Directing proteins to nucleus by fusion to nuclear localization signal tags

Cytoplasmic-translocated Ku70 senses intracellular DNA and mediates interferon-lambda1 induction

We have previously identified that human Ku70, a nuclear protein, serves as a cytosolic DNA sensor. Upon transfection with DNA or infection with DNA virus, Ku70 translocates from the nucleus into the cytoplasm and then predominately induces interferon lambda1 (IFN-λ1) rather than IFN-alpha or IFN-beta, through a STING-dependent signalling pathway. However, a detailed mechanism for Ku70 cytoplasmic translocation and its correlation with IFN-λ1 induction have not been fully elucidated. Here, we observed that cytoplasmic translocation of Ku70 only occurred in DNA-triggered IFN-λ1-inducible cells. Additionally, infection by Herpes simplex virus type-1 (HSV-1), a DNA virus, induces cytoplasmic translocation of Ku70 and IFN-λ1 induction in a strain-dependent manner: the translocation and IFN-λ1 induction were detected upon infection by HSV-1 McKrae, but not MacIntyre, strain. A kinetic analysis indicated that cytoplasmic translocation of Ku70 was initiated right after DNA transfection and was peaked at 6 hr after DNA stimulation. Furthermore, treatment with leptomycin B, a nuclear export inhibitor, inhibited both Ku70 translocation and IFN-λ1 induction, suggesting that Ku70 translocation is an essential and early event for its cytosolic DNA sensing. We further confirmed that enhancing the acetylation status of the cells promotes Ku70's cytoplasmic accumulation, and therefore increases DNA-mediated IFN-λ1 induction. These findings provide insights into the molecular mechanism by which the versatile sensor detects pathogenic DNA in a localization-dependent manner.

Role of SUMO:SIM-mediated protein-protein interaction in non-homologous end joining

Although post-translational modifications by the small ubiquitin-like modifiers (SUMO) are known to be important in DNA damage response, it is unclear whether they have a role in double-strand break (DSB) repair by non-homologous end joining (NHEJ). Here, we analyzed various DSB repair pathways upon inhibition of SUMO-mediated protein-protein interactions using peptides that contain the SUMO-interaction motif (SIM) and discriminate between mono- and SUMO-chain modifications. The SIM peptides specifically inhibit NHEJ as shown by in vivo repair assays and radio-sensitivity of cell lines deficient in different DSB repair pathways. Furthermore, mono-SUMO, instead of SUMO-chain, modifications appear to be involved in NHEJ. Immunoprecipitation experiments also showed that the SIM peptide interacted with SUMOylated Ku70 after radiation. This study is the first to show an important role for SUMO:SIM-mediated protein-protein interactions in NHEJ, and provides a mechanistic basis for the role of SIM peptide in sensitizing genotoxic stress of cancer cells.

Involvement of protein synthesis and degradation in long-term potentiation of Schaffer collateral CA1 synapses

Expression of synaptic plasticity involves the translation of mRNA into protein and, probably, active protein degradation via the proteasome pathway. Here, we report on the rapid activation of synthesis and degradation of a probe protein with the induction of long-term potentiation (LTP) in the hippocampal Schaffer collateral CA1 pathway. The proteasome inhibitor MG132 significantly reduced the field EPSP slope potentiation and LTP maintenance without acutely affecting basal synaptic transmission. To visualize protein dynamics, CA1 pyramidal cells of hippocampal slices were transfected with Semliki Forest virus particles expressing a recombinant RNA. This RNA contained the coding sequence for a degradable green fluorescence protein with a nuclear localization signal (NLS-d1EGFP) followed by a 3'- untranslated region dendritic targeting sequence. NLS-d1EGFP fluorescence remained stable in the low-frequency test stimulation but increased with LTP induction in the cell body and in most dendritic compartments of CA1 neurons. Applying anisomycin, a protein synthesis inhibitor, caused NLS-d1EGFP levels to decline; a proteasome inhibitor MG132 reversed this effect. In the presence of anisomycin, LTP induction accelerated the degradation of NLS-d1EGFP. When both inhibitors were present, NLS-d1EGFP levels remained unaffected by LTP induction. Moreover, LTP-induced acceleration of NLS-d1EGFP synthesis was blocked by rapamycin, which is consistent with the involvement of dendritic mammalian target of rapamycin in LTP-triggered translational activity. Our results clearly demonstrate that LTP induction not only leads to a rapid increase in the rate of protein synthesis but also accelerates protein degradation via the proteasome system.

Binding of chromium(VI) to histones: implications for chromium(VI)-induced genotoxicity

The first evidence has been obtained for Cr(VI) (chromate) binding to isolated calf thymus (CT) histones under physiological conditions (pH 7.4, Cl(-) concentration 152 mM, 310 K). No significant Cr(VI) binding under the same conditions was observed for other extracellular and intracellular proteins, including albumin, apo-transferrin and G-actin, as well as for CT DNA. The mode of Cr(VI) binding to histones was studied by vibrational, electronic and X-ray absorption (X-ray absorption near-edge structure and X-ray absorption fine structure) spectroscopies and molecular mechanics calculations. A proposed binding mechanism includes electrostatic interactions of CrO(4) (2-) with protonated Lys and Arg residues of histones, as well as the formation of hydrogen bonds with the protein backbone. Similarly, Cr(VI) can bind to nuclear localization signals (typically, Lys- and Arg-rich fragments) of other nuclear proteins. Selective binding of Cr(VI) to newly synthesized nuclear proteins (including histones) in the cytoplasm is likely to be responsible for the active transport of Cr(VI) into the nuclei of living cells.