p53 N-terminal phosphorylation: a defining layer of complex regulation

Abstract

The p53 tumor suppressor is a critical component of the cellular response to stress. As it can inhibit cell growth, p53 is mutated or functionally inactivated in most tumors. A multitude of protein-protein interactions with transcriptional cofactors are central to p53-dependent responses. In its activated state, p53 is extensively modified in both the N- and C-terminal regions of the protein. These modifications, especially phosphorylation of serine and threonine residues in the N-terminal transactivation domain, affect p53 stability and activity by modulating the affinity of protein-protein interactions. Here, we review recent findings from in vitro and in vivo studies on the role of p53 N-terminal phosphorylation. These modifications can either positively or negatively affect p53 and add a second layer of complex regulation to the divergent interactions of the p53 transactivation domain.

Fig. 1.

Domain structure of p53 showing known sites of posttranslational modification. Documented sites of p53 posttranslational modification are shown; known modifying (black arrow) or unmodifying (red arrow) enzyme(s) are indicated above the modification. Circle: serine (yellow) or threonine (orange) phosphorylation; hexagon: ubiquitin or ubiquitin-like modification; square: acetylation; oval: N-acetylglucosamine; octagon: poly ADP-ribosylation; pentagon: arginine (gray) or lysine mono- (lavender) or di- (violet) methylation; star: nitration.

Fig. 2.

Phenotypes of p53 knock-in mice. (A) Sequence of human and murine p53, showing sites of mutation in p53 knock-in mouse models. (B) Summary of phenotypic observations for various knock-in mice.

Fig. 3.

Effect of phosphorylation on p53 complexes. (A) Sequence of p53 TAD showing regions of helical structure in the different complexes. The helical boundaries are taken from the description in the header of the Protein Data Bank structure file. The Φ-X-X-Φ-Φ motif is shown for TAD1 and TAD2, and the dominant modifications in each domain are marked. (B) Changes in the affinity of p53 TAD complexes for domains of CBP/p300. The general increase in affinity for modified forms of p53 is shown; changes are summarized from (60,62).

Fig. 4.

Hydrophobic interactions in p53 complexes. In each structure, p53 is shown as a blue ribbon and the binding partner is shown as gray surfaces. Hydrophobic residues from p53 are highlighted as magenta spheres. (A) Mdm2 (PDB: 1YCQ); (B) p300 Taz2 (PDB: 2K8F); (C) CBP NCBD (PDB: 2L14); (D) Tfb1 (PDB: 2GS0); (E) RPA70 (PDB: 2B3G) PDB, Protein Data Bank.

Fig. 5.

Models of phosphorylated p53 in complexes. The models shown are derived from the structures of the unmodified complexes with phosphate groups added on the specified residues. In each panel, p53 is shown as a blue ribbon and the binding partner is shown as a gray surface. The oxygen and phosphorus atoms of the phosphate group are in red and orange, respectively. Basic residues of each binding partner that could interact with phosphorylated sites on p53 are indicated. In (A), Glu69 of Mdm2, one of the residues mutated in the study by Brown et al. (69), is not indicated as it is obscured by Asp68 in this view.