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Aminoacyl-tRNA synthetases recognize cognate amino acids and tRNAs from their noncognate counterparts and catalyze the formation of aminoacyl-tRNAs. Halofuginone (HF), a coccidiostat used in veterinary medicine, exerts its effects by acting as a high-affinity inhibitor of the enzyme glutamyl-prolyl-tRNA synthetase (EPRS). In order to elucidate the precise molecular basis of this inhibition mechanism of human EPRS, the crystal structures of the prolyl-tRNA synthetase domain of human EPRS (hPRS) at 2.4 Å resolution (hPRS-apo), of hPRS complexed with ATP and the substrate proline at 2.3 Å resolution (hPRS-sub) and of hPRS complexed with HF at 2.62 Å resolution (hPRS-HF) are presented. These structures show plainly that motif 1 functions as a cap in hPRS, which is loosely opened in hPRS-apo, tightly closed in hPRS-sub and incorrectly closed in hPRS-HF. In addition, the structural analyses are consistent with more effective binding of hPRS to HF with ATP. Mutagenesis and biochemical analysis confirmed the key roles of two residues, Phe1097 and Arg1152, in the HF inhibition mechanism. These structures will lead to the development of more potent and selective hPRS inhibitors for promoting inflammatory resolution.

Peptoids are a rapidly developing class of biomimetic polymers based on oligo-N-substituted glycine backbones, designed to mimic peptides and proteins. Inspired by natural antimicrobial peptides, a group of cationic amphipathic peptoids has been successfully discovered with a potent and broad-spectrum activity against pathogenic bacteria; however, there are limited studies to address the in vivo pharmacokinetics of the peptoids. Herein, 64Cu labeled DOTA conjugates of three different peptoids and two control peptides were synthesized and assayed in vivo by both biodistribution studies and small animal positron emission tomography (PET). The study was designed in a way to assess how structural differences of the peptidomimetics affect in vivo pharmacokinetics. As amphipathic molecules, major uptake of the peptoids occurred at the liver. Increased kidney uptake was observed by deleting one hydrophobic residue in the peptoid, and 64Cu-3 achieved the highest kidney uptake of all the conjugates tested in this study. In comparison to peptides, our data indicated that peptoids had general in vivo properties of higher tissue accumulation, slower elimination, and higher in vivo stability. Different administration routes (intravenous, intraperitoneal, and oral) were investigated with peptoids. When administered orally, the peptoids showed poor bioavailability, reminiscent to that of peptide. But, remarkably longer passage through the gastrointestinal (GI) tract without rapid digestion was observed for peptoids. These unique in vivo properties of peptoids were rationalized by efficient cellular membrane permeability and protease resistance of peptoids. The results observed in the biodistribution studies could be confirmed by the PET imaging, which provides a reliable way to evaluate in vivo pharmacokinetic properties of peptoids noninvasively and in real time. The pharmacokinetic data presented here can provide an insight for further development of the antimicrobial peptoids as pharmaceuticals.

In higher eukaryotes, one of the two arginyl-tRNA synthetases (ArgRSs) has evolved to have an extended N-terminal domain that plays a crucial role in protein synthesis and cell growth and in integration into the multisynthetase complex (MSC). Here, we report a crystal structure of the MSC subcomplex comprising ArgRS, glutaminyl-tRNA synthetase (GlnRS), and the auxiliary factor aminoacyl tRNA synthetase complex-interacting multifunctional protein 1 (AIMP1)/p43. In this complex, the N-terminal domain of ArgRS forms a long coiled-coil structure with the N-terminal helix of AIMP1 and anchors the C-terminal core of GlnRS, thereby playing a central role in assembly of the three components. Mutation of AIMP1 destabilized the N-terminal helix of ArgRS and abrogated its catalytic activity. Mutation of the N-terminal helix of ArgRS liberated GlnRS, which is known to control cell death. This ternary complex was further anchored to AIMP2/p38 through interaction with AIMP1. These findings demonstrate the importance of interactions between the N-terminal domains of ArgRS and AIMP1 for the catalytic and noncatalytic activities of ArgRS and for the assembly of the higher-order MSC protein complex.arginyl-tRNA synthetase | multisynthetase complex | crystal structure | AIMP1 | glutaminyl-tRNA synthetase

This study reports the physical and functional interplay between Fas-associated factor 1 (FAF1), a death-promoting protein, and parkin, a key susceptibility protein for Parkinson's disease (PD). We found that parkin acts as an E3 ubiquitin ligase to ubiquitinate FAF1 both in vitro and at cellular level, identifying FAF1 as a direct substrate of parkin. The loss of parkin function due to PD-linked mutations was found to disrupt the ubiquitination and degradation of FAF1, resulting in elevated FAF1 expression in SH-SY5Y cells. Moreover, FAF1-mediated cell death was abolished by wild-type parkin, but not by PD-linked parkin mutants, implying that parkin antagonizes the death potential of FAF1. This led us to investigate whether FAF1 participates in the pathogenesis of PD. To address this, we used a gene trap mutagenesis approach to generate mutant mice with diminished levels of FAF1 (Faf1(gt/gt)). Using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mouse model of PD, we found that FAF1 accumulated in the substantia nigra pars compacta (SNc) of MPTP-treated PD mice, and that MPTP-induced dopaminergic cell loss in the SNc was significantly attenuated in Faf1(gt/gt) mice versus Faf1(+/+) mice. MPTP-induced reduction of locomotor activity was also lessened in Faf1(gt/gt) mice versus Faf1(+/+) mice. Furthermore, we found that FAF1 deficiency blocked PD-linked biochemical events, including caspase activation, ROS generation, JNK activation and cell death. Taken together, these results suggest a new role for FAF1: that of a positive modulator for PD.

This study presents a molecular inhibitory mechanism by Fasassociated factor 1 (FAF1) on IB kinase ( Fas-associated factor 1 (FAF1)3 is evolutionarily conserved from flies to mammals (1-4) and is involved in various key biological processes. FAF1 potentiates the Fas pathway as a member of the Fas death-inducing signaling complex and mediates chemotherapeutic-induced cell death (5, 6). FAF1 also functions as an integral regulatory component of the transient receptor potential vanilloid type 1 (TRPV1) signaling pathway (7). FAF1 is involved in the ubiquitination pathway and interacts with ubiquitin and valosin-containing protein, which is a multiubiquitin chain-targeting factor (8). In addition, FAF1 inhibits the chaperone activities of the heat-shock proteins Hsc70 and Hsp70 (9). FAF1 is also involved in the nuclear factor-B (NF-B) signaling pathway. FAF1 inhibits NF-B activation in HEK 293 cells by binding to NF-B p65 (10). NF-B inhibition by FAF1has also been reported in Drosophila (4). Caspar, a fly homolog of human FAF1, selectively suppresses the immune deficiency (Imd) pathway. Loss-of-function caspar mutants constitutively expressed antibacterial genes in the absence of bacterial infections, indicating that caspar is an endogenous suppressor of the Imd pathway. Selective involvement of IB kinase (IKK) complex in the fly Imd pathway led us to examine the regulatory mechanism, if any, between FAF1 and the IKK complex.The IKK complex is mainly composed of two catalytic subunits, IKK␣/IKK1 and IKK␤/IKK2, and a regulatory subunit, IKK␥/NF-B essential modulator (NEMO)/IKKAP1. Knockout mouse studies have demonstrated that IKK␤ has a dominant role in NF-B activation induced by proinflammatory cytokines, whereas IKK␣ is essential for morphogenic signaling (11). Although IKK␥ lacks the catalytic function, IKK␥ is essential for activation of the IKK complex. Both IKK␣ and IKK␤ contain an N-terminal kinase domain, a central leucine-zipper (LZ) domain, and a C-terminal helix-loop-helix (HLH) domain (12)(13)(14). Homo-and hetero-oligomerizations between IKK␣ and IKK␤ occur through their LZ domains, and the HLH domains mediate recruitment of IKK␥ to the IKK complex.This study reveals a novel function of FAF1: as an endogenous suppressor of IKK activation. FAF1 disrupts IKK complex assembly through physical interaction with the LZ domain of IKK␤. Association between FAF1 and IKK␤ was induced by proinflammatory stimuli. Such an induced interaction indicates that FAF1 is an NF-B pathway suppressor with a unique mode of action.

TRPV1, a cloned capsaicin receptor, is a molecular sensor for detecting adverse stimuli and a key element for inflammatory nociception and represents biophysical properties of native channel. However, there seems to be a marked difference between TRPV1 and native capsaicin receptors in the pharmacological response profiles to vanilloids or acid. One plausible explanation for this overt discrepancy is the presence of regulatory proteins associated with TRPV1. Here, we identify Fas-associated factor 1 (FAF1) as a regulatory factor, which is coexpressed with and binds to TRPV1 in sensory neurons. When expressed heterologously, FAF1 reduces the responses of TRPV1 to capsaicin, acid, and heat, to the pharmacological level of native capsaicin receptor in sensory neurons. Furthermore, silencing FAF1 by RNA interference augments capsaicin-sensitive current in native sensory neurons. We therefore conclude that FAF1 forms an integral component of the vanilloid receptor complex and that it constitutively modulates the sensitivity of TRPV1 to various noxious stimuli in sensory neurons.

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