Ann Taylor p53 sandbox
p53 Tumor Suppressor Protein
p53, also known as TP53 (for Tumor Protein 53) was named "Molecule of the Year" by Science Magazine for its important role in both cell cycle regulation and apoptosis(). The name p53 reference to it apparent molecular mass. It runs as a 53 kDa molecule on SDS-PAGE. But based on calculations from its amino acid residues, p53's mass actually 43.7 kDa. This difference is due tothe high number of proline residues in the protein, which causes it to migrate slowly on SDS-PAGE.[1]
Human p53 is 393 amino acids long and has several domains:
1. The amino-terminus part (1-44) contains the transactivation domain, which is responsible for activating downstream target genes. 2. A proline-rich domain (58-101) mediates p53 response to DNA damage through apoptosis. 3. The DNA-binding domain (102-292) is a core domain which consists of a variety of structural motifs. It is the target of 90% of p53 mutations found in human cancers as a single mutation within this domain can cause a major conformational change. 4. The oligomerization domain (325-356) consists of a β-strand, which interacts with another p53 monomer to form a dimer, followed by an α-helix which mediates the dimerization of two p53 dimers to form a tetramer. 5. Three putative nuclear localization signals (NLS) have been identified in the C-terminus, through sequence similarity and mutagenesis. The most N-terminal NLS (NLSI), which consists of 3 consecutive Lysine residues to a basic core, is the most active and conserved domain. 6. Two putative nuclear export signals (NES) have been identified. The leucine-rich C-terminal NES, found within the oligomerization domain, is highly conserved and it has been suggested that oligomerization can result in masking of the NES, resulting in p53 nuclear retention.
In the active form, p53 acts as a tetramer. Flexible molecules are difficult to study by x-ray crystallography because they do not form orderly crystals. So, p53 has been studied in parts, by removing the flexible regions and solving structures of the pieces that form stable structures.() The figure at the right shows the cartoon representation of DNA binding domain that has been studied most rigorously.
|
p53 Pathway and mutation
In a normal cell p53 is inactivated by its negative regulatory mdm2 (hdm2 in humans) and it is found at low levels. When DNA damage sensed p53's level rises(). p53 binds to many regulatory sites in the genome and begins production of proteins that stop cell division until the demage is repaired, or if the damage is unrepairable, p53 initiates the process programmed cell death,also known as apoptosis, permanently removing the damaged cell.()
In most of the human cancer p53 mutations has been observed. Most of the p53 mutations that cause cancer are found in and around the DNA binding surface of the protein.The most common mutation changes
that interacts with DNA when mutated to another amino acid this interaction is lost. Other residues that are commonly mutated are arginine 175, 249, 273, 282 and glycine 245.
Surface charge of the DNA binding domain
The figure at the left shows the surface charge of the p53 DNA binding domain. It is rich in arginine amino acids to interact with DNA, thus this cause its surface positively charged. This domain recognizes specific regulatory sites on the DNA and flexible structure of p53 allow it to bind to many different variant of binding site allowing it to regulate transcription at many places in the genome.()
|
There is a Zn-binding motif on p53. The p53 Zn atom (shown in red) is coordinated by residues that are located on two loops, respectively. It is conceivable that the zinc plays a role of stabilizing two loops through coordination().