High-resolution structure of the oligomerization domain of p53 by multidimensional NMR

Crystal structure of the tetramerization domain of the p53 tumor suppressor at 1.7 angstroms

The p53 protein is a tetrameric transcription factor that plays a central role in the prevention of neoplastic transformation. Oligomerization appears to be essential for the tumor suppressing activity of p53 because oligomerization-deficient p53 mutants cannot suppress the growth of carcinoma cell lines. The crystal structure of the tetramerization domain of p53 (residues 325 to 356) was determined at 1.7 angstrom resolution and refined to a crystallographic R factor of 19.2 percent. The monomer, which consists of a beta strand and an alpha helix, associates with a second monomer across an antiparallel beta sheet and an antiparallel helix-helix interface to form a dimer. Two of these dimers associate across a second and distinct parallel helix-helix interface to form the tetramer.

Folding intermediates are involved in genetic diseases?

Recent experimental data show that some human genetic diseases are due to mutations in proteins which influence their trafficking and lead to retaining of proteins in the endoplasmic reticulum or their unproper processing. In this paper a hypothesis is proposed that these mutations are connected with an incomplete protein folding, blocking it at the stage of the kinetic molten globule or even earlier. If so, the specific drugs against these diseases may be ligands and other factors which facilitate the correct protein folding.

Expression of cell cycle regulatory proteins (p53, pRb) in the human female genital tract

PURPOSE

Recent studies have shown that proliferation and differentiation of various cell types is regulated by cell-cycle-related proteins, such as protein p53 and retinoblastoma protein pRb.

METHODS

Three monoclonal antibodies to p53 (PAb240, PAb421, and PAb1801) and 3H9 monoclonal antibody to pRb were utilized for localization of proteins by peroxidase immunohistochemistry in frozen tissue sections.

RESULTS

Nuclear and nucleolar p53 expression was detected in nondividing and relatively stable cells, e.g., oocytes in primordial follicles and granulosa lutein cells. On the other hand, strong cytoplasmic p53 expression was detected in proliferating and low differentiated epithelial cells of the ovarian surface epithelium, amnion, endocervix and ectocervix, indicating enhanced p53 synthesis. Not all three p53 antibodies reacted with each tissue, perhaps due to structural and conformational changes in the p53 molecule, accompanying p53 association with other proteins, e.g., tissue specific transcription factor interactions. pRb expression was usually restricted to the cell nuclei and nucleoli. However, glandular cells of the female reproductive tract showed cytoplasmic pRb expression in juxtaluminal (secretory) segments of cells, a feature not previously described in any cell type. p53 and pRb immunoreactivities declined with advanced differentiation of cells. No p53 or pRb was detected in placental syncytiotrophoblast or terminally differentiated squamous epithelial cells.

CONCLUSION

Our data indicate that large quantities of p53 are synthesized in cells leaving the cell cycle and entering differentiation. Except in glandular cells, the pRb expression is confined to the cell nuclei and nucleoli. A unique cytoplasmic expression of pRb in juxtaluminal segments of glandular cells suggests a role for pRb in human female fertility and conception.

Biochemical properties and biological effects of p53

Over the past year, insights have been made into the biochemistry and biological effects of p53. The high-resolution three-dimensional structure has been determined for the central core and carboxy-terminal domain of the protein, important p53 target genes (such as WAF1) have been identified, and insight has been gained into the relationship between p53-mediated growth arrest and apoptosis.

Giant leap for p53, small step for drug design

We review the findings of Cho et al. on the crystal structure of a p53 tumor suppressor-DNA complex. The core DNA binding domain of p53 folds into a structure termed a beta-sandwich, which organizes two loops and a loop-sheet-helix structure on one surface of p53 to interact with the consensus DNA recognition sequence of p53. These structures help to explain the functions of wild-type p53 and the effects of tumor-associated mutations on p53 DNA binding, transactivation and suppression of cellular proliferation.

Structures of protein complexes by multidimensional heteronuclear magnetic resonance spectroscopy

With the advent of multidimensional heteronuclear-edited and -filtered NMR experiments, the field of three-dimensional structure determination by NMR has again increased in scope, making it possible to move the technology beyond the approximately 10 kDa limit inherent to conventional two-dimensional NMR to systems up to potentially 35 to 40 kDa. This article outlines the basic strategies for solving three-dimensional structures of larger systems, in particular, protein complexes and multimeric proteins using three- and four-dimensional NMR spectroscopy, summarizes the key experiments, and illustrates the power of these methods using several examples of protein-DNA, protein-peptide complexes, and oligomeric proteins from the authors' laboratories.

p53 and Rb: their cellular roles

Within the past year considerable new insights have been gained into the roles the p53 and retinoblastoma tumour suppressors play in determining the fate of cells through their regulation of cell cycle progression, apoptosis and gene expression. Key advances have been achieved in the identification and characterization of functional domains and through functional knockout studies.

Solution structure of the tetrameric minimum transforming domain of p53

We report the solution structure of the minimum transforming domain (residues 303-366) of human p53 (p53tet) determined by multidimensional NMR spectroscopy. This domain contains a number of important functions associated with p53 activity including transformation, oligomerization, nuclear localization and a phosphorylation site for p34/cdc2 kinase. p53tet forms a symmetric dimer of dimers that is significantly different from a recent structure reported for a shorter construct of this domain. Phosphorylation of Ser 315 has only minor structural consequences, as this region of the protein is unstructured. Modelling based on the p53tet structure suggests possible modes of interaction between adjacent domains in full-length p53 as well as modes of interaction with DNA.

Forewarned is four-armed

A new solution structure of the oligomerization domain of p53 provides a framework for interpreting a wealth of biochemical data but casts doubt on details of an earlier structure.

Allosteric activation of latent p53 tetramers

BACKGROUND

The DNA-binding activity of p53 is essential to its function as a tumour suppressor. Point mutations that abolish this activity have been found to occur frequently in the p53 genes of human cancer cells. Wild-type p53 protein assembles into oligomers with latent DNA-binding activity that can be activated in vitro by phosphorylation of a carboxy-terminal regulatory region, catalyzed by protein kinase C or casein kinase II. We have investigated the mechanism underlying this post-translational regulation of p53. Specifically, we have asked the following questions. First, whether the carboxy-terminal regulatory site contributes to p53's ability to form tetramers. Second, whether the latent DNA-binding activity of p53 can be activated in vivo. And third, whether the activation of p53 is reversible.

RESULTS

Biophysical molecular-sizing analysis shows that both latent and activated forms of p53 are tetramers. Using a novel method, we have further established that p53 remains tetrameric when bound to DNA. We have also found that p53 can indeed be activated in vivo: p53 prepared from cells can be separated into activated and latent forms. Finally, we generated a monoclonal antibody specific for the casein kinase II target site in the carboxy-terminal regulatory region of p53, and used it to demonstrate the allosteric inhibition of in vitro and in vivo activated forms of p53.

CONCLUSIONS

p53 protein assembles naturally as a tetramer that can be converted between latent and activated forms by a concerted, allosteric transition. The highly purified, reconstituted system that we have developed, in which the DNA-binding activity of p53 can be reversibly regulated, should facilitate the discovery of agents that can modulate the DNA-binding activity of p53--particularly those that can activate mutant p53 proteins and that may have potential in the design of anti-cancer drugs.

p53: a cellular Achilles' heel revealed

Many human cancers result from the inactivation of p53, a protein central to DNA repair. Recent papers reporting the structures of two p53 domains help to rationalize the wealth of information about this protein.