A switch from α‐helical to β‐strand conformation during co‐translational protein folding

Cellular proteins begin to fold as they emerge from the ribosome. The folding landscape of nascent chains is not only shaped by their amino acid sequence but also by the interactions with the ribosome. Here, we combine biophysical methods with cryo‐EM structure determination to show that folding of a β‐barrel protein begins with formation of a dynamic α‐helix inside the ribosome. As the growing peptide reaches the end of the tunnel, the N‐terminal part of the nascent chain refolds to a β‐hairpin structure that remains dynamic until its release from the ribosome. Contacts with the ribosome and structure of the peptidyl transferase center depend on nascent chain conformation. These results indicate that proteins may start out as α‐helices inside the tunnel and switch into their native folds only as they emerge from the ribosome. Moreover, the correlation of nascent chain conformations with reorientation of key residues of the ribosomal peptidyl‐transferase center suggest that protein folding could modulate ribosome activity.     

GnRH-II analogs for selective activation and inhibition of non-mammalian and type-II mammalian GnRH receptors

Recently, we identified three types of non-mammalian gonadotropin-releasing hormone receptors (GnRHR) in the bullfrog (designated bfGnRHR-1-3), and a mammalian type-II GnRHR in green monkey cell lines (denoted gmGnRHR-2). All these receptors responded better to GnRH-II than GnRH-I, while mammalian type-I GnRHR showed greater sensitivity to GnRH-I than GnRH-II. In the present study, we designed new GnRH-II analogs and examined whether they activated or inhibited non-mammalian and mammalian type-II GnRHRs. [D-Ala6]GnRH-II, with D-Ala substituted for Gly6 in GnRH-II, increased inositol phosphate (IP) production in cells stably expressing non-mammalian GnRHRs more effectively than native GnRH-II. However, it exhibited lower activity for mammalian type-I GnRHR than GnRH-I itself. Trptorelix-1, a GnRH-II antagonist, inhibited GnRH-induced IP production in cells expressing non-mammalian GnRHRs more effectively than Cetrorelix, a GnRH-I antagonist. Trptorelix-1, however, had lower potency for mammalian type-I GnRHR than Cetrorelix. Ligand-receptor binding assays revealed that [D-Ala6]GnRH-II and Trptorelix-1 have higher affinities for non-mammalian GnRHRs but lower affinities for mammalian type-I GnRHR than GnRH-II and Cetrorelix, respectively. Moreover, [D-Ala6]GnRH-II and Trptorelix-1 had a higher affinity for gmGnRHR-2 than GnRH-II and Cetrorelix, respectively. These results indicate that [D-Ala6]GnRH-II and Trptorelix-1 are highly effective agonist and antagonist, respectively, for non-mammalian and type-II mammalian GnRHRs.     

Hoechst 33258 binds to G-quadruplex in the promoter region of human c-myc

In vitro binding of Hoechst 33258 to the promoter region of human c-myc, d(GG GGAGGG TGG GGA GGG TGG GGA AGG TGG GG) which forms G-quadruplex, both in vitro and in vivo in the presence of metal ions, was investigated by equilibrium absorption, fluorescence, and kinetic surface plasmon resonance methods. Hypochromic effect in UV absorption spectra and blue shift in fluorescence emission maxima of Hoechst in the presence of quadruplex revealed that Hoechst binds to the quadruplex. Analysis of UV and fluorescence titration data revealed that Hoechst binds to quadruplex with binding affinity of the order of 10(6). Anisotropy measurements and higher lifetime obtained from time-resolved decay experiments revealed that quadruplex-bound Hoechst is rotationally restricted in a less polar environment than the bulk buffer medium. From surface plasmon resonance studies, we obtained kinetic association (k(a)) and dissociation (k(d)) of 1.23+/-0.04 x 10(5)M(-1)s(-1) and 0.686+/-0.009 s(-1), respectively. As Hoechst is known to bind A-T-rich region of duplex DNA, here we propose the likelihood of Hoechst interacting with the AAGGT loop of the quadruplex.     

RPE65 operates in the vertebrate visual cycle by stereospecifically binding all-trans-retinyl esters

RPE65 is a major protein of unknown function found associated with the retinyl pigment epithelial (RPE) membranes [Hamel, C. P., Tsilou, E., Pfeffer, B. A., Hooks, J. J., Detrick, B., and Redmond, T. M. (1993) J. Biol. Chem. 268, 15751-15757; Bavik, C. O., Levy, F., Hellman, U., Wernstedt, C., and Eriksson, U. (1993) J. Biol. Chem. 268, 20540-20546]. RPE65 knockouts fail to synthesize 11-cis-retinal, the chromophore of rhodopsin, and accumulate all-trans-retinyl esters in the RPE. Previous studies have also shown that RPE65 is specifically labeled with all-trans-retinyl ester based affinity labeling agents, suggesting a retinyl ester binding role for the protein. In the present work, we show that purified RPE65 binds all-trans-retinyl palmitate (tRP) with a K(D) = 20 pM. These quantitative experiments are performed by measuring the quenching of RPE65 fluorescence by added tRP. The binding for tRP is highly specific because 11-cis-retinyl palmitate binds with a K(D) = 14 nM, 11-cis-retinol binds with a K(D) = 3.8 nM, and all-trans-retinol (vitamin A) binds with a K(D) = 10.8 nM. This stereospecificity for tRP is to be compared to the binding of retinoids to BSA, where virtually no discrimination is found in the binding of the same retinoids. This work provides further evidence that RPE65 functions by binding to and mobilizing the highly hydrophobic all-trans-retinyl esters, allowing them to enter the visual cycle.     

The amyloid beta peptide (Abeta(1-40)) is thermodynamically soluble at physiological concentrations

Precipitation of the 39-43-residue amyloid beta peptide (Abeta) is a crucial factor in Alzheimer's disease (AD). In normal as well as in AD-afflicted brain, the Abeta concentration is estimated to be a few nanomolar. Here we show that Abeta(1-40) precipitates in vitro only if the dissolved concentration is >14 microM. Using fluorescence correlation spectroscopy, we further show that the precipitation is complete in 1 day, after which the size distribution of Abeta monomer/oligomers in the solution phase becomes stationary in time and independent of the starting Abeta concentration. Mass spectra confirm that both the solution phase and the coexisting precipitate contain chemically identical Abeta molecules. Incubation at 68 degrees C for 1 h reduces the solubility by <12%. Together, these results show that the thermodynamic saturation concentration (C(sat)) of Abeta(1-40) in phosphate-buffered saline (PBS) at pH 7.4 has a well-defined lower limit of 15.5 +/- 1 microM. Divalent metal ions (believed to play a role in AD) at near-saturation concentrations in PBS reduce C(sat) only marginally (2 mM Mg(2+) by 6%, 2.5 microM Ca(2+) by 7%, and 4 microM Zn(2+) by 11%). Given that no precipitation is possible at concentrations below C(sat), we infer that coprecipitant(s), and not properties of Abeta(1-40) alone, are key factors in the in vivo aggregation of Abeta.     

Influence of DNA structures on the conversion of sanguinarine alkanolamine form to iminium form

Sanguinarine exhibits pH dependent structural equilibrium between iminium form (structure I) and alkanolamine form (structure II) with a pKa of 7.4 as revealed from spectrophotometric titration. The titration data show that the compound exists almost exclusively as structure I and structure II in the pH range 1 to 6 and 8.5 to 11, respectively. The interaction of structure I and structure II to several B-form natural and synthetic double and single stranded DNAs has been studied by spectrophotometric, spectrofluorimetric and circular dichroic measurements in buffers of pH 5.2 and pH 10.4 where the physicochemical properties of DNA remain in B-form structure. The results show that structure I bind strongly to all B-form DNA structures showing typical hypochromism and bathochromism of the alkaloid's absorption maximum, quenching of steady-state fluorescence intensity and perturbations in circular dichroic spectrum. The structure II does not bind to DNA, but in presence of large amount of DNA significant population of structure I is generated, which binds to DNA and forms a structure I-DNA intercalated complex. The nature and magnitude of the spectral pattern are very much dependent on the structure as well as base composition of each DNA. The generation of the structure I from structure II is significantly affected by increasing ionic strength of the medium. The conversion of structure II to structure I in presence of high concentration of DNA in solution is explained through formation of a binding equilibrium process between structure II and structure I-DNA intercalated complex.     

Differential desensitization and internalization of three different bullfrog gonadotropin-releasing hormone receptors

We previously demonstrated the presence of three distinct types of the gonadotropin-releasing hormone receptor (GnRHR) in a bullfrog (denoted bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3). The bfGnRHRs exhibited differential tissue distribution and ligand selectivity. In the present study, we demonstrated the desensitization and internalization kinetics of these receptors in both transiently-transfected HEK293 cells and retrovirus-mediated stable cells. The time-course accumulation of the inositol phosphate in response to GnRH revealed that bfGnRHR-1 and -2 were rapidly desensitized, whereas bfGnRHR-3 was slowly desensitized. A comparison of the internalization kinetics revealed the most rapid rate and highest extent of internalization of bfGnRHR-2 among the three receptors. Interestingly, the mechanisms that underlie the receptor internalization appear to differ from each other. Internalization of bfGnRHR-1 was dependent on both dynamin and beta-arrestin, whereas those of bfGnRHR-2 and -3 were dependent on dynamin, but not on arrestin. These results, therefore, suggest that differential regulatory mechanisms for desensitization and internalization of the GnRHR are involved in diverse cellular and physiological responses to GnRH stimulation.     

Lithium action on adrenomedullary and adrenocortical functions and serum ionic balance in different age-groups of albino rats

The aim of the current study was to investigate lithium action on adrenomedullary and adrenocortical functions and on serum ionic balance in rats. Three age-groups of male rats (juvenile: 30 days, adult: 100 days and aged: 3 years) were used. Each age-group of animal was exposed to short- (10 days) and long-term (25 days) treatments with lithium. Each age-group of rat received lithium at a dose 2mEq/kg body weight daily for 10 and 25 days. Each daily dose (2mEq) was divided equally into half (1 mEq) and each half was injected intraperitoneally twice (at 9 am and 9 pm) for both the durations of experiments. Control animals received physiological saline for similar duration of experiments. Thirty animals were used for each age-group and they were divided equally into 6 groups with 5 each. After termination of all the experiments rats were sacrificed and, adrenal glands were quickly dissected out and processed for epinephrine, norepinephrine and corticosterone estimations and, 3 beta-hydroxysteroid dehydrogenase (3 beta-HSDH) activity of the adrenal gland. Blood was drawn from the heart of each rat and, serum was collected and stored at -20 degrees C until assayed for lithium, calcium, sodium, potassium and corticosterone concentrations. The findings revealed that lithium in both short- and long-term treatments was maintained well within the therapeutic range (0.3-0.8 mEq/l) in all the age-groups of rats. This alkali metal caused depletions of both epinephrine and norepinephrine concentrations from adrenal glands, and elevations of corticosterone in both adrenal and blood serum of each age-group of rat (juvenile, adult and aged). Additionally adrenal 3beta-HSDH activity was also increased in all the age-groups of rats irrespective of duration of the treatments. Short-term treatment of lithium elevated only serum K+ level in juvenile and adult rats and, Ca+ level only in adult animals. Significant elevations of serum K+ and Ca+ levels were observed following long-term treatments of lithium in all the age group of rats. No significant change in serum Na+ level was recorded after lithium treatment, irrespective of duration of treatments, in any age-group of rats. The findings suggest that lithium action, in respect of adrenomedullary and adrenocortical functions and, serum ionic balance, may not be largely related to the age-group of rats and that, lithium acts on adrenomedullary activity probably by stimulating the release mechanism of epinephrine and norepinephrine from the adrenal gland of rats, but stimulates adrenocortical activity by stimulating both synthesis (including 3 beta-HSDH activity) and release of corticorterone. Simultaneously, lithium disturbs normal ionic balance by elevating K+ and Ca+ levels in all the age-group of rats. Thus, the antimanic drug certainly disturbs both adrenomedullary and adrenocortical functions and, serum ionic balance in all the age-group of rats.