Crystal structure of Arabidopsis thaliana RabA1a.

RabGTPase is a member of the Ras superfamily of small GTPases, which share a GTP-binding pocket containing highly conserved motifs that promote GTP hydrolysis. In Arabidopsis, the RabA group, which corresponds to the Rab11 group in animals, functions in the recycling of endosomes that control docking and fusion during vesicle transport. However, their molecular mechanisms remain unknown. In this study, we determined the crystal structures of the GDP-bound inactive form and both GppNHp- and GTP-bound active forms of RabA1a, at resolutions of 2.8, 2.6, and 2.6 Å, respectively. A bound sulfate ion in the active site of the GDP-bound structure stabilized Switch II by bridging the interaction between a magnesium ion and Arg74. Comparisons of the two states of RabA1a with Rab11 proteins revealed clear differences in the Switch I and II loops. These results suggested that conformational change of the Switch regions of RabA1a, derived by GTP or GDP binding, could maintain subcellular membrane traffic through the specific interaction of effector molecules.

Molecular mechanism for the inhibition of DXO by adenosine 3',5'-bisphosphate.

The decapping exoribonuclease DXO functions in pre-mRNA capping quality control, and shows multiple biochemical activities such as decapping, deNADding, pyrophosphohydrolase, and 5'-3' exoribonuclease activities. Previous studies revealed the molecular mechanisms of DXO based on the structures in complexes with a product, substrate mimic, cap analogue, and 3'-NADP+. Despite several reports on the substrate-specific reaction mechanism, the inhibitory mechanism of DXO remains elusive. Here, we demonstrate that adenosine 3', 5'-bisphosphate (pAp), a known inhibitor of the 5'-3' exoribonuclease Xrn1, inhibits the nuclease activity of DXO based on the results of structural and biochemical experiments. We determined the crystal structure of the DXO-pAp-Mg2+ complex at 1.8 Šresolution. In comparison with the DXO-RNA product complex, the position of pAp is well superimposed with the first nucleotide of the product RNA in the vicinity of two magnesium ions. Furthermore, biochemical assays showed that the inhibition by pAp is comparable between Xrn1 and DXO. Collectively, these structural and biochemical studies reveal that pAp inhibits the activities of DXO by occupying the active site to act as a competitive inhibitor.

Structural Basis for the Selective Inhibition of Cdc2-Like Kinases by CX-4945.

Cdc2-like kinases (CLKs) play a crucial role in the alternative splicing of eukaryotic pre-mRNAs through the phosphorylation of serine/arginine-rich proteins (SR proteins). Dysregulation of this processes is linked with various diseases including cancers, neurodegenerative diseases, and many genetic diseases. Thus, CLKs have been regarded to have a potential as a therapeutic target and significant efforts have been exerted to discover an effective inhibitor. In particular, the small molecule CX-4945, originally identified as an inhibitor of casein kinase 2 (CK2), was further revealed to have a strong CLK-inhibitory activity. Four isoforms of CLKs (CLK1, CLK2, CLK3, and CLK4) can be inhibited by CX-4945, with the highest inhibitory effect on CLK2. This study aimed to elucidate the structural basis of the selective inhibitory effect of CX-4945 on different isoforms of CLKs. We determined the crystal structures of CLK1, CLK2, and CLK3 in complex with CX-4945 at resolutions of 2.4 Å, 2.8 Å, and 2.6 Å, respectively. Comparative analysis revealed that CX-4945 was bound in the same active site pocket of the CLKs with similar interacting networks. Intriguingly, the active sites of CLK/CX-4945 complex structures had different sizes and electrostatic surface charge distributions. The active site of CLK1 was somewhat narrow and contained a negatively charged patch. CLK3 had a protruded Lys248 residue in the entrance of the active site pocket. In addition, Ala319, equivalent to Val324 (CLK1) and Val326 (CLK2), contributed to the weak hydrophobic interactions with the benzonaphthyridine ring of CX-4945. In contrast, the charge distribution pattern of CLK2 was the weakest, favoring its interactions with benzonaphthyridine ring. Thus, the relatively strong binding affinities of CX-4945 with CLK2 are consistent with its strong inhibitory effect defined in the previous study. These results may provide insights into structure-based drug discovery processes.

Crystal Structure of Histidine Triad Nucleotide-Binding Protein from the Pathogenic Fungus Candida albicans.

Histidine triad nucleotide-binding protein (HINT) is a member of the histidine triad (HIT) superfamily, which has hydrolase activity owing to a histidine triad motif. The HIT superfamily can be divided to five classes with functions in galactose metabolism, DNA repair, and tumor suppression. HINTs are highly conserved from archaea to humans and function as tumor suppressors, translation regulators, and neuropathy inhibitors. Although the structures of HINT proteins from various species have been reported, limited structural information is available for fungal species. Here, to elucidate the structural features and functional diversity of HINTs, we determined the crystal structure of HINT from the pathogenic fungus Candida albicans (CaHINT) in complex with zinc ions at a resolution of 2.5 Å. Based on structural comparisons, the monomer of CaHINT overlaid best with HINT protein from the protozoal species Leishmania major. Additionally, structural comparisons with human HINT revealed an additional helix at the C-terminus of CaHINT. Interestingly, the extended C-terminal helix interacted with the N-terminal loop (α1-ß1) and with the α3 helix, which appeared to stabilize the dimerization of CaHINT. In the C-terminal region, structural and sequence comparisons showed strong relationships among 19 diverse species from archea to humans, suggesting early separation in the course of evolution. Further studies are required to address the functional significance of variations in the C-terminal region. This structural analysis of CaHINT provided important insights into the molecular aspects of evolution within the HIT superfamily.

Crystal Structure and Comparative Sequence Analysis of GmhA from Colwellia psychrerythraea Strain 34H Provides Insight into Functional Similarity with DiaA.

The psychrophilic organism Colwellia psychrerythraea strain 34H produces extracellular polysaccharide substances to tolerate cold environments. Sedoheptulose 7-phosphate isomerase (GmhA) is essential for producing d-glycero-d-mannoheptose 7-phosphate, a key mediator in the lipopolysaccharide biosynthetic pathway. We determined the crystal structure of GmhA from C. psychrerythraea strain 34H (CpsGmhA, UniProtKB code: Q47VU0) at a resolution of 2.8 Å. The tetrameric structure is similar to that of homologous GmhA structures. Interestingly, one of the catalytic residues, glutamate, which has been reported to be critical for the activity of other homologous GmhA enzymes, is replaced by a glutamine residue in the CpsGmhA protein. We also found differences in the conformations of several other catalytic residues. Extensive structural and sequence analyses reveal that CpsGmhA shows high similarity to Escherichia coli DnaA initiator-associating protein A (DiaA). Therefore, the CpsGmhA structure reported here may provide insight into the structural and functional correlations between GmhA and DiaA among specific microorganisms.

Valproic acid decreases the proliferation of telencephalic cells in zebrafish larvae.

Valproic acid (VPA) is used as an antiepileptic drug (AED) or mood stabilizer. Recent studies have shown that exposure to VPA during embryonic development alters neural progenitor cell (NPC) proliferation, and can also lead to behavioral impairment in adult animals. The main goal of this investigation was to evaluate the effects of treatment with 2 mM VPA for 3h on cell proliferation in the telencephalic area of zebrafish larvae of 5 days post-fertilization (dpf) using immunohistochemistry (IHC). It was also determined whether VPA exposure affects the learning ability and anxiety levels of zebrafish during adulthood using bottom-dwelling behavior and passive avoidance tests. Result of the study demonstrated that VPA exposure during development transiently decreased neuronal cell proliferation without inducing apoptosis. Additionally, quantitative real-time PCR (qRT-PCR) data indicated that mRNA expression levels of wnt signaling pathway-related factors such as ß-catenin, lef1, and gsk3ß were altered in the zebrafish treated with VPA. Interestingly, these effects were reversed over time after VPA treatment had ceased. Alterations of passive avoidance learning and bottom dwelling behavior were not observed during adulthood after developmental VPA exposure. These results may be due to the restoration of cell proliferation during the recovery period after VPA treatment.