Mitochondrial protein import. Requirement of presequence elements and tom components for precursor binding to the TOM complex

Protein translocation across the outer mitochondrial membrane is mediated by the translocator called the TOM (translocase of the outer mitochondrial membrane) complex. The TOM complex possesses two presequence binding sites on the cytosolic side (the cis site) and on the intermembrane space side (the trans site). Here we analyzed the requirement of presequence elements and subunits of the TOM complex for presequence binding to the cis and trans sites of the TOM complex. The N-terminal 14 residues of the presequence of subunit 9 of F(0)-ATPase are required for binding to the trans site. The interaction between the presequence and the cis site is not sufficient to anchor the precursor protein to the TOM complex. Tom7 constitutes or is close to the trans site and has overlapping functions with the C-terminal intermembrane space domain of Tom22 in the mitochondrial protein import.     

Acid-induced denaturation of Escherichia coli ribonuclease HI analyzed by CD and NMR spectroscopies

Acid-induced denaturation of the ribonuclease HI protein from Escherichia coli was analyzed by CD and NMR spectroscopies. The CD measurement revealed that the acid denaturation at 10 degrees C proceeds from the native state (N-state) to a molten globule-like state (A-state), through an apparently more unfolded state (U(A)-state). In (1)H-(15)N heteronuclear single-quantum coherence (HSQC) spectra, cross peaks from the N-state and those from the other two states are distinctively observed, while the U(A)-state and A-state are not distinguished from each other. Cross peaks from the U(A)/A-states showed a small pH dependence, which suggests a similarity in the backbone structure between the two states. The direct hydrogen-deuterium (H-D) exchange measurement at pH with the largest population of U(A)-state revealed that at least alpha-helix I is highly protected in the structure of the U(A)-state. A pH-jump H-D exchange analysis showed that the protection of alpha-helix I is highest also in the A-state. The profile of hydrogen-bond protection indicated that the structure of the A-state is closely related to that of the kinetic folding intermediate.     

Overexpressed mortalin (mot-2)/mthsp70/GRP75 and hTERT cooperate to extend the in vitro lifespan of human fibroblasts

The lifespan of human foreskin fibroblasts (HFF5), cultured under standard in vitro conditions (including ambient atmospheric oxygen tension), was extended slightly by expression of exogenous mortalin (mot-2)/mthsp70/Grp75, but not by the catalytic subunit of telomerase, hTERT. Together, mot-2 and hTERT permitted bypass of senescence, a substantial extension of lifespan, and possibly immortalization. This is the first demonstration that mot-2 and telomerase can cooperate in the immortalization process.     

Purification and properties of an intracellular 3-hydroxybutyrate-oligomer hydrolase (PhaZ2) in Ralstonia eutropha H16 and its identification as a novel intracellular poly(3-hydroxybutyrate) depolymerase

An intracellular 3-hydroxybutyrate (3HB)-oligomer hydrolase (PhaZ2(Reu)) of Ralstonia eutropha was purified from Escherichia coli harboring a plasmid containing phaZ2(Reu). The purified enzyme hydrolyzed linear and cyclic 3HB-oligomers. Although it did not degrade crystalline poly(3-hydroxybutyrate) (PHB), the purified enzyme degraded artificial amorphous PHB at a rate similar to that of the previously identified intracellular PHB (iPHB) depolymerase (PhaZ1(Reu)). The enzyme appeared to be an endo-type hydrolase, since it actively hydrolyzed cyclic 3HB-oligomers. However, it degraded various linear 3HB-oligomers and amorphous PHB in the fashion of an exo-type hydrolase, releasing one monomer unit at a time. PhaZ2 was found to bind to PHB inclusion bodies and as a soluble enzyme to cell-free supernatant fractions in R. eutropha; in contrast, PhaZ1 bound exclusively to the inclusion bodies. When R. eutropha H16 was cultivated in a nutrient-rich medium, the transient deposition of PHB was observed: the content of PHB was maximized in the log growth phase (12 h, ca. 14% PHB of dry cell weight) and decreased to a very low level in the stationary phase (ca. 1% of dry cell weight). In each phaZ1-null mutant and phaZ2-null mutant, the PHB content in the cell increased to ca. 5% in the stationary phase. A double mutant lacking both phaZ1 and phaZ2 showed increased PHB content in the log phase (ca. 20%) and also an elevated PHB level (ca. 8%) in the stationary phase. These results indicate that PhaZ2 is a novel iPHB depolymerase, which participates in the mobilization of PHB in R. eutropha along with PhaZ1.     

Conversion of the aminocrotonate intermediate limits the rate of gamma-elimination reaction catalyzed by L-cystathionine gamma-lyase of the yeast Saccharomyces cerevisiae

L-Cystathionine gamma-lyase [EC 4.4.1.1] of Saccharomyces cerevisiae was shown to bind cofactor pyridoxal 5'-phosphate, up to 2 molecules/subunit. The association constants of the enzyme for the cofactor were estimated to be 3.67 x 10(5) M(-1) and 9.05 x 10(3) M(-1). However, the latter value was too small for the binding to play a catalytic role. Changes in the absorption spectra of the enzyme in gamma-elimination reaction mixtures with various amino acids as substrates were observed at 10 degrees C to elucidate the reaction mechanism of the enzyme. The enzyme formed a chromophore exhibiting absorption at approximately 480 nm, which is characteristic of an aminocrotonate intermediate with O-succinyl-L-homoserine, L-cystathionine, L-homoserine, or O-acetyl-L-homoserine, at rates in this order. The intermediate was consumed at much lower rates than those of formation. The order of the rates of consumption was the same as the order of the formation rates and the order of the gamma-elimination activity of the enzyme with the above-mentioned substrates. These results strongly suggested that the intermediate was essential for gamma-elimination and that the reaction was rate-limited by its conversion into the product alpha-ketobutyrate. L-Cysteine sensitively inhibited the alpha, gamma-elimination activity of the enzyme, and also retarded the formation of the chromophore when it was provided to the enzyme together with a substrate. The reason for these phenomena is discussed.     

Association of cathepsin E deficiency with development of atopic dermatitis

Atopic dermatitis (AD) is a pruritic inflammatory skin diseases associated with a family history of atropy. Here we show that mice lacking the endolysosomal aspartic proteinase cathepsin E spontaneously develop skin lesions similar to those of humans with AD when reared under conventional conditions but not under specific pathogen-free conditions. These mice showed the increase in the ratio of CD4+/CD8+ T cells, the strong polarization of na?ve T cells to T helper 2 cells, and the systemic accumulation of IL-18 and IL-1beta accompanied by a marked increase in IL-4, IL-5, and IgE. The relative rates of degradation of IL-18 and IL-1beta were significantly lower in cathepsin E-deficient mice than wild-type mice. These results strongly suggest that the development of AD in cathepsin E-deficient mice is initiated by systemic accumulation of IL-18 and IL-1beta, mainly due to their reduced turnover rates. In addition, the reduced expression of cathepsin E was also observed in erythrocytes of both humans with AD and the AD mouse model NC/Nga. Cathepsin E deficiency might thus be responsible for the induction of AD in humans and mice.     

Purification and characterization of Chinese hamster phosphatidylserine synthase 2

Phosphatidylserine (PtdSer) in mammalian cells is synthesized through the action of PtdSer synthase (PSS) 1 and 2, which catalyze the conversion of phosphatidylcholine and phosphatidylethanolamine, respectively, to PtdSer. The PtdSer synthesis in intact cells and an isolated membrane fraction is inhibited by exogenous PtdSer, indicating that inhibition of PtdSer synthases by PtdSer is important for the regulation of PtdSer biosynthesis. In this study, to examine whether the inhibition occurs through the direct interaction of PtdSer with the synthases or is mediated by unidentified factor(s), we purified a FLAG and HA peptide-tagged form of Chinese hamster PSS 2 to near homogeneity. The purified enzyme, as well as the crude enzyme in a membrane fraction, was inhibited on the addition of PtdSer to the enzyme assay mixture. In contrast to PtdSer, phosphatidylcholine and phosphatidylethanolamine did not significantly inhibit the purified enzyme. Furthermore, PtdSer-resistant PtdSer synthesis was observed on cell-free assaying of the membrane fraction prepared from a Chinese hamster ovary cell strain whose PtdSer synthesis in vivo is not inhibited by exogenous PtdSer. These results suggested that the interaction of PtdSer with PSS 2 or a very minor protein co-purified with PSS 2 was critical for the regulation of PSS 2 activity in intact cells.     

Tamalin is a scaffold protein that interacts with multiple neuronal proteins in distinct modes of protein-protein association

Tamalin is a scaffold protein that comprises multiple protein-interacting domains, including a 95-kDa postsynaptic density protein (PSD-95)/discs-large/ZO-1 (PDZ) domain, a leucine-zipper region, and a carboxyl-terminal PDZ binding motif. Tamalin forms a complex with metabotropic glutamate receptors and guanine nucleotide exchange factor cytohesins and promotes intracellular trafficking and cell surface expression of group 1 metabotropic glutamate receptors. In the present study, using several different approaches we have shown that tamalin interacts with multiple neuronal proteins through its distinct protein-binding domains. The PDZ domain of tamalin binds to the PDZ binding motifs of SAP90/PSD-95-associated protein and tamalin itself, whereas the PDZ binding motif of tamalin is capable of interacting with the PDZ domain of S-SCAM. In addition, tamalin forms a complex with PSD-95 and Mint2/X11beta/X11L by mechanisms different from the PDZ-mediated interaction. Tamalin has the ability to assemble with these proteins in vivo; their protein complex with tamalin was verified by coimmunoprecipitation of rat brain lysates. Interestingly, the distinct protein-interacting domains of tamalin are evolutionarily conserved, and mRNA expression is developmentally up-regulated at the postnatal period. The results indicate that tamalin exists as a key element that forms a protein complex with multiple postsynaptic and protein-trafficking scaffold proteins.