The retinoblastoma protein (RB) suppresses human cell division byarresting cells in the G₁ phase of the cell cycle and preventingprogression to the next phase. It accomplishes this task by bindingto another protein, E2F, a transcription factor needed for furtherprogression through the cell cycle. Normal progression through thecell cycle is accomplished when cyclin-dependent kinases (CDKs)phosphorylate RB, preventing its binding to E2F.Many viruses can induce abnormal exit from G, using viral proteinsthat bind to RB at a motif at the N-terminal called LXCXE. Anexample is the E7 papilloma protein, which causes the excessiveproliferation of cells in warts.The site at which LXCXE proteins bind is called the pocket domainand is highly conserved on RB and related proteins in plants andanimals. The configuration of the pocket domain is well established.Mutant experimental RB proteins are available with alterations in theconserved amino acids of the pocket domain.A simple explanation for the ability of E7 to inactivate RB would bethat both E7 and E2F bind to the pocket domain. To test this theory,wild-type (the naturally occurring or nonmutant variation) and pocketdomain mutant RB were expressed in an RB-deficient cell line. Figure1 shows the change in G, content of a population of cellstransfected with wild-type or with three RB mutants (RB6, RB9, andRB10), or cotransfected with wild-type or the three RB mutants andE7. Figure 2 shows the percentage of G, cells following transfectionwith wild-type or mutant RB, and cotransfection of wild-type ormutant RB with the CDK4 kinase, which adds phosphate groups toRB.Change in percentage ofcells in G₁ phase120T100+80+60+40-20-0Percentage of cells in G₁ phaseWTRB6RB9Figure 1 Change in percentage of cells in G, after transfection withRB and after transfection with RB and E7. Note that a high number ofcells in G, suggests that arrest has occurred. Change aftertransfection with wild-type RB is arbitrarily set at 100% in thisanalysis. (WT = wild-type; other abbreviations designate the threedifferent mutant RB proteins.)100-90-80-70-60-50-40-30-20-10-LLLLRB6RB9 RB100WTFigure 2 Percentage of cells in G, after transfection with RB andafter transfection with RB and CDK4 (WT = wild-type; otherabbreviations designate the three different mutant RB proteins.)If the RB mutants cannot bind to E2F, then:RB10O A. cell division arrest will not occur.O B. E7 will not bind to these mutants.O C. CDK4 cannot phosphorylate these mutants.OD. E2F will bind to E7.Comparing the bars for RB transfection with those of RB and CDK4transfection in Figure 2 shows that:RBORBO A. mutant RB is better at arresting cells in G₁ than wild-type RB.O B. CDK4 transfection causes more mutant RB to bephosphorylated.O c. more cells are G, arrested in CDK4-transfected cells.O D. CDK4 transfection results in fewer cells being G₁ arrested.RBORB + (O A. vital to an organism's survival.OB. chemically incapable of mutation.O c. stored in vesicles for later secretion.O D. stored in cell compartments where they are unlikely to besecreted.The LXCXE motif is described as being "highly conserved." Certain partsof the genome, such as that encoding the LXCXE motif, are highlyconserved because they are:These experiments suggest that the mutant RB proteins do not:O A. bind to E7.O B. bind to E2F.OC. arrest cells in G₁.OD. permit exit from G₁.

Question

The retinoblastoma protein (RB) suppresses human cell division byarresting cells in the G₁ phase of the cell cycle and preventingprogression to the next phase. It accomplishes this task by bindingto another protein, E2F, a transcription factor needed for furtherprogression through the cell cycle. Normal progression through thecell cycle is accomplished when cyclin-dependent kinases (CDKs)phosphorylate RB, preventing its binding to E2F.Many viruses can induce abnormal exit from G, using viral proteinsthat bind to RB at a motif at the N-terminal called LXCXE. Anexample is the E7 papilloma protein, which causes the excessiveproliferation of cells in warts.The site at which LXCXE proteins bind is called the pocket domainand is highly conserved on RB and related proteins in plants andanimals. The configuration of the pocket domain is well established.Mutant experimental RB proteins are available with alterations in theconserved amino acids of the pocket domain.A simple explanation for the ability of E7 to inactivate RB would bethat both E7 and E2F bind to the pocket domain. To test this theory,wild-type (the naturally occurring or nonmutant variation) and pocketdomain mutant RB were expressed in an RB-deficient cell line. Figure1 shows the change in G, content of a population of cellstransfected with wild-type or with three RB mutants (RB6, RB9, andRB10), or cotransfected with wild-type or the three RB mutants andE7. Figure 2 shows the percentage of G, cells following transfectionwith wild-type or mutant RB, and cotransfection of wild-type ormutant RB with the CDK4 kinase, which adds phosphate groups toRB.Change in percentage ofcells in G₁ phase120T100+80+60+40-20-0Percentage of cells in G₁ phaseWTRB6RB9Figure 1 Change in percentage of cells in G, after transfection withRB and after transfection with RB and E7. Note that a high number ofcells in G, suggests that arrest has occurred. Change aftertransfection with wild-type RB is arbitrarily set at 100% in thisanalysis. (WT = wild-type; other abbreviations designate the threedifferent mutant RB proteins.)100-90-80-70-60-50-40-30-20-10-LLLLRB6RB9 RB100WTFigure 2 Percentage of cells in G, after transfection with RB andafter transfection with RB and CDK4 (WT = wild-type; otherabbreviations designate the three different mutant RB proteins.)If the RB mutants cannot bind to E2F, then:RB10O A. cell division arrest will not occur.O B. E7 will not bind to these mutants.O C. CDK4 cannot phosphorylate these mutants.OD. E2F will bind to E7.Comparing the bars for RB transfection with those of RB and CDK4transfection in Figure 2 shows that:RBORBO A. mutant RB is better at arresting cells in G₁ than wild-type RB.O B. CDK4 transfection causes more mutant RB to bephosphorylated.O c. more cells are G, arrested in CDK4-transfected cells.O D. CDK4 transfection results in fewer cells being G₁ arrested.RBORB + (O A. vital to an organism's survival.OB. chemically incapable of mutation.O c. stored in vesicles for later secretion.O D. stored in cell compartments where they are unlikely to besecreted.The LXCXE motif is described as being &#34;highly conserved.&#34; Certain partsof the genome, such as that encoding the LXCXE motif, are highlyconserved because they are:These experiments suggest that the mutant RB proteins do not:O A. bind to E7.O B. bind to E2F.OC. arrest cells in G₁.OD. permit exit from G₁.

Accepted Answer

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If the RB mutants cannot bind to E2F, then: O A. cell division arrest will not occur. O B. E7 will not bind to these mutants. O C. CDK4 cannot phosphorylate these mutants. O D. E2F will bind to E7.