Structure of p73 DNA-binding domain tetramer modulates p73 transactivation

p73 is a 636 amino acid protein that acts as a dimer of dimers. The N-terminus is responsible for transcriptional activation and the C-terminus facilitates oligomerisation. The DNA binding domain is found in the centre of the protein and has a loop-sheet-helix motif. The protein also contains a coordinated zinc ion that is vital for both DNA binding and oligomerisation. The zinc atom is coordinated by 4 residues; Cys194, His197, Cys258 and Cys262 (see fig 1).

 Fig 1: shows the Zinc atom coordinated by the 4 residues, the 2 zinc atoms visible in the picture are from 2 different monomers of the protein. 

The DNA binding domain binds both the phosphate backbone and specific residues in the DNA response element; 3 residues contact the DNA bases of both the response element and the complementary chain via the major groove in DNA binding; these are Arg300, Cys297 and Lys138 (see fig 2). Of the residues that they bind, the cytosine in position 4 of the response element is the only conserved residue, this is because both it and its complementary guanine share H-bonds with Arg300, positions 2 and 3 can be either of the purines because Cys297 and Lys138 are flexible and so can accommodate the different hydrogen bonds formed by the 2 different bases.

Fig 2: shows Arg300, Cys297 and Lys138 binding bases via the major groove of the response element.

To stabilise the protein-DNA complex, 5 residues also contact the phosphate backbone; Lys138 and Ala296 approach from the major groove side, Ser261, Arg268 and Arg293 approach from the minor groove (see fig 3).

Fig 3: shows all contacts made between protein residues in the DBD of p73 and DNA. 

 

The structure of p73 bound and unbound to DNA is slightly different; the structure of the DNA binding domain of p73 without DNA bound had already been solved by a team at Alex Bullock Laboratory at Structural Genomics Consortium, Oxford University (paper not published, PDB ref: 2XWC). The only difference between the 2 structures was found in the loops involved in tetramerisation; 4 residues (Cys194, His197, Cys258 and Cys262) which were unordered before DNA binding are ordered after DNA binding (see fig 4). This helps demonstrate that p73 is a monomer in absence of DNA and conformational changes take place that allow it to dimerise when it binds DNA. 

 Fig 4: shows the 4 residues ordered, upon DNA binding.