G.T. Kim

Leucyl-tRNA synthetase is an intracellular leucine sensor for the mTORC1-signaling pathway.

Amino acids are required for activation of the mammalian target of rapamycin (mTOR) kinase, which regulates protein translation, cell size, and autophagy. However, the amino acid sensor that directly couples intracellular amino acid-mediated signaling to mTORC1 is unknown. Here we show that leucyl-tRNA synthetase (LRS) plays a critical role in amino acid-induced mTORC1 activation by sensing intracellular leucine concentration and initiating molecular events leading to mTORC1 activation. Mutation of LRS amino acid residues important for leucine binding renders the mTORC1 pathway insensitive to intracellular levels of amino acids. We show that LRS directly binds to Rag GTPase, the mediator of amino acid signaling to mTORC1, in an amino acid-dependent manner and functions as a GTPase-activating protein (GAP) for Rag GTPase to activate mTORC1. This work demonstrates that LRS is a key mediator for amino acid signaling to mTORC1.

Field-effect transistor made of individual V2O5 nanofibers

A field-effect transistor (FET) with a channel length of ∼100 nm was constructed from a small number of individual V2O5 fibers of the cross section 1.5 nm×10 nm. At low temperature, the conductance increases as the gate voltage is changed from negative to positive values, characteristic of a FET with n-type enhancement mode. The carrier mobility, estimated from the low-field regime, is found to increase from 7.7×10−5 cm2/V s at T=131 K to 9.6×10−3 cm2/V s at T=192 K with an activation energy of Ea=0.18 eV. The nonohmic current/voltage dependence at high electric fields was analyzed in the frame of small polaron hopping conduction, yielding a nearest-neighbor hopping distance of ∼4 nm.

Nanotransport in polyacetylene single fiber: Toward the intrinsic properties

The electrical transport properties of iodine doped polyacetylene (PA) nanofibers were measured as function of temperature in micron and sub-micron scale. Polyacetylenc fiber network measured in micron scale shows weaker temperature dependence of resistivity and smaller negative magnetoresistance (MR) at T=1.5K compared to those of hulk PA film. The reaction of Au electrodes with dopant became serious in submicron experiment, so that stripes of Pt electrodes with 100nm separation were patterned on top of the SiO 2 substrate to prevent the reaction. Non-ohmic I-V charactersitics are observed in PA nanofiber. The gate dependence shows the charge carrier to be hole with mobility μ FET ∼ 4.4×10 -5 cm 2 /Vs at 233K. The non-ohmic I-V dependence at high electric fields could be originated from the soliton tunneling conduction in PA nanofiber.

Downregulation of lamin A by tumor suppressor AIMP3⁄p18 leads to a progeroid phenotype in mice

Although AIMP3/p18 is normally associated with the macromolecular tRNA synthetase complex, recent reports have revealed a new role of AIMP3 in tumor suppression. In this study, we generated a transgenic mouse that overexpresses AIMP3 and characterized the associated phenotype in vivo and in vitro. Surprisingly, the AIMP3 transgenic mouse exhibited a progeroid phenotype, and the cells that overexpressed AIMP3 showed accelerated senescence and defects in nuclear morphology. We found that overexpression of AIMP3 resulted in proteasome‐dependent degradation of mature lamin A, but not of lamin C, prelamin A, or progerin. The resulting imbalance in the protein levels of lamin A isoforms, namely altered stoichiometry of prelamin A and progerin to lamin A, appeared to be responsible for a phenotype that resembled progeria. An increase in the level of endogenous AIMP3 has been observed in aged human tissues and cells. The findings in this report suggest that AIMP3 is a specific regulator of mature lamin A and imply that enhanced expression of AIMP3 might be a factor driving cellular and/or organismal aging.

Magneto thermoelectric power of the doped polyacetylene

Abstract Thermoelectric power (TEP) and electrical conductivity of the metallic polyacetylene (PA) doped with AuCl 3 and I 2 are measured under high magnetic field up to 17 Tesla. We have observed the following two remarkable behaviors in the TEP results; (1) the temperature dependence of TEP for the iodine and metal-halide doped polyacetylene show clear difference at T

Simple technique for the simultaneous measurements of the four-probe resistivity and the thermoelectric power

We propose simple configurations to measure the thermoelectric power and the four-probe resistivity simultaneously for different types of samples.

Toll-like receptor 4-mediated c-Jun N-terminal kinase activation induces gp96 cell surface expression via AIMP1 phosphorylation.

The presentation of the endoplasmic reticulum resident chaperone protein, gp96 on the cell surface have been considered as a phenomenon of the immunogenic process activation. Previously, we showed aminoacyl-tRNA synthetase-interacting multifunctional protein 1 (AIMP1) can form a molecular complex with gp96, regulate the ER retention of gp96 through KDEL receptor, and suppress its cell surface expression. However, the physiological conditions that modulate AIMP1-gp96 interaction and cell surface expression of gp96 are not known. In this study, we investigated the process that which can modulate dissociation of AIMP1 and gp96 by using Toll-like receptor (TLR) activation. MyD88 pathway by LPS-mediated TLR4 activation induced the cell surface presentation of gp96 through c-Jun N-terminal kinase (JNK). AIMP1 was phosphorylated by JNK upon LPS stimulation and gp96 was dissociated from phosphorylated AIMP1. We further demonstrated that serine-140 residue of AIMP1 was phosphorylated by JNK and alanine mutation of serine-140 suppressed LPS-induced cell surface expression of gp96. Altogether, these results suggest that AIMP1 is phosphorylated by JNK through TLR-MyD88 pathway and lose the regulatory activity for ER retention of gp96, resulting in the increase of cell surface expression of gp96, and provide a new molecular mechanism underlying TLR-mediated gp96 regulation.

Magnetoresistance of the metallic polyacetylene

Abstract The temperature dependence of the zero field resistivity, ρ ( T ), and the magnetoresistance (MR) of polyacetylene (PA) doped with iodine, metal-halide (AuCl 3 , FeCl 3 ) and perchlorate are measured. The results of doped PA are different for each dopant and for the degree of aging. The ρ ( T ) of the metal-halide and the perchlorate doped PAs show a resistivity minimum near T *≈200 K and the weak temperature dependence at low temperature. In particular, some of the perchlorate doped PA samples show the positive temperature coefficient of resistivity (TCR) from T =1.5 K to T =300 K with resistivity ratio ρ ( T =1.5 K)/ ρ ( T =300 K)=0.5∼0.7 depending on the samples. The MR is negative for all of the metallic PA samples. Both transverse ( J ⊥ H ) and longitudinal ( J ‖ H ) MR show similar magnetic field dependencies, although the magnitude of the transverse MR is larger than that of the longitudinal MR. For the aged samples, the ρ ( T ) increase more rapidly upon cooling than that of the fresh samples. The MR of the aged samples is positive. The localization–interaction theory is examined for the observed ρ ( T ) and MR data of doped PA. But the model of interchain charge transfer bridged by the dopants seems to explain the negative MR as well as the metallic temperature dependence of resistivity for the perchlorate doped polyacetylene.

Conductivity and magnetoresistance of polyacetylene fiber network

Abstract The four-probe conductivity and magnetoresistance (MR) in micron-scale for the iodine-doped polyacetylene (PA) fiber network were measured. A remarkably weaker temperature dependence of resistivity compared to that of the bulk PA film was observed. The transverse MR at T =1.5 K was negative and its relative magnitude was approximately 0.1% at H =10 T. The slow oscillatory tendency with small amplitude was suggested in MR. The results suggest the importance of contact barriers in the nanosize PA fiber network.

Magnetothermopower of single wall carbon nanotube newtwork

Abstract We have measured the electrical resistivity and thermoelectric power (TEP) of single wall carbon nanotube (SWCN) network. The resistivity result can be fitted to the variable range hopping (VRH) type conduction formula. The room temperature TEP is about 30~60 μV/K and the overall temperature dependence of the TEP is similar to that of the disordered alloy which shows the electron-phonon enhancement of TEP. By applying the magnetic field up to 6 Tesla, the magnitude of the TEP is reduced by ~2 μV/K while the overall temperature dependence is more or less the same. The results are discussed by the heterogeneous model which was adapted to the conducting polymers.