Dimerization is important for the GTPase activity of chloroplast translocon components atToc33 and psToc159

Arabidopsis Toc33 (atToc33) is a GTPase and a member of the Toc (translocon at the outer-envelope membrane of chloroplasts) complex that associates with precursor proteins during protein import into chloroplasts. By inference from the crystal structure of psToc34, a homologue in pea, the arginine at residue 130 (Arg(130)) has been implicated in the formation of the atToc33 dimer and in intermolecular GTPase activation within the dimer. Here we report the crystal structure at 3.2-A resolution of an atToc33 mutant, atToc33(R130A), in which Arg(130) was mutated to alanine. Both in solution and in crystals, atToc33(R130A) was present in its monomeric form. In contrast, both wild-type atToc33 and another pea Toc GTPase homologue, pea Toc159 (psToc159), were able to form dimers in solution. Dimeric atToc33 and psToc159 had significantly higher GTPase activity than monomeric atToc33, psToc159, and atToc33(R130A). Molecular modeling using the structures of psToc34 and atToc33(R130A) suggests that, in an architectural dimer of atToc33, Arg(130) from one monomer interacts with the beta-phosphate of GDP and several other amino acids of the other monomer. These results indicate that Arg(130) is critical for dimer formation, which is itself important for GTPase activity. Activation of GTPase activity by dimer formation is likely to be a critical regulatory step in protein import into chloroplasts.     

Effect of dimethyl sulphoxide on the crystal structure of porcine pepsin

The structure of porcine pepsin crystallized in the presence of dimethyl sulphoxide has been analysed by X-ray crystallography to obtain insights into the structural events that occur at the onset of chemical denaturation of proteins. The results show that one dimethyl sulphoxide molecule occupies a site on the surface of pepsin interacting with two of its residues. An increase in the average temperature factor of pepsin in the presence of dimethyl sulphoxide has been observed indicating protein destabilization induced by the denaturant. Significant increase in the temperature factor and weakening of the electron density have been observed for the catalytic water molecule located between the active aspartates. The conformation of pepsin remains unchanged in the crystal structure. However, the enzyme assay and circular dichroism studies indicate that dimethyl sulphoxide causes a slight change in the secondary structure and complete loss of activity of pepsin in solution.     

N- and C-terminal cooperation in rotavirus enterotoxin: novel mechanism of modulation of the properties of a multifunctional protein by a structurally and functionally overlapping conformational domain

Rotavirus NSP4 is a multifunctional endoplasmic reticulum (ER)-resident nonstructural protein with the N terminus anchored in the ER and about 131 amino acids (aa) of the C-terminal tail (CT) oriented in the cytoplasm. Previous studies showed a peptide spanning aa 114 to 135 to induce diarrhea in newborn mouse pups with the 50% diarrheal dose approximately 100-fold higher than that for the full-length protein, suggesting a role for other regions in the protein in potentiating its diarrhea-inducing ability. In this report, employing a large number of methods and deletion and amino acid substitution mutants, we provide evidence for the cooperation between the extreme C terminus and a putative amphipathic alpha-helix located between aa 73 and 85 (AAH73-85) at the N terminus of DeltaN72, a mutant that lacked the N-terminal 72 aa of nonstructural protein 4 (NSP4) from Hg18 and SA11. Cooperation between the two termini appears to generate a unique conformational state, specifically recognized by thioflavin T, that promoted efficient multimerization of the oligomer into high-molecular-mass soluble complexes and dramatically enhanced resistance against trypsin digestion, enterotoxin activity of the diarrhea-inducing region (DIR), and double-layered particle-binding activity of the protein. Mutations in either the C terminus, AAH73-85, or the DIR resulted in severely compromised biological functions, suggesting that the properties of NSP4 are subject to modulation by a single and/or overlapping highly sensitive conformational domain that appears to encompass the entire CT. Our results provide for the first time, in the absence of a three-dimensional structure, a unique conformation-dependent mechanism for understanding the NSP4-mediated pleiotropic properties including virus virulence and morphogenesis.     

Erbium(III) ion-doped borate-based glasses for 1.53 μm broad band applications

Novel erbium(III) ion-doped borate-based glasses (Er³⁺:BCNF) by conventional melt-quenching technique were designed and synthesized. The glasses were characterized for their structural, vibrational and spectroscopic properties. The nephelauxetic ratio, bonding parameters, and Judd–Ofelt (JO) intensity parameters (Ω_λ λ = 2, 4 and 6) were determined by using absorption spectrum of 1 mol% Er₂O₃ doped glass. These JO parameters were utilized to derive radiative properties for various excited states of erbium(III) ions. Emission cross-section for ⁴I_13/2 → ⁴I_15/2 transition of erbium(III) ions was computed through McCumber theory. The decay curves for (²H_11/2, ⁴S_3/2) and ⁴I_13/2 levels were recorded and analysed. All the results of Er³⁺:BCNF glasses revealed that the studied glasses are efficient and thermally stable and could be suitable for display devices, optical amplification and green laser applications.     

Toc GTPases

Chloroplasts import more than 90% of their protein constituents from the cytosol. The import is mediated by translocon complexes located in the chloroplast envelope and the stroma. This review focuses on the two GTPases in the Toc (translocon at the outer envelope membrane of chloroplasts) complex. Hypotheses are presented about gating across the outer membrane and the possible functional states of the GTPases. This article is for the Special Issue of IMB 20th anniversary.     

Orange-red luminescence of samarium-doped bismuth–germanium–borate glass for light-emitting devices

Samarium (Sm³⁺)-doped glass has sparked a rising interest in demonstrating a noticeable emission in the range of 400–700, which is advantageous in solid-state lasers in the visible region, colour displays, undersea communication, and optical memory devices. This study reports the fabrication of Sm³⁺-doped bismuth–germanium–borate glasses were established using a standard melt-quenching technique and inspection by absorption, steady-state luminescence, and transient studies. The typical peaks of Sm³⁺ ions were detected in the visible range under 403 nm excitation. A strong emission band was detected at 599 nm that resembles the ⁴G_5/2→⁶H_7/2 transition of Sm³⁺ ions for BGBiNYSm_0.5 glass. Furthermore, a reddish-orange (coral) luminescence at 646 nm that resembles the ⁴G_5/2→⁶H_9/2 transition was also perceived. The stimulated emission cross-section of ⁴G_5/2 level for BGBiNYSm_0.5 glass was 0.39 × 10⁻²² cm². Lifetime of the ⁴G_5/2 level was enhanced for the BGBiNYSm_0.5 glass and decreased with an increase in active ion concentrations. The lifetime quenching of ions at the metastable state was because of energy transfer among Sm³⁺ ions by cross-relaxation channels. Commission Internationale de l'Éclairage (CIE) coordinates were evaluated from the emission spectra. Moreover, all the findings recommend these glass as light-emitting materials in the coral region at 599 nm for solid-state lighting applications.     

Plasmepsin inhibitors: design, synthesis, inhibitory studies and crystal structure analysis.

Plasmepsin group of enzymes are key enzymes in the life cycle of malarial parasites. As inhibition of plasmepsins leads to the parasite's death, these enzymes can be utilized as potential drug targets. Although many drugs are available, it has been observed that Plasmodium falciparum, the species that causes most of the malarial infections and subsequent death, has developed resistance against most of the drugs. Based on the cleavage sites of hemglobin, the substrate for plasmepsins, we have designed two compounds (p-nitrobenzoyl-leucine-beta-alanine and p-nitrobenzoyl-leucine-isonipecotic acid), synthesized them, solved their crystal structures and studied their inhibitory effect using experimental and theoretical (docking) methods. In this paper, we discuss the synthesis, crystal structures and inhibitory nature of these two compounds which have a potential to inhibit plasmepsins.