A Lynch syndrome-associated mutation at a Bergerat ATP-binding fold destabilizes the structure of the DNA mismatch repair endonuclease MutL

Identification of a Catalytic Lysine Residue Conserved Among GHKL ATPases: MutL, GyrB, and MORC

DNA mismatch repair endonuclease MutL is a member of GHKL ATPase superfamily. Mutations of MutL homologs are causative of a hereditary cancer, Lynch syndrome. We characterized MutL homologs from human and a hyperthermophile, Aquifex aeolicus, (aqMutL) to reveal the catalytic mechanism for the ATPase activity. Although involvement of a basic residue had not been conceived in the catalytic mechanism, analysis of the pH dependence of the aqMutL ATPase activity revealed that the reaction is catalyzed by a residue with an alkaline pKa. Analyses of mutant aqMutLs showed that Lys79 is the catalytic residue, and the corresponding residues were confirmed to be critical for activities of human MutL homologs, on the basis of which a catalytic mechanism for MutL ATPase is proposed. These and other results described here would contribute to evaluating the pathogenicity of Lynch syndrome-associated missense mutations. Furthermore, it was confirmed that the catalytic lysine residue is conserved among DNA gyrases and microrchidia ATPases, other members of GHKL ATPases, indicating that the catalytic mechanism proposed here is applicable to these members of the superfamily.

An extensive ion-pair/hydrogen-bond network contributes to the thermostability of the MutL ATPase domain from Aquifex aeolicus

Proteins from hyperthermophiles often contain a large number of ionic interactions. Close examination of the previously determined crystal structure of the ATPase domain of MutL from a hyperthermophile, Aquifex aeolicus, revealed that the domain contains a continuous ion-pair/hydrogen-bond network consisting of 11 charged amino acid residues on a β-sheet. Mutations were introduced to disrupt the network, showing that the more extensively the network was disrupted, the greater the thermostability of the protein was decreased. Based on urea denaturation analysis, a thermodynamic parameter, energy for the conformational stability, was evaluated, which indicated that amino acid residues in the network contributed additively to the protein stability. A continuous network rather than a cluster of isolated interactions would pay less entropic penalty upon fixing the side chains to make the same number of ion pairs/hydrogen bonds, which might contribute more favorably to the structural formation of thermostable proteins.

Association of Hsp90 with p53 and Fizzy related homolog (Fzr) synchronizing Anaphase Promoting Complex (APC/C): An unexplored ally towards oncogenic pathway

The intricate molecular interactions leading to the oncogenic pathway are the consequence of cell cycle modification controlled by a bunch of cell cycle regulatory proteins. The tumor suppressor and cell cycle regulatory proteins work in coordination to maintain a healthy cellular environment. The integrity of this cellular protein pool is perpetuated by heat shock proteins/chaperones, which assist in proper protein folding during normal and cellular stress conditions. Among these versatile groups of chaperone proteins, Hsp90 is one of the significant ATP-dependent chaperones that aid in stabilizing many tumor suppressors and cell cycle regulator protein targets. Recently, studies have revealed that in cancerous cell lines, Hsp90 stabilizes mutant p53, 'the guardian of the genome.' Hsp90 also has a significant impact on Fzr, an essential regulator of the cell cycle having an important role in the developmental process of various organisms, including Drosophila, yeast, Caenorhabditis elegans, and plants. During cell cycle progression, p53 and Fzr coordinately regulate the Anaphase Promoting Complex (APC/C) from metaphase to anaphase transition up to cell cycle exit. APC/C mediates proper centrosome function in the dividing cell. The centrosome acts as the microtubule organizing center for the correct segregation of the sister chromatids to ensure perfect cell division. This review examines the structure of Hsp90 and its co-chaperones, which work in synergy to stabilize proteins such as p53 and Fizzy-related homolog (Fzr) to synchronize the Anaphase Promoting Complex (APC/C). Dysfunction of this process activates the oncogenic pathway leading to the development of cancer. Additionally, an overview of current drugs targeting Hsp90 at various phases of clinical trials has been included.