Featured Articles

p53 degradation: N-acetylglucosamine strikes back

Serine- or threonine-linked N-acetylglucosamine (GlcNAc) is an abundant modulator of nucleoplasmic protein function. In this role, GlcNAcylation can alternate with phosphorylation as both modifications target hydroxylated amino acid residues. GlcNAcylation of the tumor suppressor protein p53, which can carry many other post-translational modifications, has been shown to diminish its ubiquitination and proteosomal degradation. Yang and colleagues in Nature Cell Biology now single out one exact location of GlcNAcylation and reveal its interaction with a neighboring phosphorylation site.

Yang et al. used the breast cancer cell line MCF-7 as an in vivo system and treated cells with either streptozotocin (STZ), an inhibitor of the glycosidase that removes O-linked GlcNAc, or doxorubicin (Dox), a DNA-damaging agent known to stabilize p53. STZ administration heightened the level of GlcNAcylated p53 but did not increase the amount of p53 mRNA, suggesting that STZ does not activate p53 transcription. This finding indicates that GlcNAcylation of p53 diminished the rate of its proteosomal degradation, which was also confirmed by an observed reduction in ubiquitinated p53 in STZ-grown MCF-7 cells.

Phosphorylation at the N-terminal region of p53 attenuates its interaction with the E3 ubiquitin ligase family member MDM2, thereby diminishing ubiquitination and proteosomal degradation. N-terminal addition of phosphate groups was increased by Dox, but not altered by STZ. Thus, the reduced p53 ubiquitination after STZ administration was not due to increased N-terminal phosphorylation. However, phosphorylation of Thr 155, which is known to facilitate ubiquitination, was decreased after STZ treatment.

Using tandem mass spectrometric analysis, Yang et al. showed that the p53 fragment peptide 140-156 carried a GlcNAcylation position at Ser 149. Administration of STZ to cells expressing a Histidine-tagged p53 containing a serine to alanine mutation at residue 149 (His-p53 S149A) led to a decrease in p53 compared with cells expressing wild-type His-p53. Phosphorylation of Thr 155 was unaffected in His-p53 S149A cells, but drastically diminished in His-p53 wild-type cells.

Taken together, these results demonstrate that O-GlcNAcylation of p53 at Ser 149 stabilizes p53 by preventing ubiquitin-dependent proteolysis. The authors speculate that as Ser 149 and Thr 155 are localized on a flexible loop, the close distance between them may be enough to block Thr 155 phosphorylation and ubiquitin-dependent degradation of p53 after Ser 149 GlcNAcylation. This study encourages further work on the precise spatial correlations of other GlcNAcylation and phosphorylation sites of p53.

Author: Mirko von Elstermann