Discovery of nigerose phosphorylase from Clostridium phytofermentans

A novel phosphorylase from Clostridium phytofermentans belonging to the glycoside hydrolase family (GH) 65 (Cphy1874) was characterized. The recombinant Cphy1874 protein produced in Escherichia coli showed phosphorolytic activity on nigerose in the presence of inorganic phosphate, resulting in the release of d-glucose and β-d-glucose 1-phosphate (β-G1P) with the inversion of the anomeric configuration. Kinetic parameters of the phosphorolytic activity on nigerose were k _cat = 67 s⁻¹ and K _m = 1.7 mM. This enzyme did not phosphorolyze substrates for the typical GH65 enzymes such as trehalose, maltose, and trehalose 6-phosphate except for a weak phosphorolytic activity on kojibiose. It showed the highest reverse phosphorolytic activity in the reverse reaction using d-glucose as the acceptor and β-G1P as the donor, and the product was mostly nigerose at the early stage of the reaction. The enzyme also showed reverse phosphorolytic activity, in a decreasing order, on d-xylose, 1,5-anhydro-d-glucitol, d-galactose, and methyl-α-d-glucoside. All major products were α-1,3-glucosyl disaccharides, although the reaction with d-xylose and methyl-α-d-glucoside produced significant amounts of α-1,2-glucosides as by-products. We propose 3-α-d-glucosyl-d-glucose:phosphate β-d-glucosyltransferase as the systematic name and nigerose phosphorylase as the short name for this Cphy1874 protein.     

N-glycans are not required for the efficient degradation of the mutant Saccharomyces cerevisiae CPY* in Schizosaccharomyces pombe

In eukaryotic cells, aberrant proteins generated in the endoplasmic reticulum (ER) are degraded by the ER-associated degradation (ERAD) pathway. Here, we report on the ERAD pathway of the fission yeast Schizosaccharomyces pombe. We constructed and expressed Saccharomyces cerevisiae wild-type CPY (ScCPY) and CPY-G255R mutant (ScCPY*) in S. pombe. While ScCPY was glycosylated and efficiently transported to the vacuoles in S. pombe, ScCPY* was retained in the ER and was not processed to the matured form in these cells. Cycloheximide chase experiments revealed that ScCPY* was rapidly degraded in S. pombe, and its degradation depended on Hrd1p and Ubc7p homologs. We also found that Mnl1p and Yos9p, proteins that are essential for ERAD in S. cerevisiae, were not required for ScCPY* degradation in S. pombe. Moreover, the null-glycosylation mutant of ScCPY, CPY*0000, was rapidly degraded by the ERAD pathway. These results suggested that N-linked oligosaccharides are not important for the recognition of luminal proteins for ERAD in S. pombe cells.     

Histone chaperone activity of Fanconi anemia proteins, FANCD2 and FANCI, is required for DNA crosslink repair

Fanconi anaemia (FA) is a rare hereditary disorder characterized by genomic instability and cancer susceptibility. A key FA protein, FANCD2, is targeted to chromatin with its partner, FANCI, and plays a critical role in DNA crosslink repair. However, the molecular function of chromatin-bound FANCD2-FANCI is still poorly understood. In the present study, we found that FANCD2 possesses nucleosome-assembly activity in vitro. The mobility of histone H3 was reduced in FANCD2-knockdown cells following treatment with an interstrand DNA crosslinker, mitomycin C. Furthermore, cells harbouring FANCD2 mutations that were defective in nucleosome assembly displayed impaired survival upon cisplatin treatment. Although FANCI by itself lacked nucleosome-assembly activity, it significantly stimulated FANCD2-mediated nucleosome assembly. These observations suggest that FANCD2-FANCI may regulate chromatin dynamics during DNA repair.     

Chk1 phosphorylates the tumour suppressor Mig-6, regulating the activation of EGF signalling

The tumour suppressor gene product Mig-6 acts as an inhibitor of epidermal growth factor (EGF) signalling. However, its posttranslational modifications and regulatory mechanisms have not been elucidated. Here, we investigated the phosphorylation of human Mig-6 and found that Chk1 phosphorylated Mig-6 in vivo as well as in vitro. Moreover, EGF stimulation promoted phosphorylation of Mig-6 without DNA damage and the phosphorylation was inhibited by depletion of Chk1. EGF also increased Ser280-phosphorylated Chk1, a cytoplasmic-tethering form, via PI3K pathway. Mass spectrometric analyses suggested that Ser 251 of Mig-6 was a major phosphorylation site by Chk1 in vitro and in vivo. Substitution of Ser 251 to alanine increased inhibitory activity of Mig-6 against EGF receptor (EGFR) activation. Moreover, EGF-dependent activation of EGFR and cell growth were inhibited by Chk1 depletion, and were rescued by co-depletion of Mig-6. Our results suggest that Chk1 phosphorylates Mig-6 on Ser 251, resulting in the inhibition of Mig-6, and that Chk1 acts as a positive regulator of EGF signalling. This is a novel function of Chk1.     

GHRP-6 mimics ghrelin-induced stimulation of food intake and suppression of locomotor activity in goldfish

Ghrelin was first identified and characterized from rat stomach as an endogenous ligand for the growth hormone secretagogue (GHS) receptor (GHS-R). Ghrelin also acts as an orexigenic factor and regulates energy balance in rodents. In goldfish, native ghrelin consists of 11 molecular variants, the major form being a 17-residue peptide with n-octanoic acid modification (n-octanoyl ghrelin17), and intraperitoneal (IP) administration of n-octanoyl ghrelin17 induces central actions such as stimulation of food intake and suppression of locomotor activity through capsaicin-sensitive afferents. Four types of GHS-Rs (1a-1, 1a-2, 2a-1 and 2a-2) have been identified in goldfish, and one GHS, GHRP-6, can activate only GHS-R2a-1 in vitro. However, there is no information about the effect of GHRP-6 on food intake and locomotor activity in goldfish in vivo. Therefore, in the present study, we examined whether IP-administered GHRP-6 would mimic the orexigenic action of n-octanoyl ghrelin17 and its suppression of locomotor activity. IP administration of GHRP-6 at 1pmol/g body weight (BW) stimulated food intake, and was equipotent to the orexigenic action of n-octanoyl ghrelin17 at 10 pmol/g BW. IP-injected GHRP-6 at 1 pmol/g BW also induced a significant decrease of locomotor activity, as was the case for IP-injected n-octanoyl ghrelin17 at 10 pmol/g BW. The action of GHRP-6 was blocked by IP-preinjected capsaicin at 160 nmol/g BW. These results suggest that the central action of GHRP-6 might be mediated via the GHS-R2a-1-signaling pathway, and subsequently through capsaicin-sensitive afferents in goldfish.     

Structural insight into receptor-selectivity for lurasidone

Lurasidone is a novel antipsychotic agent with high affinity for dopamine D₂, 5-hydroxyltryptamine 5-HT_2A, and 5-HT₇ receptors. Lurasidone has negligible affinity for histamine H₁ and muscarinic M₁ receptors, which are thought to contribute to side effects such as weight gain, sedation, and worsening of cognitive deficits. Our interests focus on why lurasidone has such high selectivity for only a part of these aminergic G-protein coupled receptors (GPCRs) and the different binding profile from ziprasidone, which has the same benzisothiazolylpiperazine moiety as lurasidone. In order to address these issues, we constructed structural models of lurasidone–GPCR complexes by homology modeling of receptors, exhaustive docking of ligand, and molecular dynamics simulation-based refinement of complexes. This computational study gave reliable structural models for D₂, 5-HT_2A, and 5-HT₇, which had overall structural complementarities with a salt bridge anchor at the center of the lurasidone molecule, but not for H₁ and M₁ owing to steric hindrance between the norbornane-2,3-dicarboximide and/or cyclohexane part of lurasidone and both receptors. By comparison with the structural models of olanzapine–GPCRs and ziprasidone–GPCRs constructed using the same computational protocols, it was suggested that the bulkiness of the norbornane-2,3-dicarboximide part and the rigidity and the bulkiness of the cyclohexyl linker gave lurasidone high selectivity for the desired aminergic GPCRs. Finally, this structural insight was validated by a binding experiment of the novel benzisothiazolylpiperazine derivatives. This knowledge on the structural mechanism behind the receptor selectivity should help to design new antipsychotic agents with preferable binding profiles, and the established computational protocols realize virtual screening and structure-based drug design for other central nervous system drugs with desired selectivity for multiple targets.     

Osteocalcin- and Osteopontin-Containing Neurons in the Rat Hind Brain

Immunohistochemistry for osteocalcin (OC) and osteopontin (OPN) was performed to know their distributions in the hind brain of adult rats. OC- and OPN-immunoreactivity (-ir) were detected in neuronal cell bodies, including perikarya and proximal dendrites and the neuropil. In the cranial nerve motor nuclei, numerous OC- and OPN-immunoreactive (-ir) neurons were detected. The neuropil in the cranial motor nuclei mostly showed strong OC- and OPN-staining intensity. The cranial nerve sensory nuclei and other relay and modulating structures in the lower brain stem also contained various numbers of OC- and OPN-ir neurons. The staining intensities in the neuropil were varied among these regions. In the cerebellar cortex, Purkinje cells and granule cells showed OPN-ir but not OC-ir. However, OC- and OPN-ir neurons were abundantly distributed throughout the cerebellar nuclei. The neuropil in the cerebellar nuclei showed moderate OC-ir and strong OPN-ir staining intensities. These findings indicate that the distribution patterns of OC- and OPN-ir neurons were similar in many structures within the hind brain. OC may play a role in modulating neuroprotective function of OPN.     

Acetylation-dependent regulation of Skp2 function

Aberrant Skp2 signaling has been implicated as a driving event in tumorigenesis. Although the underlying molecular mechanisms remain elusive, cytoplasmic Skp2 correlates with more aggressive forms of breast and prostate cancers. Here, we report that Skp2 is acetylated by p300 at K68 and K71, which is a process that can be antagonized by the SIRT3 deacetylase. Inactivation of SIRT3 leads to elevated Skp2 acetylation, which leads to increased Skp2 stability through impairment of the Cdh1-mediated proteolysis pathway. As a result, Skp2 oncogenic function is increased, whereby cells expressing an acetylation-mimetic mutant display enhanced cellular proliferation and tumorigenesis in vivo. Moreover, acetylation of Skp2 in the nuclear localization signal (NLS) promotes its cytoplasmic retention, and cytoplasmic Skp2 enhances cellular migration through ubiquitination and destruction of E-cadherin. Thus, our study identifies an acetylation-dependent regulatory mechanism governing Skp2 oncogenic function and provides insight into how cytoplasmic Skp2 controls cellular migration.     

Telomeric 3' overhangs derive from resection by Exo1 and Apollo and fill-in by POT1b-associated CST

A 3' overhang is critical for the protection and maintenance of mammalian telomeres, but its synthesis must be regulated to avoid excessive resection of the 5' end, which could cause telomere shortening. How this balance is achieved in mammals has not been resolved. Here, we determine the mechanism for 3' overhang synthesis in mouse cells by evaluating changes in telomeric overhangs throughout the cell cycle and at leading- and lagging-end telomeres. Apollo, a nuclease bound to the shelterin subunit TRF2, initiates formation of the 3' overhang at leading-, but not lagging-end telomeres. Hyperresection by Apollo is blocked at both ends by the shelterin protein POT1b. Exo1 extensively resects both telomere ends, generating transient long 3' overhangs in S/G2. CST/AAF, a DNA polα.primase accessory factor, binds POT1b and shortens the extended overhangs produced by Exo1, likely through fill-in synthesis. 3' overhang formation is thus a multistep, shelterin-controlled process, ensuring functional telomeric overhangs at chromosome ends.