Passage
The tumor suppressor p53 is a homotetrameric protein with an N-terminal transactivation domain (TAD) , a proline-rich domain, a DNA-binding domain (DBD) , a tetramerization domain (TD) , and a C-terminal regulatory domain (REG) on each subunit (Figure 1) .
Figure 1 Organization of the p53 domains (1-393 amino acids) Human double minute 2 (Hdm2) is a lysine-specific ubiquitin protein ligase that can change the half-life of some proteins, including p53, by signaling for their proteolytic cleavage. Under normal circumstances, Hdm2 binds p53 at the AD1 region within the TAD and attaches ubiquitin to a lysine residue. However, errors in DNA replication induce the phosphorylation of residue T18 on p53, which inhibits p53's association with Hdm2. The overexpression of Hdm2 has been correlated with the downregulation of tumor suppressor p53 in certain types of cancers. Changes in the NMR chemical shift (ppm) of amino acids in the AD1 region shown in Figure 2 provide evidence for the formation of a p53-Hdm2 complex.
Figure 2 ¹H NMR spectrum of the AD1 region in ¹⁵N-labeled p53 during G₀ and G₂ phases of the cell cycleHdm2 also binds to Nedd8 ultimate buster 1 (NUB1) , which enhances signaling for the degradation of Nedd8 proteins. Preliminary data suggest that Hdm2 interacts differently with wild-type NUB1 (WTNUB1) than with mutant K159RNUB1. To assess the effect of Hdm2 on NUB1 and p53, the half-lives of p53 and Nedd8 protein variants were compared in cells that overexpress Hdm2 (Figure 3) .
Figure 3 Half-lives of p53, phosphorylated p53 (p53-PO₄) , and Nedd-8 proteins in cells with wild-type or mutant NUB1 over time
Adapted from Ferreon JC, Lee CW, Arai M, Martinez-yamout MA, Dyson HJ, Wright PE. Cooperative regulation of p53 by modulation of ternary complex formation with CBP/p300 and HDM2. Proc Natl Acad Sci USA. 2009;106(16) :6591-6.
-P53 displays positive cooperativity when it binds to promoters on DNA. Which statement regarding p53 structure and function accurately explains this observation?

Question

Passage
The tumor suppressor p53 is a homotetrameric protein with an N-terminal transactivation domain (TAD) , a proline-rich domain, a DNA-binding domain (DBD) , a tetramerization domain (TD) , and a C-terminal regulatory domain (REG) on each subunit (Figure 1) .

Figure 1 Organization of the p53 domains (1-393 amino acids) Human double minute 2 (Hdm2) is a lysine-specific ubiquitin protein ligase that can change the half-life of some proteins, including p53, by signaling for their proteolytic cleavage. Under normal circumstances, Hdm2 binds p53 at the AD1 region within the TAD and attaches ubiquitin to a lysine residue. However, errors in DNA replication induce the phosphorylation of residue T18 on p53, which inhibits p53's association with Hdm2. The overexpression of Hdm2 has been correlated with the downregulation of tumor suppressor p53 in certain types of cancers. Changes in the NMR chemical shift (ppm) of amino acids in the AD1 region shown in Figure 2 provide evidence for the formation of a p53-Hdm2 complex.

Figure 2 ¹H NMR spectrum of the AD1 region in ¹⁵N-labeled p53 during G₀ and G₂ phases of the cell cycleHdm2 also binds to Nedd8 ultimate buster 1 (NUB1) , which enhances signaling for the degradation of Nedd8 proteins. Preliminary data suggest that Hdm2 interacts differently with wild-type NUB1 (WTNUB1) than with mutant K159RNUB1. To assess the effect of Hdm2 on NUB1 and p53, the half-lives of p53 and Nedd8 protein variants were compared in cells that overexpress Hdm2 (Figure 3) .

Figure 3 Half-lives of p53, phosphorylated p53 (p53-PO₄) , and Nedd-8 proteins in cells with wild-type or mutant NUB1 over time
Adapted from Ferreon JC, Lee CW, Arai M, Martinez-yamout MA, Dyson HJ, Wright PE. Cooperative regulation of p53 by modulation of ternary complex formation with CBP/p300 and HDM2. Proc Natl Acad Sci USA. 2009;106(16) :6591-6.
-P53 displays positive cooperativity when it binds to promoters on DNA. Which statement regarding p53 structure and function accurately explains this observation?

Quizplus · Accepted Answer

11ecba2d_0e00_a32f_a3bf_2158b949fb0b_MD0008_00 Proteins that change their shape or conformation when a substrate or ligand binds are referred to as allosteric proteins. Some proteins have quaternary structure, meaning they are composed of multiple polypeptide chains known as subunits, which allows for a special kind of allosteric behavior known as cooperativity. In cooperative proteins, the binding of one subunit induces a conformational change in adjacent subunits, which results in a new protein shape with a higher (R-state) or lower (T-state) affinity for the substrate or ligand. Cooperativity can be positive or negative: proteins with subunits that increase in affinity when other subunits are bound are said to exhibit positive cooperativity whereas those that lose affinity exhibit negative cooperativity. Proteins that do not change affinity are noncooperative. p53 is a homotetrameric (four identical subunits) protein that displays positive cooperativity for DNA. Therefore, the binding affinity of the DBD in each subunit will increase after one subunit has bound to the DNA. (Choice A) If the affinity of each subunit were independent of the binding states of other subunits, p53 would be noncooperative. (Choice B) Post-translational modifications such as phosphorylation can induce conformational changes but are not involved in cooperativity because phosphorylation is a covalent attachment, not a binding event. (Choice D) A decrease in binding affinity of the DBD of one subunit when another subunit binds would be an example of negative cooperativity. Educational objective: Cooperativity refers to the conformational changes in one subunit of a protein that increase or decrease the binding affinity of the other subunits. Ligand/substrate binding that increases the binding affinity of other subunits is referred to as positive cooperativity, and binding that decreases affinity is negative cooperativity.

Passage the Tumor Suppressor P53 Is a Homotetrameric Protein with an an N-Terminal