DUX4 (Double Homeobox Protein 4) is a nuclear transcription factor encoded at each D4Z4 unit of a tandem-repeat array at human chromosome 4q35. DUX4 constitutes a major candidate pathogenic protein for facioscapulohumeral muscular dystrophy (FSHD), the third most common form of inherited myopathy. A low-level expression of DUX4 compromises cell differentiation in myoblasts and its overexpression induces apoptosis in cultured cells and living organisms. In this work we explore potential molecular determinants of DUX4 mediating nuclear import and cell toxicity. Deletion of the hypothetical monopartite nuclear localization sequences RRRR23, RRKR98 and RRAR148 (i.e. NLS1, NLS2 and NLS3, respectively) only partially delocalizes DUX4 from the cell nuclei. Nuclear entrance guided by NLS1, NLS2 and NLS3 does not follow the classical nuclear import pathway mediated by α/β importins. NLS and homeodomain mutants from DUX4 are dramatically less cell-toxic than the wild type molecule, independently of their subcellular localization. A triple ΔNLS1-2-3 deletion mutant is still partially localized in the nuclei, indicating that additional sequences in DUX4 contribute to nuclear import. Deletion of ≥111 amino acids from the C-terminal of DUX4, on a ΔNLS1-2-3 background, almost completely re-localizes DUX4 to the cytoplasm, indicating that the C-ter tail contributes to subcellular trafficking of DUX4. Also, C-terminal deletion mutants of DUX4 on a NLS wild type background are less toxic than wild type DUX4. Results reported here indicate that DUX4 possesses redundant mechanisms to assure nuclear entrance and that its various transcription-factor associated domains play an essential role in cell toxicity.

In this work, we used the ReaxFF force field to investigate the dynamics of different network structures of trimesic acid (TMA) molecules on graphene as a function of temperature. We considered the so-called honeycomb, filled honeycomb, flower, zigzag, and close-packed TMA motifs. The thermal stability was investigated using molecular dynamics simulations with the constant number of molecules, volume, and temperature and force-biased Monte Carlo calculations up to 650 K. Our simulations provide detailed atomistic insights into the intermolecular and molecule−substrate interactions responsible for the self-assembly or the breakage of the TMA networks at different temperatures. The dynamics of hydrogen bonding were followed by counting the number of hydrogen bonds as well as by analyzing OH radial distribution functions. According to the melting temperatures obtained, the honeycomb structure has a higher stability than the high-coverage zigzag and close-packed structures. Guest TMA molecules within the pores of the honeycomb motif further increase its thermal stability, thus showing the beneficial effect of host−guest interactions. The twisting and rotation of carboxylic groups with increasing temperature are responsible for the breakage of hydrogen bonds, which ultimately leads to the melting of the networks. Partial TMA desorption observed at the onset of network disordering was attributed to the intermolecular vibrational energy transfer between the molecules. For the high-coverage close-packed network and for an island of TMA molecules with a close-packed structure, we observed a phase transition to the honeycomb structure as a consequence of the stronger dimeric −COOH bonding of the latter. The energetics of the formation of the different networks from TMA molecules in the gas phase was also investigated. Intermolecular interactions and TMA−graphene interactions have similar magnitudes. The stability of the different networks cannot be fully understood only based on energetic considerations, and in the case of the dense close-packed structure, MD simulations show how it is rapidly destabilized.

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Escherichia coli MutS, MutL and MutH proteins act sequentially in the MMRS (mismatch repair system). MutH directs the repair system to the newly synthesized strand due to its transient lack of Dam (DNA-adenine methylase) methylation. Although Pseudomonas aeruginosa does not have the corresponding E. coli MutH and Dam homologues, and consequently the MMRS seems to work differently, we show that the mutL gene from P. aeruginosa is capable of complementing a MutL-deficient strain of E. coli. MutL from P. aeruginosa has conserved 21 out of the 22 amino acids known to affect functioning of E. coli MutL. We showed, using protein affinity chromatography, that the C-terminal regions of P. aeruginosa and E. coli MutL are capable of specifically interacting with E. coli MutH and retaining the E. coli MutH. Although, the amino acid sequences of the C-terminal regions of these two proteins are only 18% identical, they are 88% identical in the predicted secondary structure. Finally, by analysing (E. coli-P. aeruginosa) chimaeric MutL proteins, we show that the N-terminal regions of E. coli and P. aeruginosa MutL proteins function similarly, in vivo and in vitro. These new findings support the hypothesis that a large surface, rather than a single amino acid, constitutes the MutL surface for interaction with MutH, and that the N- and C-terminal regions of MutL are involved in such interactions.