p53 Protein Family

The p53 protein family includes the p53, p63 and p73 proteins, which all have a high degree of homology. Overall, p53 family proteins bind the same response elements and activate different pathways. How the members of this family trigger different cellular responses while binding similar response elements remains an open question.

GENERAL STRUCTURE

Although the protein sizes can vary from 393 amino acids (for p53) to 636 amino  acids (for p73) , all p53 family proteins have a similar general structure (see here), made up of a N-terminal transactivation domain, a central immunoglobulin-like DNA binding domain (DBD), and a C terminus domain promoting oligomerization. 

 For the shorter p53 protein, the C terminal forms a 30 amino acid regulatory domain that binds DNA nonspecifically, whereas p73 and p63 have more than 200 extra amino acids. The most conserved domain is the DBD, with 58% sequence identity between p63 and p73.

DNA BINDING

Experiments have shown that p53, p63 and p73 all bind DNA and form oligomers in a similar manner. All p53 protein family members are monomers in the absence of DNA, then dimerize upon quarter-site binding.

Two dimers in close proximity then interact to form a tetramer. Although different proteins will yield slightly different tetramers, the overall structure of the oligomer remains very similar.

Recent Experiments have shown that p53 DBD binds DNA with an affinity 20 to 100 times greater than for p63 and p73. This significant difference is believed to be due to the structural differences of the tetramers, as only 50% of residues are conserved at tetramerization interfaces between p73 and p53.

The p53 dimer has a particular intermolecular salt bridge between Glu180 from the L2 loop from one DBD monomer with Arg 181 from the other monomer. In p73, Arg181 is replaced by Leu199 and no salt bridge can form, causing a smaller interaction surface between dimers and lower affinity for DNA (see Figures 12 and 13).




Figure 12: CLustalW comparison of p53 and p73 DNA sequences, focused on residues 132-350.

 






Figure 13: p53 and p73 Dimer-Dimer interface residues and interactions. The p53 dimer-dimer interface, on the left, has many residues involved in dimer-dimer interactions. The P73 dimer-dimer interface, on the right, does not have a salt bridge and less residues are involved in dimer-dimer interactions.



RESPONSE ELEMENTS

Although there is a large redundancy in the response elements the p53 proteins bind to, over 100 genes from the stress pathway regulated by p73 and p63 are not activated by p53. All three proteins have a conserved DNA recognition motif.

They recognize the same 5’-Pur1-Pur2-Pur3-Cyt4-Ade5/Thy5-Ade6/Thy6-Gua7-Pyr8-Pyr9-Pyr10-3’ consensus sequence and bind to the same quarter sites (see here). P53 proteins tend to bind to response elements with no or small spacer sequences.

TRANSCRIPTIONAL ACTIVITY

Experiments have shown that the members of the p53 protein family respond differently to spacer length. It has first been determined that longer spacer sequences decrease transcriptional activity of the p53 family proteins (see here). Spacer sequences are therefore believed to monitor p53 family proteins activity and to regulate transcription.

 It has also been found that transactivation drops more rapidly for p73 than for p53, probably because of the smaller tetramer interfaces, as previously discussed.