Active Site Mutations in Yeast Protein Disulfide Isomerase Cause Dithiothreitol Sensitivity and a Reduced Rate of Protein Folding in the Endoplasmic Reticulum

Aspects of protein disulfide isomerase (PDI) function have been studied in yeast in vivo. PDI contains two thioredoxin-like domains, a and a′, each of which contains an active-site CXXC motif. The relative importance of the two domains was analyzed by rendering each one inactive by mutation to SGAS. Such mutations had no significant effect on growth. The domains however, were not equivalent since the rate of folding of carboxypeptidase Y (CPY) in vivo was reduced by inactivation of the a domain but not the a′ domain. To investigate the relevance of PDI redox potential, the G and H positions of each CGHC active site were randomly mutagenized. The resulting mutant PDIs were ranked by their growth phenotype on medium containing increasing concentrations of DTT. The rate of CPY folding in the mutants showed the same ranking as the DTT sensitivity, suggesting that the oxidative power of PDI is an important factor in folding in vivo. Mutants with a PDI that cannot perform oxidation reactions on its own (CGHS) had a strongly reduced growth rate. The growth rates, however, did not correlate with CPY folding, suggesting that the protein(s) required for optimal growth are dependent on PDI for oxidation. pdi1-deleted strains overexpressing the yeast PDI homologue EUG1 are viable. Exchanging the wild-type Eug1p C(L/I)HS active site sequences for C(L/I)HC increased the growth rate significantly, however, further highlighting the importance of the oxidizing function for optimal growth.     

Ethanol Specifically Inhibits NMDA Receptors with Affinity for Ifenprodil in the Low Micromolar Range in Cultured Cerebellar Granule Cells

Abstract: The effect of ethanol on the intracellular Ca²⁺ concentration response to NMDA in rat cerebellar granule cells grown in low or high KCI concentrations has been studied using image analysis. The cells grown in low KCI displayed high sensitivity for glycine. The subtype-selective antagonist ifenprodil inhibited the response with high (in the low micromolar range) and low (in the high micromolar range) potency. Ethanol affected the high-potency component in these cultures. In cells grown in high KCI the glycine sensitivity was lower, and a low potency for ifenprodil (high micromolar) dominated. These cells were not significantly sensitive to ethanol. The results indicate that the component displaying potency for ifenprodil in the low micromolar range with properties of the NR2B subunit is the target for ethanol action on the NMDA receptor.     

Quantitative assessment of pair formation behavior in captive whooping cranes (Grus americana)

Instantaneous scan sampling for mean distance and synchronous action patterns and all-occurrence sampling for unison call, dance, strut, and hoover-up behaviors were conducted for five potential whooping crane pairs at Patuxent Environmental Science Center, Laurel, Maryland. Dance, strut, and hoover-up differed among pairs, as did total frequency of social behaviors. It was unclear whether or not total frequency of social behaviors during pair formation can be used as an index for potential breeding success. The relative importance of different action patterns should be used as indices of pair compatibility in captive whooping cranes. © 1995 Wiley-Liss, Inc.     

Dynamic structure of metallothionein

Molecular sieve chromatography of rabbit liver metallothionein at different electrolyte concentrations revealed that this protein undergoes an increase in Stokes radius from 1.50 to 1.78 nm when the ionic strength is lowered from 0.5 to 0.015 indicating a change in molecular shape and/or hydration. The variation in ionic strength also affects the far-UV circular dichroism of metallothionein reflecting a conformational transition in the protein. The effects are attributed to changes in intramolecular repulsion between the strongly negatively charged metal-thiolate clusters of the protein. It is suggested that metallothionein exists in at least two interchangeable conformational states which differ in hydrodynamic properties and whose equilibrium concentrations are determined by the electrostatic free energy of the system.     

The enzymatic malonylation of 1-aminocyclopropane-1-carboxylic acid in homogenates of mung-bean hypocotyls

Homogenates of hypocotyls of light-grown mung-bean (Vigna radiata (L.) Wilczek) seedlings catalyzed the formation of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC) from the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and malonyl-coenzyme A. Apparent K_m values for ACC and malonyl-CoA were found to be 0.17 mM and 0.25 mM, respectively. Free coenzyme A was an uncompetitive inhibitor with respect to malonyl-CoA (apparent K_i=0.3 mM). Only malonyl-CoA served as an effective acyl donor in the reaction. The d-enantiomers of unpolar amino acids inhibited the malonylation of ACC. Inhibition by d-phenylalanine was competitive with respect to ACC (apparent K_i=1.2 mM). d-Phenylalanine and d-alanine were malonylated by the preparation, and their malonylation was inhibited by ACC. When hypocotyl segments were administered ACC in the presence of certain unpolar d-amino acids, the malonylation of ACC was inhibited while the production of ethylene was enhanced. Thus, a close-relationship appears to exist between the malonylation of ACC and d-amino acids. The cis- as well as the trans-diastereoisomers of 2-methyl- or 2-ethyl-substituted ACC were potent inhibitors of the malonyltransferase. Treatment of hypocotyl segments with indole-3-acetic acid or CdCl₂ greatly increased their content of ACC and MACC, as well as their release of ethylene, but had little, or no, effect on their extractable ACC-malonylating activity.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - MACC 1-(malonylamino)-cyclopropane-1-carboxylic acid Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

X-inactivation patterns in lymphocytes and skin fibroblasts of three cases of X-autosome translocations with abnormal phenotypes

Summary X-inactivation patterns were studied by replication analyses both in lymphocytes and skin fibroblasts of two patients carrying balanced X-autosome translocations, t(X;10)-(pter;q11) and t(X;17)(q11;q11), and one patient with an unbalanced translocation t(X;22)(p21;q11). Preferential late replication of the normal X chromosome was found in lymphocytes of both patients carrying balanced translocations and in skin fibroblasts of the patient carrying the translocation t(X;17). However, skin fibroblasts of the patient with a translocation t(X;10) showed preferential late replication of the abnormal der(X) chromosome with no spreading of late replication to the autosomal segment. In the case of unbalanced translocation t(X;22) there was preferential late replication of the der(X) chromosome both in lymphocytes and skin fibroblasts. The abnormal phenotype of the patients is discussed in relation to the observed X-inactivation patterns and the variability of the patterns in different tissues.     

Replication of poly dA and poly rA by a Drosophila DNA polymerase

The activity of a 7.3S-8.3S Drosophila DNA polymerase was characterized in detail using poly dA.p(dT)[unk] and poly rA.p(dT)[unk]. With poly dA.p(dT)[unk], Mg(2+) ion was the preferred divalent cation, and enzyme activity was inhibited by K(+) ion and by spermidine. With poly rA.p(dT)[unk], Mn(2+) ion was the preferred divalent cation and enzyme activity was stimulated by K(+) ion and by spermidine. The dependence of enzyme activity on the concentration of primer-template and on the ratio of primer to template was the same in both reactions. The two enzyme activities were identically inhibited by N-ethylmaleimide. Poly dA was replicated extensively and poly rA was replicated partially. The activation energy for poly dA replication was twice that for poly rA replication. Enzyme activity with poly dA.p(dT)[unk] was more stable to thermal inactivation than was enzyme activity with poly rA.p(dT)[unk]. These studies suggest that the same enzyme responds to both the deoxy- and the ribohomopolymer template but that the mechanisms of replication may be different.