Consequences of point mutations in melanoma-associated antigen 4 (MAGE-A4) protein: Insights from structural and biophysical studies.

The Melanoma-Associated Antigen A4 (MAGE-A4) protein is a target for cancer therapy. The function of this protein is not well understood. We report the first comprehensive study on key cancer-associated MAGE-A4 mutations and provide analysis on the consequences of these mutations on the structure, folding and stability of the protein. Based on Nuclear Magnetic Resonance and Circular Dichroism, these mutations had no significant effects on the structure and the folding of the protein. Some mutations affected the thermal stability of the protein remarkably. Native mass spectrometry of wild-type MAGE-A4 showed a broad charge state distribution suggestive of a structurally dynamic protein. Significant intensity was found in relatively low charge states, indicative of a predominantly globular form and some population in more extended states. The latter is supported by Ion Mobility measurements. The MAGE-A4 mutants exhibited similar features. These novel molecular insights shed further light on better understanding of these proteins, which are implicated in a wide range of human cancers.

Insights into the conformations of three structurally diverse proteins: cytochrome c, p53, and MDM2, provided by variable-temperature ion mobility mass spectrometry.

Thermally induced conformational transitions of three proteins of increasing intrinsic disorder-cytochrome c, the tumor suppressor protein p53 DNA binding domain (p53 DBD), and the N-terminus of the oncoprotein murine double minute 2 (NT-MDM2)-have been studied by native mass spectrometry and variable-temperature drift time ion mobility mass spectrometry (VT-DT-IM-MS). Ion mobility measurements were carried out at temperatures ranging from 200 to 571 K. Multiple conformations are observable over several charge states for all three monomeric proteins, and for cytochrome c, dimers of significant intensity are also observed. Cytochrome c [M + 5H](5+) ions present in one conformer of CCS ∼1200 Å(2), undergoing compaction in line with the reported Tmelt = 360.15 K before slight unfolding at 571 K. The more extended [M + 7H](7+) cytochrome c monomer presents as two conformers undergoing similar compaction and structural rearrangements, prior to thermally induced unfolding. The [D + 11H](11+) dimer presents as two conformers, which undergo slight structural compaction or annealing before dissociation. p53 DBD follows a trend of structural collapse before an increase in the observed collision cross section (CCS), akin to that observed for cytochrome c but proceeding more smoothly. At 300 K, the monomeric charge states present in two conformational families, which compact to one conformer of CCS ∼1750 Å(2) at 365 K, in line with the low solution Tmelt = 315-317 K. The protein then extends to produce either a broad unresolved CCS distribution or, for z > 9, two conformers. NT-MDM2 exhibits a greater number of structural rearrangements, displaying charge-state-dependent unfolding pathways. DT-IM-MS experiments at 200 K resolve multiple conformers. Low charge state species of NT-MDM2 present as a single compact conformational family centered on CCS ∼1250 Å(2) at 300 K. This undergoes conformational tightening in line with the solution Tmelt = 348 K before unfolding at the highest temperatures. The more extended charge states present in two or more conformers at room temperature, undergoing thermally induced unfolding before significant structural collapse or annealing at high temperatures. Variable-temperature IM-MS is here shown to be an exciting approach to discern protein unfolding pathways for conformationally diverse proteins.

Structure and stability insights into tumour suppressor p53 evolutionary related proteins.

The p53 family of genes and their protein products, namely, p53, p63 and p73, have over one billion years of evolutionary history. Advances in computational biology and genomics are enabling studies of the complexities of the molecular evolution of p53 protein family to decipher the underpinnings of key biological conditions spanning from cancer through to various metabolic and developmental disorders and facilitate the design of personalised medicines. However, a complete understanding of the inherent nature of the thermodynamic and structural stability of the p53 protein family is still lacking. This is due, to a degree, to the lack of comprehensive structural information for a large number of homologous proteins and to an incomplete knowledge of the intrinsic factors responsible for their stability and how these might influence function. Here we investigate the thermal stability, secondary structure and folding properties of the DNA-binding domains (DBDs) of a range of proteins from the p53 family using biophysical methods. While the N- and the C-terminal domains of the p53 family show sequence diversity and are normally targets for post-translational modifications and alternative splicing, the central DBD is highly conserved. Together with data obtained from Molecular Dynamics simulations in solution and with structure based homology modelling, our results provide further insights into the molecular properties of evolutionary related p53 proteins. We identify some marked structural differences within the p53 family, which could account for the divergence in biological functions as well as the subtleties manifested in the oligomerization properties of this family.

Mutants of the tumour suppressor p53 L1 loop as second-site suppressors for restoring DNA binding to oncogenic p53 mutations: structural and biochemical insights.

To assess the potential of mutations from the L1 loop of the tumour suppressor p53 as second-site suppressors, the effect of H115N and S116M on the p53 'hot spot' mutations has been investigated using the double-mutant approach. The effects of these two mutants on the p53 hot spots in terms of thermal stability and DNA binding were evaluated. The results show that: (i) the p53 mutants H115N and S116M are thermally more stable than wild-type p53; (ii) H115N but not S116M is capable of rescuing the DNA binding of one of the most frequent p53 mutants in cancer, R248Q, as shown by binding of R248Q/H115N to gadd45 (the promoter of a gene involved in cell-cycle arrest); (iii) the double mutant R248Q/H115N is more stable than wild-type p53; (iv) the effect of H115N as a second-site suppressor to restore DNA-binding activity is specific to R248Q, but not to R248W; (v) molecular-dynamics simulations indicate that R248Q/H115N has a conformation similar to wild-type p53, which is distinct from that of R248Q. These findings could be exploited in designing strategies for cancer therapy to identify molecules that could mimic the effect of H115N in restoring function to oncogenic p53 mutants.

Molecular interactions of ASPP1 and ASPP2 with the p53 protein family and the apoptotic promoters PUMA and Bax.

The apoptosis stimulating p53 proteins, ASPP1 and ASPP2, are the first two common activators of the p53 protein family that selectively enable the latter to regulate specific apoptotic target genes, which facilitates yes yet unknown mechanisms for discrimination between cell cycle arrest and apoptosis. To better understand the interplay between ASPP- and p53-family of proteins we investigated the molecular interactions between them using biochemical methods and structure-based homology modelling. The data demonstrate that: (i) the binding of ASPP1 and ASPP2 to p53, p63 and p73 is direct; (ii) the C-termini of ASPP1 and ASPP2 interact with the DNA-binding domains of p53 protein family with dissociation constants, K(d), in the lower micro-molar range; (iii) the stoichiometry of binding is 1:1; (iv) the DNA-binding domains of p53 family members are sufficient for these protein-protein interactions; (v) EMSA titrations revealed that while tri-complex formation between ASPPs, p53 family of proteins and PUMA/Bax is mutually exclusive, ASPP2 (but not ASPP1) formed a complex with PUMA (but not Bax) and displaced p53 and p73. The structure-based homology modelling revealed subtle differences between ASPP2 and ASPP1 and together with the experimental data provide novel mechanistic insights.

The p73 DNA binding domain displays enhanced stability relative to its homologue, the tumor suppressor p53, and exhibits cooperative DNA binding.

The p53 protein family is involved in the control of an intricate network of genes implicated in cell cycle, through to germ line integrity and development. Although the role of p53 is well-established, the intrinsic nature of its homologue p73 has yet to be fully elucidated. Here, the biochemical characterization and homology-based modeling of the p73 protein is presented and the implications for its function(s) examined. The DNA binding domains (DBDs) of p53, p63, and p73 bind to the specific target site of a 30-mer gadd45 dsDNA, as tested by EMSA. The monomeric DBDs bind cooperatively forming tetrameric complexes. However, a larger construct consisting of p73 DBD plus TET domain (p73 CT) and the corresponding p53 DBD plus TET domain (p53 CT) bind gadd45 differently than the respective DBDs. Significantly, p73 DBD exhibited enhanced thermodynamic stability relative to the p53 DBD but not compared to p63 DBD as shown by DSC, CD, and equilibrium unfolding. The p73 CT is less stable than p73 DBD. The modeling data show distinct electrostatic surfaces of p73 and p53 dimers when bound to DNA. Specifically, the p73 surface is less complementary for DNA binding, which may account for the differences in affinity and specificity for p53 REs. These stability and DNA binding data for p73 in vitro enhance and complement our understanding of the role of the p73 protein in vivo and could be exploited in designing strategies for cancer therapy in places where p53 is mutated.

A peptide that binds and stabilizes p53 core domain: chaperone strategy for rescue of oncogenic mutants.

Conformationally compromised oncogenic mutants of the tumor suppressor protein p53 can, in principle, be rescued by small molecules that bind the native, but not the denatured state. We describe a strategy for the rational search for such molecules. A nine-residue peptide, CDB3, which was derived from a p53 binding protein, binds to p53 core domain and stabilizes it in vitro. NMR studies showed that CDB3 bound to p53 at the edge of the DNA binding site, partly overlapping it. The fluorescein-labeled peptide, FL-CDB3, binds wild-type p53 core domain with a dissociation constant of 0.5 microM, and raises the apparent melting temperatures of wild-type and a representative oncogenic mutant, R249S core domain. gadd45 DNA competes with CDB3 and displaces it from its binding site. But this competition does not preclude CDB3 from being a lead compound. CDB3 may act as a "chaperone" that maintains existing or newly synthesized destabilized p53 mutants in a native conformation and then allows transfer to specific DNA, which binds more tightly. Indeed, CDB3 restored specific DNA binding activity to a highly destabilized mutant I195T to close to that of wild-type level.