8 Регуляция экспрессии генов. Система передачи сигнала (1160077), страница 2
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The G proteins thus modified fail to respond to normal hormonal stimuli.The pathology of both diseases results from defective regulation of adenylate cyclase and overproduction of cAMP.776Part III Bioenergetics and MetabolismRetrovirusNormal cellis infected withretrovirus.Gene for regulatorygrowth protein(proto-oncogene)Host cell now hasretroviral genomeincorporated nearproto-oncogene.Forming virusencapsulatesproto-oncogeneand viral genome.Retrovirus withproto-oncogeneInfection cyclesMutation createsoncogene.Retrovirus withoncogene invadesnormal cell.Transformed cell,producing defectiveregulatory proteinFigure 22-36 Conversion of a normal regulatorygene into a viral oncogene.
(T) A normal cell is infected by a retrovirus, which (2) inserts its owngenome into the chromosome of the host cell, nearthe gene for a regulatory protein (the proto-oncogene). (3) Virus particles released from the infectedcell infrequently "capture" a host gene, in this casethe proto-oncogene that encodes a regulatory protein. (4) During several cycles of infection, a mutation occurs in the viral proto-oncogene, convertingit into an oncogene. (5) When the virus subsequently infects a normal cell, it introduces the oncogene into the host-cell DNA. Transcription of theoncogene leads to the production of a defective regulatory protein that continuously gives the signalfor host-cell division, overriding normal mechanisms for limiting cell division.
Host cells infectedwith oncogene-carrying viruses therefore undergounregulated cell division—they form tumors.Proto-oncogenes can also undergo mutation to oncogenes without the intervention of a retrovirus;these cellular oncogenes also confer unregulatedgrowth on the cells in which they occur.result of truncation or some other mutation.
During a subsequent infection, when this viral oncogene is expressed in its host cell, the abnormal protein product interferes with normal regulation of cellgrowth, and the unregulated growth can result in a tumor. Oncogenescan also arise from proto-oncogenes without viral involvement. Chromosomal rearrangements, chemical agents, radiation, or other factorscan cause mutations in the genes that encode signal-transducing proteins. The resulting oncogenes express defective proteins and defectivesignaling, once again leading to tumor growth.Many viral oncogenes encode unregulated tyrosine kinase activities, and in some cases the oncogene product is nearly identical to anormal animal-cell receptor, but with the normal signal-binding sitedefective or missing. For example, the erbB oncogene product, a protein called ErbB, is essentially identical to the normal receptor forepidermal growth factor, except that ErbB lacks the domain that normally binds EGF (Fig.
22-37, p. 777). The erbB2 oncogene is commonlyassociated with adenocarcinomas (cancers) of the breast, stomach, andovary.Other signal-transducing proteins with oncogene analogs are theGTP-binding (G) proteins. One well-characterized oncogene, ras, encodes a protein with normal GTP binding but no GTPase activity.When the Ras protein (p. 682) is produced in an animal cell, it remainsalways in the activated form, regardless of the signals coming throughnormal receptors.
Again, the result is unregulated growth—cancer.Mutations in ras are associated with 30 to 50% of lung and colon carcinomas and over 90% of pancreatic carcinomas.The action of a group of compounds known as tumor promoterscan also be understood in the light of what we know of signal transduction. The best understood of these compounds, phorbol esters, are777Chapter 22 Integration and Hormonal Regulation of Mammalian Metabolismchemically synthesized compounds that are potent activators of protein kinase C.
They apparently mimic cellular diacylglycerol as secondmessengers (Fig. 22-32), but unlike naturally occurring diacylglycerols they are not rapidly metabolized. By permanently activating protein kinase C, these synthetic tumor promoters interfere with the normal regulation of cell growth and division.Protein Phosphorylation and DephosphorylationAre Central to Cellular ControlOne common denominator in signal transductions—whether they involve adenylate cyclase, a transmembrane receptor-tyrosine kinase,phospholipase C, or an ion channel—is the eventual regulation of theactivity of a protein kinase.
We have seen examples of kinases activated by cAMP, insulin, Ca2+/calmodulin, Ca2+/diacylglycerol, and byphosphorylation catalyzed by another protein kinase. The number ofknown protein kinases has grown remarkably since their discovery byEdwin G. Krebs and Edmond H. Fischer in 1959. Hundreds of differentprotein kinases, each with its own specific activator and its own specific protein target(s), may be present in eukaryotic cells. Althoughmany other types of covalent modifications are known to occur on proteins, it is clear that phosphorylations make up the vast majority ofknown regulatory modifications of proteins.The addition of a phosphate group to a Ser, Thr, or Tyr residueintroduces a bulky, highly charged group into a region that was onlymoderately polar.
When the modified side chain is located in a regionof the protein critical to its three-dimensional structure, phosphorylation can be expected to have dramatic effects on protein conformationand thus on the catalytic activity of the protein. As a result of evolution, the kinase-phosphorylated Ser, Thr, and/or Tyr residues of regulated proteins occur within common structural motifs (consensus sequences) that are recognized by their specific protein kinases (Table22-9).Table 22-9 Consensus sequences for protein kinasesProtein kinaseProtein kinase AConsensus sequence*-X-R-(R/K)-X-(S/T)-X-Protein kinase G-X-(R/K) 2 _ 3 -X-(S/T)-X-Protein kinase C-X-tR/KLs, Xo_2)-(S/T)-(Xo_2,Ca2+/calmodulin kinase IIPhosphorylase b kinaseInsulin receptor kinaseEGF receptor kinase-X-R-X-X-(S/T)-X-K-R-K-Q-I-(S/T)-V-R-T-R-D-I-Y-E-T-D-Y-Y-R-K-T-A-E-N-A-E-Y-L-R-V-A-PSource: Data from Kemp, B.E.
& Pearson, R.B. (1990) Protein kinase recognition sequence motifs. Trends Biochem. Sci. 15, 342-346; and Kennelly, P.J. & Krebs, E.G. (1991) Consensus sequences as substrate specificity determinants for protein kinases and protein phosphatases.J. Biol. Chem. 266, 15555-15558.* (S/T) and Y are the Ser (or Thr) and Tyr residues that are phosphorylated. X is a less essentialresidue; any of several amino acids may be at this position.
Essential residues are indicated bytheir one-letter abbreviations (see Table 5-1). The notation -(WKi_3, XQ_2)- means that at thisposition there are from one to three amino acids, which can be R (Arg) or K (Lys), as well aszero to two of any amino acids, in any sequence (the comma indicates that no sequence is implied).CHoOHMyristoylphorbol acetate(a phorbol ester)Extracellular spaceEGF-bindingdomainTyrosine kinasedomainEGF binding site Binding of EGFempty; tyrosineactivateskinase is inactive, tyrosine kinase.Normal EGF receptorTyrosinekinase isconstantlyactive.ErbB proteinFigure 22-37 The product of the erbB oncogene(the ErbB protein) is a truncated version of thenormal receptor for epidermal growth factor (EGF).Its intracellular domain has the structure normallyinduced by EGF binding, but the protein lacks theextracellular binding site for EGF.
Unregulated byEGF, ErbB continuously signals cell division.Part III Bioenergetics and MetabolismFigure 22—38 The enzyme glycogen synthase contains at least nine separate sites in five designatedregions susceptible to phosphorylation by one of thecellular protein kinases.
The activity of this enzymeis therefore capable of modulation in response to avariety of second messengers produced in responseto different extracellular signals. Thus regulation isa matter not of binary (on/off) switching but offinely tuned modulation of the activity over a widerange.GlycogensynthasemoleculeA BABC451I IA BCOO"KinasecAMP-dependentprotein kinasecGMP-dependentprotein kinasePhosphorylase b kinaseCa2+/calmodulin-dependentkinaseGlycogen synthasekinase 3Glycogen synthasekinase 4Casein kinase IICasein kinase IProtein kinase CGlycogen synthasesites phosphorylatedDegree ofsynthaseinactivation1A, IB, 2 , 41A, IB, 22+IB, 2+3A, 3B, 3C+ ++2+50At least 9 sites1A+ + 4- ++Not all cases of regulation by phosphorylation are as simple asthose we have described.
Some proteins have consensus sequences recognized by several different protein kinases, each of which can phosphorylate the protein and alter its enzymatic activity. For example,glycogen synthase is inactivated by cAMP-dependent phosphorylationof specific Ser residues, and is also modulated by at least four otherprotein kinases that phosphorylate four other sites in the protein (Fig.22-38). Some of the phosphorylations inhibit the enzyme more thanothers, and some combinations of phosphorylation are cumulative. Theresult of all of these regulations is the potential for extremely subtlemodulation of the activity of glycogen synthase, allowing very finelytuned responses to varying metabolic circumstances.The end effect of epinephrine's interaction with the /3-adrenergicreceptor is the phosphorylation of several cellular enzymes, includingglycogen synthase and glycogen phosphorylase.
To serve as an effectiveregulatory mechanism, this phosphorylation must be reversible, allowing the regulated enzymes to return to their prestimulus level whenthe hormonal signal stops. In muscle, for example, the enzyme phosphoprotein phosphatase-1 dephosphorylates glycogen phosphorylase,phosphorylase b kinase, and glycogen synthase (see Figs. 14-17, 1915), reversing the effects of cAMP on the activities of these enzymes.This enzyme (sometimes called phosphorylase a phosphatase, synthase phosphatase, or kinase phosphatase to indicate its substratespecificity) is regulated by another protein, phosphoprotein phosphatase inhibitor.
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