Probing novel elements for protein splicing in the yeast Vma1 protozyme: a study of replacement mutagenesis and intragenic suppression

Protein splicing is a compelling chemical reaction in which two proteins are produced posttranslationally from a single precursor polypeptide by excision of the internal protein segment and ligation of the flanking regions. This unique autocatalytic reaction was first discovered in the yeast Vma1p protozyme where the 50-kD site-specific endonuclease (VDE) is excised from the 120-kD precursor containing the N- and G-terminal regions of the catalytic subunit of the vacuolar H(+)-ATPase. In this work, we randomized the conserved valine triplet residues three amino acids upstream of the C-terminal splicing junction in the Vma1 protozyme and found that these site-specific random mutations interfere with normal protein splicing to different extents. Intragenic suppressor analysis has revealed that this particular hydrophobic triplet preceding the C-terminal splicing junction genetically interacts with three hydrophobic residues preceding the N-terminal splicing junction. This is the first evidence showing that the N-terminal portion of the V-ATPase subunit is involved in protein splicing. Our genetic evidence is consistent with a structural model that correctly aligns two parallel beta-strands ascribed to the triplets. This model delineates spatial interactions between the two conserved regions both residing upstream of the splicing junctions.

Site-directed mutagenesis of maize recombinant C4-pyruvate,orthophosphate dikinase at the phosphorylatable target threonine residue

A key regulatory enzyme of the C4-photosynthetic pathway is stromal pyruvate,orthophosphate dikinase (PPDK, EC 2.7.9.1). As a pivotal enzyme in the C4 pathway, it undergoes diurnal light-dark regulation of activity which is mediated by a single bifunctional regulatory protein (RP). RP specifically inactivates PPDK in the dark by an ADP-dependent phosphorylation of an active-site Thr residue (Thr-456 in maize). Conversely, RP activates inactive PPDK in the light by phosphorolytic dephosphorylation of this target Thr-P residue. We have employed a His-tagged maize recombinant C4 PPDK for directed mutagenesis of this active-site regulatory Thr. Three such mutants (T456V, T456S, T456D) were analyzed with respect to overall catalysis and regulation by exogenous maize RP. Substitution with Val and Ser at this position does not affect overall catalysis, whereas Asp abolishes enzyme activity. With respect to regulation by RP, it was found that Ser can effectively substitute for the wild-type Thr residue in that mutant enzyme is phosphorylated and inactivated by RP. The T456V mutant, however, could not be phosphorylated and was, thus, resistant to ADP-dependent inactivation by RP.

The carboxyl-terminal valine residues of proTGF alpha are required for its efficient maturation and intracellular routing

Soluble forms of transforming growth factor-alpha (TGF alpha) are derived by proteolytic processing of an integral membrane glycoprotein precursor (pro TGF alpha). Previous studies indicated that phorbol ester-induced cleavage of pro TGF alpha in CHO cells is dependent on the presence of a valine residue located at the carboxyl terminus of the precursor's cytoplasmic domain. We reassessed this requirement with epitope-tagged constructs introduced into transformed rat liver epithelial cells that normally express and process TGF alpha. We found that pro TGF alpha mutants lacking the terminal valine residues showed greatly reduced maturation to the fully glycosylated form. Additionally, they were present at substantially reduced levels on the cell surface and, instead, accumulated in the endoplasmic reticulum. Consistent with these results, enzyme-linked immunosorbent assay (ELISA) and Western blot analyses revealed little or no soluble TGF alpha in medium conditioned by cells expressing the mutant constructs. Finally, a truncated pro TGF alpha mutant lacking most of the cytoplasmic domain but retaining a carboxyl-terminal valine was processed and cleaved in a near-normal manner. These results, some of which were reproduced in CHO cells, indicate that the predominant effect of the carboxyl-terminal valines is to ensure normal maturation and routing of the precursor.

Replacement of Val674 by Pro increases the sensitivity of the plasma membrane Ca2+ pump to inhibition by Mg2+

A cDNA encoding a plasma membrane Ca2+ pump mutant V674P(ct120) was constructed and expressed in COS-1 cells. Immunoblots of transfected COS-1 membranes showed that the V674P(ct120) and the wild-type hPMCA4b(ct120) proteins were expressed at similar levels. The change of Val674 to Pro reduced the activity of the hPMCA4b(ct120) to an extent similar to that observed previously in the full-length Ca2+ pump (Adamo et al. (1995) J. Biol. Chem. 270, 30111-30114). Despite its lower activity, the apparent affinity for Ca2+ of the V674P(ct120) enzyme was at least as high as that of hPMCA4b(ct120), indicating that substitution of Val674 by Pro did not impair the interaction of the enzyme with Ca2+. The sensitivity of the V674P(ct120) enzyme to inhibition by vanadate was not significantly different from that of the hPMCA4b(ct120), supporting the idea that the mutation did not alter the equilibrium between E2-E1. The study of the Mg2+ dependency of the Ca2+ transport showed that the V674P(ct120) mutant reached maximum activation at 100 microM Mg2+ in contrast with 500 microM in the hPMCA4b(ct120). Furthermore, while at 2 mM Mg2+ the hPMCA4b(ct120) showed no sign of inhibition, the activity of the mutant decreased to less than 50% of the maximum activity observed at 100 microM Mg2+. These results indicate that the decrease in the activity observed upon substitution of Val674 by Pro was due to a higher sensitivity to Mg2+ as inhibitor.

Identification of the functional importance of valine-19 residue in streptokinase by N-terminal deletion and site-directed mutagenesis

Streptokinase (SK) is a bacterial plasminogen activator of multi-domain structure. In deletion analysis of the N-terminal region of SK, the deletion of 20 amino acids (SK delta N20) resulted in the dramatic reduction of plasminogen activator activity compared to deletion of 7 (SK delta N7) and 13 amino acids (SK delta N13). The incubation time to reach maximal active site generation in an equimolar mixture of SK delta N20 and plasminogen was the same as that for wild-type SK. To identify the functional residues important in plasminogen activation, several site-directed mutations were introduced at the region spanning Ser16-Val20 of SK. The results showed that Val19 residue is important for the activity of the SK-plasminogen complex.

Two independent domains of hDlg are sufficient for subcellular targeting: the PDZ1-2 conformational unit and an alternatively spliced domain

hDlg, a human homologue of the Drosophila Dig tumor suppressor, contains two binding sites for protein 4.1, one within a domain containing three PSD-95/Dlg/ZO-1 (PDZ) repeats and another within the alternatively spliced I3 domain. Here, we further define the PDZ-protein 4.1 interaction in vitro and show the functional role of both 4.1 binding sites in situ. A single protease-resistant structure formed by the entirety of both PDZ repeats 1 and 2 (PDZ1-2) contains the protein 4.1-binding site. Both this PDZ1-2 site and the I3 domain associate with a 30-kD NH2-terminal domain of protein 4.1 that is conserved in ezrin/radixin/moesin (ERM) proteins. We show that both protein 4.1 and the ezrin ERM protein interact with the murine form of hDlg in a coprecipitating immune complex. In permeabilized cells and tissues, either the PDZ1-2 domain or the I3 domain alone are sufficient for proper subcellular targeting of exogenous hDlg. In situ, PDZ1-2-mediated targeting involves interactions with both 4.1/ERM proteins and proteins containing the COOH-terminal T/SXV motif. I3-mediated targeting depends exclusively on interactions with 4.1/ERM proteins. Our data elucidates the multivalent nature of membrane-associated guanylate kinase homologue (MAGUK) targeting, thus beginning to define those protein interactions that are critical in MAGUK function.

Evidence for a three-state model of protein folding from kinetic analysis of ubiquitin variants with altered core residues

To elucidate the kinetic importance of structural intermediates in single-domain proteins, we measured the effect of solution conditions and amino-acid changes at a central core residue of ubiquitin (Val 26) on the kinetics of folding and unfolding. Kinetic analysis in terms of a sequential three-state mechanism provides insight into the contribution of specific interactions within the ubiquitin core to the structural stability of the native and intermediate states. The observations that disruptive mutations and/or addition of denaturants result in an apparent two-state folding process with slower rates is explained by the destabilization of a partially folded intermediate, which is in rapid equilibrium with unfolded states. The model predicts that under sufficiently stabilizing conditions kinetic intermediates may become populated even for proteins showing apparent two-state kinetics.

Valine requirement of the finishing pig

Pigs weighing approximately 70 kg were used in two experiments to determine the valine requirement during the finishing period. In the first experiment, 10 gilts were allotted in two 5 x 5 Latin square designs to five semipurified diets that ranged in valine concentration from .35 to .55%. Urinary urea excretion was measured during each of the 3-d periods of the Latin square. Urea N excretion in relation to N intake and to creatinine N excretion was minimized (quadratic effect, P < .10) at valine concentrations of .45 to .50%. In Exp. 2, 36 barrows and 36 gilts were fed one of six diets containing .35 to .60% valine. The highest weight gains (not significant) and feed efficiencies (quadratic effect, P < .05) were achieved by the pigs that consumed .45% valine. Plasma urea concentration at the end of the experiment was lowest (quadratic effect, P < .05) in pigs that consumed .50% valine. Estimates of the valine requirement based on breakpoint and quadratic equation analyses ranged from .40 to .50% total valine (.33 to .43% ileal digestible valine). Pigs in Exp. 2 consumed approximately 2.5 kg/d (8,850 kcal/d of ME). Thus, the estimate of the valine requirement is approximately 11 g/d. These estimates of the valine requirements of finishing pigs are slightly higher than the National Research Council requirements when expressed as a percentage of the diet but are similar when expressed on a grams per day basis.

An essential role for Rac in Ras transformation

The GTPase Rac1 is a key component in the reorganization of the actin cytoskeleton that is induced by growth factors or oncogenic Ras1. Here we investigate the role of Rac1 in cell transformation and show that Rat1 fibroblasts expressing activated Val-12 Rac1 (Rac1 with valine at residue 12) display all the hallmarks of malignant transformation. In a focus-forming assay in NIH3T3 fibroblasts to measure the efficiency of transformation, we found that dominant-negative Asn-17 Rac1 inhibited focus formation by oncogenic Ras, but not by RafCAAX, a Raf kinase targeted to the plasma membrane by virtue of the addition of a carboxyterminal localization signal from K-Ras. This indicates that Rac is essential for transformation by Ras. In addition, Val-12 Rac1 synergizes strongly with RafCAAX in focus-formation assays, indicating that oncogenic Ras drives both the Rac and MAP-kinase pathways, which cooperate to cause transformation.

A possible role of two hydrophobic amino acids in antigen recognition by synovial T cells in rheumatoid arthritis

Synovial T cells play a crucial role in the pathogenesis of rheumatoid arthritis (RA) synovitis. We have quantitatively analyzed the T cell receptor (TcR) variable (V) region gene repertoire of freshly isolated synovial fluid (SF) T cells, comparing it with that of peripheral blood (PB) T cells in RA. The TcR V gene repertoire of PB and SF T cells in RA and osteoarthritis was heterogeneous. In contrast, V alpha 11 in SF was expressed to a greater degree in three of five RA patients, and increased levels of V beta 6, 1-3 were found in the SF of four of six RA, compared with paired PB. Of note, V beta 6, 1-3 was universally used in four RA patients with a disease duration of less than 10 years, irrespective of their HLA-DR types. This was in contrast to two other RA patients, suffering for more than 20 years, who showed different V alpha and V beta usages. beta-chain sequence analysis in RA patients with a preference for V beta 6, 1-3 has shown that a few clones dominated in SF, whereas polyclonality was observed in PB. These findings suggest oligoclonal expansion of T cells in response to specific antigen(s) in the SF of these patients with RA of relatively short duration. Concomitant use of two hydrophobic amino acids, leucine and valine in the D beta region was noticeable among the predominant SF clones. These two amino acids might directly contact a peptide specific for the induction of synovitis in RA patients. TcR-directed therapy may, therefore, be useful for the treatment of early RA synovitis.

Cold-sensitive growth and decreased GTP-hydrolytic activity from substitution of Pro17 for Val in Era, an essential Escherichia coli GTPase

A substitution mutation of Pro17 by Val (P17V) was constructed in the guanine nucleotide binding domain of Era, an essential protein in Escherichia coli. The mutation is analogous to the oncogenic activating allele at position 12 in the GTP-binding domain of p21ras. The phenotype of this mutant was analysed in a strain which exclusively expressed the mutant protein (Era-V17) in null allele chromosomal background (era1: :kan). The strain was found to be cold-sensitive for growth. Mutant Era-V17 purified from the strain was cold-sensitive for GTP-hydrolytic activity, suggesting that the GTPase activity of Era is required for cell growth since the P17V mutation resulted in both cold-sensitive growth of cells and cold-labile GTPase activity of the purified protein.

Inhibition of protein synthesis transiently stimulates initiation of minichromosome replication in Escherichia coli

Replication of oriC-dependent minichromosomes was found to be transiently stimulated when protein synthesis was inhibited by the addition of chloramphenicol. Initiation of replication was also induced by amino acid starvation of relA mutant strains and a nutritional upshift. The results are explained on the basis that these treatments rendered RNA polymerase more available for participation in the initiation process. As a consequence, the oriC duplex may be transcriptionally activated to an open form, a necessary prerequisite for DNA polymerization.

GPA1Val-50 mutation in the mating-factor signaling pathway in Saccharomyces cerevisiae

The GPA1 gene of Saccharomyces cerevisiae encodes a protein that is highly homologous to the alpha subunit of mammalian hetrotrimeric G proteins and is essential for haploid cell growth. A mutation of the GPA1 protein, GPA1Val-50, in which Gly-50 was replaced by valine, could complement the growth defect of a GPA1 disruption, gpal::HIS3. However, cells with gpa1::HIS3 expressing the GPA1Val-50 protein were supersensitive to alpha-factor in a short-term incubation but resumed growth after long-term incubation even after exposure to high concentrations of alpha-factor. The former phenotype associated with GPA1Val-50 is recessive, and the latter phenotype is dominant to GPA1+. The supersensitivity of GPA1Val-50 to alpha-factor was dependent on STE2 and STE4, which demonstrates that this GPA1Val-50-produced phenotype requires the mating-factor receptor and the beta subunit of the G protein. The double mutant of sst2-1 GPA1Val-50 recovered from division arrest, which suggested that SST2 is not required for recovery of the GPA1Val-50 mutant.

[Interaction of angiotensin analogs and fragments with rat adrenal cell receptors]

The binding of some modified angiotensin (AT) analogs and fragments to isolated rat adrenal glomerular cells was studied by radioreceptor analysis with a view of clarifying the role of C- and N-terminal amino acids in the binding of AT molecules to cell receptors. It was demonstrated that Arg2 and Val3 residues are of great importance for effective binding of the AT molecule to cell receptors. The presence of a free C-terminal carboxylic group in position 8 in the vicinity of the bulky lipophilic residue is a necessary condition for this process. The Asp and Asn residues located in position 1 of the AT molecule are not essential for the binding of the hormone molecule to adrenal cell receptors.

Evidence for a specific role for cortical NMDA receptors in slow-wave sleep

Iontophoresis of the N-methyl-D-aspartic acid (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (2-APV) was found to suppress spontaneous bursting activity of layer V cortical neurones during stage 3/4 sleep in unrestrained, normally behaving rats. Iontophoresis of NMDA, on the other hand, increased cortical burst durations and increased the number of spikes per burst. 2-APV was found not to alter cells' responses to tactile stimulation or the generation of neuronal spindling activity during stage 2 sleep. These results provide the first evidence that NMDA receptors subserve a specific function in the neocortex of the behaving animal, by gene-rating burst activity in cortical neurones during stage 3/4 of the natural sleep state. The activation of NMDA/2-APV-sensitive cortical receptors by afferents from the anterior intralaminar nuclei in the generation of bursts by cortical cells during stage 3/4 sleep is discussed.

Dissimilar effects of L and D amino acids on the growth of Tetrahymena

Of the L and D configurations of four amino acids (phenylalanine, valine, tryptophan, tyrosine) tested for influence on the growth rate of Tetrahymena, only L-tyrosine was able to induce imprinting in Tetrahymena pyriformis Zeuthen. D-valine stimulated the division of T. pyriformis NT-1, but failed to induce imprinting. The experiments have substantiated the selectivity of the amino acid receptors of T. pyriformis, and the extraordinary imprinting potential of tyrosine as well, as judged by its influence on the growth rate.

Protein and amino acid requirements of fishes

Tentative qualitative and quantitative amino acid requirements have been reported for the major species of fish reared for market or as replacement stocks for natural waters. Most work has concentrated upon juvenile fish or upon rapidly growing young market fish; these have high protein dietary requirements (30-50%) that are in direct contrast to the homothermic terrestial animals. Net protein utilization from the diet is similar or slightly better than that found in avian species, but energy needs are much lower in fish and as a result the body protein deposition in fish is larger (about 5 g protein/MJ for the chick versus about 10 g protein/MJ for young fish). Qualitative amino acid requirements appear identical for all fish species examined; arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are all required for normal growth and metabolism. Quantitative requirements differ only slightly among species that have been tested. Salmon have higher arginine requirements than other fish examined. Catfish appear to have a lower requirement for histidine and threonine, and the Japanese eel seems to need more tryptophan in the diet. However, when the quantitative requirements for indispensable amino acids are expressed as a percentage of the protein fed, then a remarkable harmony appears between values needed for maximal growth for most species examined. A review of Tables 3 and 4 will disclose the paucity of information available considering the large number of fish species reared commercially over the world. Most commercial diet formulations have relied upon the work done on salmon, catfish, and carp, and their amino acid and protein requirement values have been used. Remarkably, these diets have produced other species of fish economically. Sparing effects of one amino acid on another have only been studied with cystine-methionine and tyrosine-phenylalanine. Arginine and analogues of methionine have been used as good nitrogen sources for salmon. Isoleucine-leucine ratios have been measured and experiments indicate some growth inhibition when the isoleucine-leucine ratio was greater than 2/1. Valine at abnormally high levels also inhibited growth. Much more work needs to be done on the effects of subtle differences in amino acid ratios in the diet, and major emphasis should be placed on the important role of the dispensable amino acids in fish nutrition.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation-produced spinal inhibition from the midbrain in the rat is mediated by an excitatory amino acid neurotransmitter in the medial medulla

It has been previously established that a bulbar relay plays an important role in descending inhibition of spinal dorsal horn nociceptive neurons and nociceptive reflexes produced by stimulation in the midbrain periaqueductal gray (PAG). In the present study, selected receptor antagonists were microinjected into the medial medullary nucleus raphe magnus (NRM) to determine whether descending inhibition of the tail flick (TF) reflex in the rat produced by focal electrical stimulation in the midbrain PAG was mediated by serotonin, opioid, or glutamate receptors on bulbospinal neurons in the NRM. It was determined in initial experiments that the serotonin receptor antagonist methysergide, the opioid receptor antagonist naloxone, the local anesthetic lidocaine, and the glutamate receptor antagonists gamma-D-glutamylglycine (DGG) and DL-2-amino-5-phosphonovalerate (APV) microinjected into the medulla all significantly increased the threshold of focal electrical stimulation in the medulla required to inhibit the TF reflex. The antinociceptive efficacy of agonists at opioid, serotonin, and glutamate receptors was also tested in other experiments. The microinjection of morphine (2.5-10 micrograms) into the NRM increased significantly TF latencies in a dose-dependent manner in rats in the awake or lightly anesthetized state; morphine was more potent in awake rats. Inhibition of the TF reflex produced by the microinjection of morphine was reversed by a subsequent microinjection of naloxone into the same site in the medulla. The microinjection of serotonin (5 and 10 micrograms), however, did not affect the latency of the TF reflex in either awake or lightly anesthetized rats. Glutamate (100 microM, 0.5 microliter) microinjected into the rostral ventral medulla produced an inhibition of the TF reflex of short duration that could be blocked or attenuated significantly by the glutamate receptor antagonists DGG or APV microinjected into the same site. In subsequent experiments, a nonspecific functional block was introduced adjacent to the NRM bilaterally in the medullary reticular formations (MRFs) by the microinjection of the local anesthetic lidocaine; receptor antagonists were then microinjected into the NRM and their effect on the threshold of focal electrical stimulation in the PAG to inhibit the TF reflex determined. No increase was seen in stimulation thresholds in the PAG following the microinjection of either methysergide or naloxone into the NRM. Following the microinjection of lidocaine, DGG or APV into the NRM, the stimulation threshold in the PAG for inhibition of the TF reflex was increased significantly.(ABSTRACT TRUNCATED AT 400 WORDS)

Hemoglobin La Desirade alpha A2 beta 2 129 (H7) Ala----Val: a new unstable hemoglobin

Hemoglobin La Desirade is a new unstable hemoglobin variant arising from the substitution of beta 129 (H7) Ala for Val. Hb La Desirade exhibits a low oxygen affinity and normal heme-heme interaction. The variant was found in two unrelated black families in association with Hb S, Hb C or beta o thalassemia.

Starvation for ilvB operon leader amino acids other than leucine or valine does not increase acetohydroxy acid synthase activity in Escherichia coli

Eleven different amino acids are encoded in the ilvB leader mRNA. Starvation for leucine or valine, but not for any of the other nine amino acids, resulted in high levels of acetohydroxy acid synthase I. These results are discussed in terms of a report (C.A. Hauser and G.W. Hatfield, Proc. Natl. Acad. Sci. U.S.A. 81:76-79, 1984) which suggests that threonine and alanine, in addition to leucine and valine, are involved in the regulation of the ilvB operon.

Valine requirement of the lactating sow

An experiment was conducted using production and metabolic criteria to estimate the valine requirement of the lactating sow. Ground corn, gelatin, corn sugar, vitamins, minerals and indispensable amino acids were used to formulate a diet containing all the essential nutrients except valine. L-valine was added to provide .23 (basal), .38, .53, .68 and .83% valine. Twenty-five mature Landrace x Yorkshire sows were randomly assigned to the five dietary treatments. Milk production and average pig weight gain increased quadratically (P less than .01) and were maximized at .68% dietary valine. Total solids production (p less than .05) and protein production (P less than .01) increased with increasing level of valine, and both were maximized at .68% dietary valine. Protein content also was maximized (P less than .01) at .68% dietary valine. Fecal (P less than .05) and urinary (P less than .01) nitrogen decreased with increasing valine, yielding greatest (P less than .01) nitrogen retention (excluding milk nitrogen) at .53% dietary valine. Because daily milk nitrogen production also increased (P less than .01) with increasing valine, there were no treatment differences (P greater than .05) in nitrogen balance (including milk nitrogen). Plasma valine concentration increased (P less than .01) gradually until .53% valine was fed, after which there was a rapid increase in concentration. Plasma isoleucine and leucine decreased (P less than .01) with increasing dietary valine. Plasma urea concentration was minimal (P less than .05) with .38% dietary valine. When all the criteria measured are considered, the lactating sow's requirement of valine is between .53 and .68%. The metabolic data support .53% dietary valine for the requirement, although the maximum response in production parameters at .68% dietary valine suggests that the requirement is above .53% dietary valine.