophosphorylation while in the immune complex. However, autophosphorylation caused a release of phosphorylated enzyme from the immune complex, suggesting that dramatic conformational changes, which could even overcome the antibody constraints, took place in the phosphoregulatory region of MAPK upon enzyme activation.

Regulation of purified hepatic PC-1 (phosphodiesterase-I/nucleotide pyrophosphatase) by threonine auto(de)phosphorylation and by binding of acidic fibroblast growth factor

The plasma cell differentiation antigen PC-1 was purified to homogeneity from rat liver membranes. Denaturing electrophoresis revealed polypeptides of 118 and 128 kDa, which were both recognized by antibodies against recombinant murine PC-1. During gel filtration PC-1 migrated as a protein of about 500 kDa, suggesting a tetrameric structure. Purified PC-1 displayed a phosphodiesterase-I/nucleotide pyrophosphatase activity that could be completely blocked by EDTA, dithiothreitol and acidic fibroblast growth factor (extrapolated Ki = 1.3 nM). Purified PC-1 was also capable of threonine autophosphorylation and of phosphorylation of histone IIa. The autophosphorylation of PC-1 was inhibited by addition of histone IIa, and it was blocked by phosphodiesterase-I inhibitors (acidic fibroblast growth factor, dithiothreitol), by nucleotides (ATP, ADP, AMP), and by vanadate. When added to autophosphorylated PC-1, these compounds caused a prompt dephosphorylation. However, the same agents did not affect the (de)phosphorylation of histone IIa, which is not a substrate for the PC-1 phosphatase. These data indicate that phosphodiesterase-I inhibitors, nucleotides and vanadate affect the (de)phosphorylation of PC-1 by stimulating the PC-1 phosphatase and/or by shielding the autophosphorylation site from the PC-1 kinase. The rate of dephosphorylation of PC-1 was independent of the dilution, suggesting an autocatalytic intramolecular process. We propose that the autophosphorylation of PC-1 serves to block its nucleotide pyrophosphatase activity when extracellular ATP becomes scarce.

Phosphoryl exchange is involved in the mechanism of the insulin receptor kinase

The cytoplasmic kinase domain of the human insulin receptor (IRKD; M(r) 49 kDa) has been over-expressed in insect cells using the baculovirus expression system. To investigate the kinase mechanism, we have compared the stoichiometry of ADP formation and phosphoryl transfer. After an initial phase of autophosphorylation, ATP is consumed without a stoichiometric increase in incorporated phosphate. During substrate phosphorylation using poly(Glu:Tyr) (4:1) phosphoryl transfer comes close to ATP turnover, which is independent of the presence of the substrate, indicating an increased efficiency (i.e. ATP turnover/phosphate incorporation) of phosphoryl transfer. Autophosphorylation under pulse-chase conditions suggests the existence of a phosphoenzyme intermediate.

p56lyn catalyzes a reversible autophosphorylation reaction and a nucleoside diphosphate kinase reaction

The reversible autophosphorylation of the pp60c-src family tyrosine kinase, p56lyn has been characterized by a simple procedure that involves the examination of the enzyme catalyzed radioisotope exchange between ATP and ADP. The equilibrium constant of the reaction was determined to be 3.31 and corresponded to a standard free energy of hydrolysis of the phosphotyrosine bond in p56lyn of -8.08 kcal/mol. GDP was capable of substituting for ADP as phosphate acceptor so that p56lyn displayed a nucleoside diphosphate kinase activity.

L-asparaginase of Tetrahymena pyriformis is associated with a kinase activity

Most of L-asparaginase activity of Tetrahymena pyriformis was found to be present in microsomal membranes from which it has been purified to homogeneity (Tsirka, S.A.E. and Kyriakidis, D.A. Mol. Cell. Biochem. 83: 147-155, 1988). The native enzyme has a relative molecular weight of approximately 200 kDa, while under denaturing conditions the enzyme exhibits a subunit size of 39 kDa. Aminoacid analysis and an oligopeptide from N-terminal sequence have been determined. Dephosphorylation of L-asparaginase by alkaline phosphatase results in an activation of its catalytic activity. This enzyme also exhibits intrinsic phosphorylation activity with a Km value for ATP of 0.5 mM. Autophosphorylation with [gamma-32P] ATP of purified L-asparaginase results in the phosphorylation of tyrosine residues as well as in loss of its activity. Mg2+ and Ca2+ added together act synergistically to stimulate the kinase activity by more than 160%. The polyamines putrescine, spermidine and spermine activate the kinase approximately 100%, while neither cAMP or cGMP have any effect. These results indicate that this membrane protein with dual L-asparaginase/kinase activity must play an important role in regulating the intracellular levels of L-asparagine in Tetrahymena pyriformis.

Signal transduction in bacteria

Cells display a remarkable ability to respond to small fluctuations in their surroundings. In simple microbial systems, information from sensory receptors feeds into a circuitry of regulatory proteins that transfer high energy phosphoryl groups from histidine to aspartate side chains. This phosphotransfer network couples environmental signals to an array of response elements that control cell motility and regulate gene expression.

Specific dephosphorylation of phosphoproteins by protein-serine and -tyrosine kinases

Five protein kinases are shown to serve as specific phosphatases in the absence of ADP. Although the rates of hydrolysis are very slow compared to the forward phosphorylation rates under optimal conditions, they are of the same order as the reverse reaction in the presence of ADP. Because cells contain approximately equal to 3 mM ATP, neither the reverse reaction nor the phosphatase is likely to play a physiological role. beta-casein B phosphorylated by the catalytic subunit of cAMP-dependent protein kinase (protein kinase A) is specifically dephosphorylated by protein kinase A but not by polypeptide-dependent protein kinase (protein kinase P). beta-casein B phosphorylated by protein kinase P is specifically dephosphorylated by protein kinase P but not by protein kinase A. Histone H1 phosphorylated by protein kinase C is dephosphorylated by the same enzyme in the absence of ADP. In all cases tested addition of ADP and F1-ATPase accelerates moderately the rate of dephosphorylation. Native H+-ATPase from yeast plasma membranes is isolated mainly in the phosphorylated form. It is dephosphorylated and rephosphorylated by protein kinase P but not by protein kinase A. Protein-tyrosine kinase of the epidermal growth factor receptor phosphorylates the random synthetic polypeptide poly(Glu80Tyr20). The phosphorylated polymer is specifically dephosphorylated in the absence of ADP by epidermal growth factor receptor preparations but not by insulin receptor preparations. The same polymer phosphorylated by insulin receptor is dephosphorylated by insulin receptor but not by epidermal growth factor receptor preparations. By using a cycle of dephosphorylation-rephosphorylation, it is possible to identify proteins that are phosphorylated by these protein kinases in vivo. Should this method be applicable to additional protein kinases, it should be possible to estimate the quantitative contribution of each protein kinase to a single phosphoprotein.

Interaction of vanadate with phenol and tyrosine: implications for the effects of vanadate on systems regulated by tyrosine phosphorylation

The interaction of vanadate with phenol and N-acetyltyrosine ethyl ester in aqueous solution has been studied by using 51V nuclear magnetic resonance spectroscopy. On the basis of these studies, it has been concluded that vanadate rapidly esterifies the hydroxyl group of the aromatic ring to yield a phenyl vanadate. For phenol, the equilibrium constant for this reaction in terms of the convention that the activity of liquid water is 1.0 is K1 = [phenyl vanadate]/[phenol][vanadate] = 0.97 +/- 0.02. This value is well over 4 orders of magnitude larger than estimates from the literature for the corresponding equilibrium constant for the esterification of phenol by phosphate. The equilibrium constant for esterification of the phenol moiety of N-acetyltyrosine ethyl ester is similar to that for esterification of phenol. The relevance of these observations to processes that are regulated by reversible phosphorylation/dephosphorylation of tyrosine residues is discussed, in particular the insulin-like effect of vanadate.

The role of metarhodopsin in the generation of spontaneous quantum bumps in ultraviolet receptors of Limulus median eye. Evidence for reverse reactions into an active state

The origin of spontaneous quantum bumps has been examined in the ultraviolet photoreceptors of Limulus median eye. These cells have a rhodopsin with a lambda max at 360 nm and a stable photoproduct, metarhodopsin, with a lambda max at 470 nm. The steady state rate of spontaneous quantum bumps was found to be higher when the metarhodopsin concentration was high than when the rhodopsin concentration was high. This result implicates metarhodopsin in the generation of spontaneous quantum bumps. Furthermore, this result is consistent with the idea that the reaction which inactivates metarhodopsin (terminates the ability of metarhodopsin to initiate the reactions leading to a quantum bump) is reversible and that such reversions can be a significant source of spontaneous quantum bumps. Given that the rate of spontaneous quantum bumps is approximately 1/s under conditions where the number of inactive metarhodopsin molecules is approximately 10(9), it follows that the molecular switch that inactivates metarhodopsin reverses with a probability of less than 10(-9). A model is presented of how a molecular switch with this reliability might be constructed.

Purification of the Rous sarcoma virus src kinase by casein-agarose and tyrosine-agarose affinity chromatography

A simple and effective purification method for the src kinase, the transforming gene product of Rous sarcoma virus, has been developed by using affinity chromatography on casein-agarose and tyrosine-agarose columns. NaDodSO4/polyacrylamide gel electrophoresis and silver staining analysis showed that the purified kinase preparation was composed of a predominant polypeptide of 60,000-Da. In most of the preparations, however, three minor proteins (54,000, 52,000, and 15,000 Da) were also detected, and they were partially characterized. As one of the exogenous substrates, calmodulin was found to be phosphorylated on tyrosine by the purified src kinase.

Identification, purification, and characterization of phosphotyrosine-specific protein phosphatases from cultured chicken embryo fibroblasts

Tyrosine phosphorylation catalyzed by a unique class of protein kinases is an important process in both normal cell proliferation and oncogenic transformation. In this study, phosphoprotein phosphatases specific for the dephosphorylation of phosphotyrosine residues were partially purified from secondary chicken embryo fibroblasts, using 32P-labeled immunoglobulin G phosphorylated by pp60src as substrate. Crude cell extracts contained ca. 70% of the activity in the soluble form and ca. 30% associated with a crude membrane fraction. The soluble activity was purified by using DEAE-cellulose and carboxymethyl cellulose column chromatography and gel filtration, and at least three enzyme species of apparent Mr 55,000 (pTPI), 50,000 (pTPII), and 95,000 (pTPIII)--comprising ca. 20, 45, and 35%, respectively, of the total activity--were resolved. All three enzymes possessed somewhat similar properties. They had a pH optimum of about 7.4, they were inhibited by Zn2+, vanadate, ATP, and ADP, and they were unaffected by divalent metal cations, EDTA, and F- under standard assay conditions employing a physiological ionic strength. These properties suggest that they represent a class of enzymes distinct from well-known phosphoseryl-phosphothreonyl-protein phosphatases and that dephosphorylation of phosphotyrosine-containing proteins may be carried out by a unique family of phosphoprotein phosphatases. Transformation by Rous sarcoma virus resulted in a small increase in phosphotyrosyl-protein phosphatase activity.

The bond in the bacteriophage phi X174 gene A protein--DNA complex is a tyrosyl-5'-phosphate ester

The bacteriophage phi X174 gene A protein cleaves the viral strand of the double-stranded replicative form (RF) DNA of the phage at a specific site, the origin. It leaves a free 3'-OH at nucleotide 4305 (G) of the phi X DNA sequence and binds covalently to the DNA. The nature and position of the covalent bond have been determined using the octadecadesoxyribonucleotide CAACTTG[32P]ATATTAATAAC. This octadecamer, which corresponds to nucleotides 4299-4316 of phi X viral DNA, is cleaved by gene A protein. Gene A protein is bound to the labelled phosphate via a tyrosyl residue, indicating that binding occurs to the nucleotide corresponding to 4306 (A) of the phi X viral DNA strand.

Phosphorylation and dephosphorylation of the insulin receptor: evidence against an intrinsic phosphatase activity

We have studied the reversibility of insulin receptor phosphorylation to establish the relation between this autophosphorylation reaction and the initiation of insulin action and between dephosphorylation and the termination of insulin effects in cells. In cultured Fao hepatoma cells labeled with 32PO4(3-), insulin increased 5-fold the phosphorylation of the beta-subunit of the insulin receptor at serine, threonine, and tyrosine residues. Addition of anti-insulin antiserum to cells incubated with insulin caused dissociation of insulin from the receptor and concurrent dephosphorylation of the beta-subunit. 32PO4(3-) associated with the insulin-stimulated receptor could be decreased by the addition of sodium phosphate to the medium but with a slower time course. Insulin stimulated phosphorylation of insulin receptor purified partially on immobilized wheat germ agglutinin. This reaction utilized [gamma-32P] ATP and occurred exclusively on tyrosine residues. Addition of unlabeled ATP caused a decrease in the amount of PO4(3-) associated with the receptor. Insulin-stimulated phosphorylation was also observed if the receptors were further purified by immunoprecipitation with anti-insulin receptor antibody prior to the phosphorylation reaction; however, addition of unlabeled ATP to this system did not chase the labeled 32PO4(3-) from the beta-subunit. These data are consistent with the notion that phosphorylation and dephosphorylation of the insulin receptor parallel the onset and termination of insulin action. Phosphatase activity involved in the dephosphorylation of the insulin receptor appears to be a glycoprotein because it was retained after partial purification of the receptor on wheat germ agglutinin-agarose; however, this phosphatase activity is distinct from the insulin receptor because it was not retained after immunoprecipitation of the receptor with anti-insulin receptor antibodies.

Identification, purification, and characterization of phosphotyrosine-specific protein phosphatases from cultured chicken embryo fibroblasts

Tyrosine phosphorylation catalyzed by a unique class of protein kinases is an important process in both normal cell proliferation and oncogenic transformation. In this study, phosphoprotein phosphatases specific for the dephosphorylation of phosphotyrosine residues were partially purified from secondary chicken embryo fibroblasts, using 32P-labeled immunoglobulin G phosphorylated by pp60src as substrate. Crude cell extracts contained ca. 70% of the activity in the soluble form and ca. 30% associated with a crude membrane fraction. The soluble activity was purified by using DEAE-cellulose and carboxymethyl cellulose column chromatography and gel filtration, and at least three enzyme species of apparent Mr 55,000 (pTPI), 50,000 (pTPII), and 95,000 (pTPIII)--comprising ca. 20, 45, and 35%, respectively, of the total activity--were resolved. All three enzymes possessed somewhat similar properties. They had a pH optimum of about 7.4, they were inhibited by Zn2+, vanadate, ATP, and ADP, and they were unaffected by divalent metal cations, EDTA, and F- under standard assay conditions employing a physiological ionic strength. These properties suggest that they represent a class of enzymes distinct from well-known phosphoseryl-phosphothreonyl-protein phosphatases and that dephosphorylation of phosphotyrosine-containing proteins may be carried out by a unique family of phosphoprotein phosphatases. Transformation by Rous sarcoma virus resulted in a small increase in phosphotyrosyl-protein phosphatase activity.

Gene A protein of bacteriophage phi X174 is a highly specific single-strand nuclease and binds via a tyrosyl residue to DNA after cleavage

The sequence specificity of the endonuclease activity of gene A protein and A* protein was studied using synthetic oligonucleotides containing (part of) the sequence of the origin of phi X RF DNA replication and single-stranded (ss) DNA fragments of phi X and G4. From a comparison of the sequences that are cleaved a consensus sequence for cleavage of ssDNA by gene A protein has been deduced. This consensus sequence occurs in ssDNA of both phi X and G4 at the origin and at one additional site. This is surprising since the rolling circle mechanism demands that gene A protein cleaves at the origin only. However, it could be shown that in the presence of SSB protein the ssDNAs of phi X and G4 are only cleaved at the origin, which is probably due to a strong gene A protein binding site, the key sequence, which forms part of the 30 b.p. origin region of phi X and related bacteriophages. Gene A protein and A* protein bind covalently to the DNA at the 5'-end of the cleavage site. Using a uniquely, internally 32p-labelled oligonucleotide as a substrate, it was shown that gene A protein and A* protein are bound via a tyrosyl residue to the 5'-phosphate of the phosphodiester bond which is cleaved.