De novo and maintenance DNA methylation by a mouse plasmacytoma cell DNA methyltransferase

A DNA methyltransferase of Mr = 140,000 that is active on both unmethylated and hemimethylated DNA substrates has been purified from the murine plasma-cytoma cell line MPC 11. The maximal rate of methylation was obtained with maintenance methylation of hemimethylated Micrococcus luteus or M13 DNAs. At low enzyme concentrations, the highest rate of de novo methylation occurred with single-stranded DNA or relatively short duplex DNA containing single-stranded regions. Strong substrate inhibition was observed with hemimethylated but not unmethylated DNA substrates. Fully methylated single-stranded M13 phage DNA inhibited neither the de novo nor the maintenance reactions, but unmethylated single-stranded M13 DNA strongly inhibited the maintenance reaction. The kinetics observed with hemimethylated and single-stranded substrates could be explained if the enzyme were to bind irreversibly to a DNA molecule and to aggregate if present in molar excess. Such aggregates would be required for activity upon hemimethylated but not single-stranded DNA. For de novo methylation of duplex DNA, single-stranded regions or large amounts of methyltransferase appear to be required. The relative substrate preference for the enzyme is hemimethylated DNA greater than fully or partially single-stranded DNA greater than fully duplex DNA.     

Identification of the sites on rabbit skeletal muscle protein phosphatase inhibitor-2 phosphorylated by casein kinase-II

Inhibitor-2 was phosphorylated by casein kinase-II in vitro at a rate similar to that of glycogen synthase, a physiological substrate of this protein kinase. The major phosphorylation sites were identified as serines-86, -120 and -121, the peptide containing serines-120 and -121 being labelled about 2.5-fold more rapidly than that containing serine-86. The 13 residues C-terminal to serine-121 (SGEEDSDLSPEERE) contain seven acidic amino acids, while the six residues following serine-86 (SDTETTE) contain three. These results are consistent with the known specificity requirements of casein kinase-II. The three serines are C-terminal to the threonine (residue 72) whose phosphorylation by glycogen synthase kinase-3 is potentiated by prior phosphorylation with casein kinase-II. This reinforces the view that a C-terminal phosphoserine residue is important for the specificity of glycogen synthase kinase-3. Identification of the residues phosphorylated by casein kinase-II will facilitate further studies on the in vivo phosphorylation state of inhibitor-2.     

The protein phosphatases involved in cellular regulation. Primary structure of inhibitor-2 from rabbit skeletal muscle

The complete primary structure of inhibitor-2, a specific inhibitor of protein phosphatase-1, has been determined. The protein consists of a single polypeptide chain of 203 residues, and has a relative molecular mass of 22835 Da. This molecular mass is significantly lower than earlier estimates based on sodium dodecyl sulphate polyacrylamide gel electrophoresis. The threonyl residue phosphorylated by glycogen synthase kinase-3 is located at position 72. The molecule is very hydrophilic, lacks cysteine residues and the single tryptophanyl and phenylalanyl residues are at positions 46 and 139, respectively. The N-terminal alanyl residue is N-acetylated. Digestion with Staphylococcus aureus V8 proteinase, trypsin, or cleavage with cyanogen bromide, destroyed the biological activity of inhibitor-2, demonstrating that many large fragments (e.g. 1-49, 49-92, 67-101, 108-134, 142-182 and 163-197) are inactive. Digestion with clostripain generated a peptide comprising residues 25-114 which retained 2% of the inhibitory potency of the parent molecule. There is no sequence homology between inhibitor-2 and inhibitor-1.     

Phosphorylase a is an allosteric inhibitor of the glycogen and microsomal forms of rat hepatic protein phosphatase-1

The dephosphorylation of glycogen synthase by protein phosphatase-1 in hepatic glycogen and microsomes was inhibited by nanomolar concentrations of phosphorylase a. The I50 for phosphorylase a was 1000-fold lower than its Km as a substrate, while tryptic digestion increased the I50 1000-fold without affecting Km. Protein phosphatase-1 from skeletal muscle and protein phosphatase-2A from liver were only inhibited at 1000-fold higher concentrations. Protein phosphatase-1 became desensitized to phosphorylase a when released from hepatic microsomes, but sensitivity was partially restored by readdition of the solubilized enzyme to the microsomes. The results demonstrate that phosphorylase a is a potent allosteric inhibitor of hepatic protein phosphatase-1 and suggest that inhibition may be conferred by a novel phosphorylase a-binding subunit.     

Inherited X-chromosome inverted tandem duplication in a male traced to a grandparental mitotic error.

A male infant was referred for cytogenetic evaluation because of dysmorphic features and developmental delay. In both lymphocytes and skin fibroblasts, a modal number of 46 chromosomes was obtained with an obvious elongation of the long arm of the X chromosome (Xq+). Studies of seven members in 3 generations of this family showed that the proband's mother, sister, and maternal grandmother were phenotypically normal carriers of this abnormal X chromosome. High resolution GTG- and RBG-banding defined the extra chromatin material as an inverted duplication of Xq21----Xq24. This was supported by an approximate twofold increase in alpha-galactosidase A activity, localized to Xq21----q24, observed in the proband's lymphocytes and fibroblasts. BrdU-incorporation studies of the mother's lymphocytes showed the abnormal X to be late replicating in all 100 cells studied and normal alpha-galactosidase A levels. Cytogenetic analysis of the maternal grandmother revealed cytogenetic mosaicism with one cell line containing the abnormal X (37%), and the other, a normal female karyotype (63%). This family is instructive since: (1) it represents only the second case of a dysmorphic male demonstrating a confirmed interstitial partial Xq duplication, and (2) the origin of this familial structural rearrangement has been traced to a grandparental mitotic error.     

Activating and 'anxiogenic' effects of corticotropin releasing factor are not inhibited by blockade of the pituitary-adrenal system with dexamethasone

Central administration of corticotropin releasing factor (CRF) in rats produces pituitary-adrenal activation and a variety of "anxiogenic-like" effects. The present study was designed to explore the contribution of the peripheral pituitary-adrenocortical axis in mediating these CRF responses. Intraventricularly administered CRF produced suppression of responding in the conflict test and a marked locomotor activation. Neither behavioral effect was altered by the prior administration of dexamethasone in a dose that blocked pituitary-adrenal activation to CRF. These results support the hypothesis that behavioral effects of CRF are mediated by its action at central sites and not via an action on the pituitary-adrenocortical system.     

Biochemistry of fern spore germination: protease activity in ostrich fern spores

Protease activities were detected in quiescent and germinating spores of the ostrich fern (Matteuccia struthiopteris [L.] Todaro). Peak endopeptidase, aminopeptidase, and carboxypeptidase activities were detected 12 to 24 hours after spores began imbibing under light. There was a correlation between activities of proteases, the onset of a decline in levels of soluble protein, and an increase in levels of free amino acids. The earliest visible event of spore germination, breakage of the spore coat and protrusion of a rhizoid cell, was observed after peak protease activity, 48 to 72 hours after the start of imbibition. Results of this study demonstrate similarities in the pattern of protease activities during germination of ostrich fern spores to those of some seeds.     

Kinetics of the interaction of N-(phosphonacetyl)-L-aspartate with the catalytic subunit of aspartate transcarbamoylase. A slow conformational change subsequent to binding

The binding of the bisubstrate ligand N-(phosphonacetyl)-L-aspartate (PALA) to the active sites of both the free catalytic subunit of aspartate transcarbamoylase and the intact holoenzyme causes conformational changes which have been studied extensively. However, no kinetic information has been available about the sequence of events occurring during the formation or dissociation of the complexes. Stopped flow kinetics, 31P saturation transfer NMR spectroscopy, and presteady-state kinetics were used to monitor the interaction of PALA with the catalytic subunit (or a derivative containing nitrotyrosyl chromophores which served as spectral probes). The various experimental approaches lead to a mechanism that includes a rapid binding of PALA with an "on" rate of about 10(8)M-1s-1 and an "off" rate of 28 s-1, followed by a much slower isomerization of the complex with a forward rate constant of 0.18 s-1. Analysis of the presteady-state bursts of enzyme activity when the protein is added to a mixture of substrates and PALA and of the lag in activity when the PALA complex with catalytic subunit is added to substrates yielded a rate constant for the reverse isomerization of 0.018s-1. Thus, the conformational change subsequent to PALA binding leads to a 10-fold increase in the equilibrium constant for complex formation. Stopped flow kinetic measurements of the spectral change resulting from mixing the complex of PALA and nitrated protein with native enzyme showed a slow process with a t1/2 of about 11 s, whereas 31P saturation transfer NMR experiments yielded at t1/2 of about 260 ms for the dissociation of PALA from the complex. This apparent disparity is understood in terms of the two-step binding scheme where rapid dissociation of the initial ligand X enzyme complex is measured by the NMR technique and the slow isomerization of the complex is responsible for the bulk of the stopped flow signal.     

Rapid declines in cyclic GMP of rod outer segments of intact frog photoreceptors after illumination

Considerable disagreement has resulted from experiments designed to test whether light-induced falls in cGMP in outer segment (OS) of photoreceptors precede their light-induced electrical responses. Different studies have reported initial declines at 50 ms, at s, or not at all for physiological stimuli. Such studies have employed whole retinas, isolated rod OS, or isolated rod OS with attached inner segments and involved a variety of techniques. We developed an apparatus that illuminates intact pieces of dark-adapted frog retinas at 22 degrees C for known brief durations and then rapidly (47 ms) presses their OS surface against a copper mirror cooled by liquid helium. Freezing occurs in less than 2 ms. Cyclic GMP was then assayed in cryostat sections of the OS layer. Six illumination intensities that bleached from 90 to 9 X 10(8) rhodopsin molecules per s were delivered for durations of 0.1-2 s. Compared to dark-adapted values, progressive losses of cGMP were seen with all illumination intensities. Because a significant loss in cGMP was seen after a 100 ms exposure to our dimmest stimulus, it appears that a loss of cGMP could play a role in rod visual transduction.     

Location of ligand-binding sites on the nicotinic acetylcholine receptor alpha-subunit

The portions of the Torpedo californica nicotinic acetylcholine receptor (AChR) alpha-subunit that contribute to the allosteric antagonist-binding site and to the agonist-binding site have been localized by affinity labeling and proteolytic mapping. [3H]Meproadifen mustard was employed as an affinity label for the allosteric antagonist-binding site and [3H]tubocurare as a photoaffinity label for the agonist-binding site. Both labels were found in a 20-kDa proteolytic fragment generated from the AChR alpha-subunit by Staphylococcus aureus V8 protease. This 20-kDa peptide also contains the 3H-labeled 4-(N-maleimido)-alpha-benzyltrimethylammonium iodide-reactive site and binds 125I-alpha-bungarotoxin. N-terminal sequencing established that the 20-kDa fragment began at Ser-173 of the alpha-subunit. Fluorescein isothiocyanate-conjugated concanavalin A could be bound to the second of the two major V8 cleavage products, an 18-kDa peptide. This peptide was also sensitive to treatment with endo-beta-N-acetyl-glucosaminidase H, consistent with the presence of N-linked carbohydrate on this fragment. The N terminus of this peptide was found to be Val-46 of the alpha-subunit sequence. Experiments designed to map disulfide bonds within the AChR alpha-subunit indicate that no bonds exist between the 18-kDa fragment (containing Cys-128 and Cys-142) and the 20-kDa fragment (containing Cys-192, Cys-193, and Cys-222). These results establish that the 20-kDa fragment contributes to both the acetylcholine and the allosteric antagonist-binding sites, whereas there is no evidence that the 18-kDa fragment is part of either site.