Specific sequences from the carboxyl terminus of human p53 gene product form anti-parallel tetramers in solution

Human p53 is a tumor-suppressor gene product associated with control of the cell cycle and with growth suppression, and it is known to form homotetramers in solution. To investigate the relationship of structure to tetramerization, nine peptides corresponding to carboxyl-terminal sequences in human p53 were chemically synthesized, and their equilibrium associative properties were determined by analytical ultracentrifugation. Secondary structure, as determined by circular dichroism measurements, was correlated with oligomerization properties of each peptide. The sedimentation profiles of peptides 319-393 and 319-360 fit a two-state model of peptide monomers in equilibrium with peptide tetramers. Successive deletion of amino- and carboxyl-terminal residues from 319-360 reduced tetramer formation. Further, substitution of alanine for Leu-323, Tyr-327, and Leu-330 abolished tetramerization. Circular dichroism studies showed that peptide 319-351 had the highest alpha-helix content, while the other peptides that did not form tetramers had low helical structure. These studies define a minimal region and identify certain critical residues involved in tetramerization. Cross-linking studies between monomer units in the tetramer suggest that the helices adopt an anti-parallel arrangement. We propose that conformational shifts in the helical structure of the p53 tetramerization domain result in a repositioning of subunits relative to one another. This repositioning provides an explanation relating conformational changes at the carboxyl terminus with changes in sequence-specific DNA binding by the highly conserved central domain.