A mutation in the small (alpha) subunit of glycyl-tRNA synthetase affects amino acid activation and subunit association parameters

A strategy was designed to isolate mutants of glycyl-tRNA synthetase that are altered at the amino acid binding site, including a class with altered amino acid specificity. For this purpose, the plasmid pBR322 was mutated so that the codon (AGC) of the active site Ser-68 in the beta-lactamase gene was changed to the glycine codon GGC to inactivate the encoded enzyme. Suppressors that increase the amount of beta-lactamase activity of the Gly-68 allele of beta-lactamase were isolated and some mapped to the gene encoding glycyl-tRNA synthetase (glyS). While in vitro misaminoacylation of tRNA(Gly) with serine was not detected for any of the mutants, glycyl-tRNA synthetase activity was altered. One severely affected glyS mutant (N302) was studied in more detail. For this mutant, a single Pro-61----Leu substitution in the alpha chain confers an elevation of the Km values for glycine (25-fold) and for ATP (45-fold) in the aminoacylation reaction, but only a minor perturbation of the Km for tRNA. There also was a severely reduced adenylate synthesis activity (greater than 100-fold). In addition, a nonlinear dependence between aminoacylation activity and enzyme concentration was observed which implies that the alpha chain Pro-61----Leu mutation has disrupted the functionally essential subunit interactions of the holoenzyme. The results of the preceding paper have shown that the alpha chain and parts of the beta chain are required for aminoacylation and adenylate synthesis activity. The results of this study suggest that the alpha chain specifically contributes to amino acid and to ATP binding in a way that is affected by proper subunit interactions.     

Subunit topography of RNA polymerase from Escherichia coli. A cross-linking study with bifunctional reagents.

The quaternary structures of Escherichia coli DNA-dependent RNA polymerase holenzyme (alpha 2 beta beta' sigma) and core enzyme (alpha 2 beta beta') have been investigated by chemical cross-linking with a cleavable bifunctional reagent, methyl 4-mercaptobutyrimidate, and noncleavable reagents, dimethyl suberimidate and N,N'-(1,4-phenylene)bismaleimide. A model of the subunit organization deduced from cross-linked subunit neighbors identified by dodecyl sulfate-polyacrylamide gel electrophoresis indicates that the large beta and beta' subunits constitute the backbone of both core and holoenzyme, while sigma and two alpha subunits interact with this structure along the contact domain of beta and beta' subunits. In holoenzyme, sigma subunit is in the vicinity of at least one alpha subunit. The two alpha subunits are close to each other in holoenzyme, core enzyme, and the isolated alpha 2 beta complex. Cross-linking of the "premature" core and holoenzyme intermediates in the in vitro reconstitution of active enzyme from isolated subunits suggests that these species are composed of subunit complexes of molecular weight lower than that of native core and holoenzyme, respectively. The structural information obtained for RNA polymerase and its subcomplexes has important implications for the enzyme-promoter recognition as well as the mechanism of subunit assembly of the enzyme.     

Studies of the antigenic properties of the catalytic and glycoprotein subunits of Na+,K+-ATPase

Antibodies were raised against isolated, delipidated catalytic [alpha] and glycoprotein [beta] subunits of the Na+,K+-dependent ATPase purified from lamb kidney medulla. The specificity of each antiserum was confirmed by agar double-diffusion precipitation, immunoelectrophoresis, and polyacrylamide gel electrophoresis. A solid phase adsorption assay was also employed to determine antibody binding titers and to further test the specificity of these antisera. Antibodies raised to the alpha subunit had a strong reactivity and similar titer values for both the holoenzyme and the alpha subunit and a low-affinity cross-reactivity with the beta subunit. In contrast, beta-subunit-directed antibodies had little reactivity or binding with the holoenzyme and a low-affinity cross-reactivity with the alpha subunit. Competition binding studies revealed that about 80% of the alpha-subunit-specific antibodies bound to the holoenzyme, indicating that similar sets of antigenic sites are exposed in the lipid-embedded holoenzyme complex and in the isolated alpha subunit. Competition binding studies also suggest that the subunit cross-reactivities of the antisera may not result from simple contamination of the respective antigens, but that there may be partial homologies of some antigenic sites. In addition, the beta-directed antibodies had no effect on Na+,K+-ATPase activity, while the alpha-directed antibodies were effective inhibitors of activity. This indicates that at least some functionally important antigenic sites of the alpha subunit may be unaltered by its isolation and delipidation.     

Expression and purification of the alpha and beta subunits of Drosophila casein kinase II using a baculovirus vector.

Two recombinant baculoviruses that express the alpha and beta subunits of Drosophila melanogaster casein kinase II, respectively, have been constructed. The expressed proteins are similar to the authentic Drosophila subunits in size and are recognized by antisera raised against the Drosophila holoenzyme. Extracts derived from cells infected with the alpha subunit-expressing virus display elevated casein kinase II activity in vitro. This activity is markedly enhanced in extracts of cells infected with both viruses, or when alpha and beta subunit-containing extracts are mixed in vitro following lysis. Recombinant holoenzyme and the alpha subunit were purified to near homogeneity using phosphocellulose column chromatography. The specific activity of the purified recombinant holoenzyme was very similar to that of the native enzyme, and was fivefold higher than that of the purified free alpha subunit. The Stokes radius of the recombinant holoenzyme was estimated to be 50 A, a value similar to that reported for the native enzyme, whereas the alpha subunit demonstrated a Stokes radius of 26.5 A. Studies using sucrose density gradient centrifugation showed that, under conditions of high ionic strength, the quaternary structure of the purified holoenzyme was tetrameric (like the native enzyme), whereas the structure of the alpha subunit was monomeric. At lower ionic strength the recombinant holoenzyme had a significantly higher sedimentation coefficient, characteristic of the formation of filaments found for the native enzyme. Interestingly, the purified catalytic subunit also displayed a higher S value under conditions of low ionic strength, revealing the formation of alpha subunit aggregates.     

Modification of phenylalanyl-tRNA synthetase from baker's yeast by proteolytic cleavage and properties of the trypsin-modified enzyme.

Earlier studies have shown that native phenylalanyl-tRNA synthetase from baker's yeast contains two different kinds of subunits, alpha of molecular weight 73000 and beta of molecular weight 63000. The enzyme is an asymmetric tetramer alpha-2beta-2, which binds two moles of each ligand per mole. Incubation of the purified enzyme with trypsin results in an irreversible conversion: the alpha-subunit remains apparently unchanged but beta is rapidly degraded and yields a lighter species beta of molecular weight 41000. The trypsin-modified enzyme is an alpha-2beta-2 molecule which can still activate phenylalanine but cannot transfer it to tRNA-Phe; furthermore it does not bind tRNA-Phe but its kinetic parameters are identical to those of the native enzyme with respect to ATP and phenylalanine. Therefore the two beta subunits play a critical part in tRNA binding. Isolated alpha or beta subunits exhibit no significant activity and both types of subunit seem to be required for phenylalanine activation.     

On the binding of tRNA to Escherichia coli RNA polymerase.

The fixation of tRNA to Escherichia coli RNA polymerase has been investigated. Bound and free tRNA have been separated and quantified after filtration through cellulose nitrate filters, centrifugation or sucrose gradients or electrophoresis in polyacrylamide gels. We detect no differences between the fixation of E. coli fMet-tRNAfMet, Met-tRNAmMet or uncharged unfractionated tRNA to RNA polymerase. Tight complexes, with a long residence time, are formed between core enzyme and tRNA with a dissociation constant of less than 1 nM. Complexes exist between tRNA and both monomer and dimer forms of the core enzyme. In the monomer complex, one tRNA is bound per alpha 2 beta beta' unit, whereas in the dimer complex only 0.5 tRNA molecule is fixed per alpha 2 beta beta' unit. In contrast to the core enzyme, very little tRNA fixes tightly to the holoenzyme at salt concentrations greater than 80 mM. At lower salt concentrations tRNA fixation results in a loss of sigma subunit from the holo enzyme to the resulting core enzyme where it binds tightly. DNA fixation reduces the binding of tRNA to RNA polymerase and tRNA fixation reduces the binding of DNA. However, binding of DNA to polymerase is not competitive with binding of tRNA, and ternary complexes between RNA polymerase, DNA and tRNA are shown to exist. Our results are discussed in relation to other studies concerning the effects of tRNA upon RNA polymerase.     

Immunochemical comparison of cardiac glycoside-sensitive (lamb) and -insensitive (rat) kidney (Na+ + K+)-ATPase

The immunological cross-reactivity of the ouabain-sensitive lamb kidney and the ouabain-insensitive rat kidney (Na+ + K+)-ATPase (EC 3.6.1.37) was examined using polyclonal and monoclonal antibodies. Studies using rabbit antisera prepared against both the lamb kidney and rat kidney holoenzymes showed the existence of substantial antigenic differences as well as similarities between the holoenzymes and the respective denatured alpha and beta subunits of these two enzymes. Quantitation of the extent of cross-reactivity using holoenzyme-directed antibodies showed a 40-60% cross-reactivity. In addition, rabbit antisera monospecific to the purified, denatured alpha and beta subunits of the lamb kidney enzyme showed about a 50% cross-reactivity towards the respective subunit of the rat enzyme. In contrast to the cross-reactivity observed using the polyclonal antibodies, six monoclonal antibodies specific for the alpha subunit of the lamb holoenzyme exhibited no cross-reactivity with the rat holoenzyme. Four of these monoclonal antibodies, however, showed substantial cross-reactivity with rat alpha subunit as resolved by SDS-polyacrylamide gel electrophoresis. A fifth antibody did not bind to the denatured alpha subunit of either the lamb or the rat enzyme. Another monoclonal antibody (M7-PB-E9), which is specific for an epitope previously implicated in the regulation of both ATP and ouabain binding to (Na+ + K+)-ATPase (Ball, W.J., Jr. (1984) Biochemistry 2275-2281) was found to bind to the denatured lamb alpha but not to the rat alpha. This antibody has identified a region of the lamb alpha that has an altered amino acid sequence in the ouabain-insensitive rat enzyme. These immunological studies indicate that there are substantial antigenic differences between the lamb and rat kidney (Na+ + K+)-ATPases. The majority of these antigenic differences appear to be due to variations in the tertiary structures rather than to variations in the primary structures of the alpha subunits.     

Immunochemical studies of (Na+ + K+)-ATPase using site-specific, synthetic peptide directed antibodies

Rabbit antisera were raised against a series of synthetic peptides corresponding to regions of the alpha subunit of lamb kidney (Na+ + K+)-ATPase which chemical labeling studies and hydropathy plots of the amino-acid sequence suggest are exposed, accessible regions of the enzyme and may comprise the cation selectivity region, the ATP and cardiac glycoside binding sites, and the phosphorylation site. Five of six peptides tested (11-15 residues in length) were immunogenic and the antisera to four peptides recognized the intact, electroblotted (Western blot analysis) alpha subunit. Immunization with peptides conjugated to keyhole limpet haemocyanin (KLH) produced antipeptide antibodies for seven of nine conjugates. Antisera to four peptide conjugates recognized the native enzyme, confirming predictions that these sequence regions are exposed regions of the holoenzyme. In addition, a collection of four polyclonal antisera and five monoclonal antibodies raised to native holoenzyme were tested for their ability to bind to the peptide conjugates. In this way, two NH2-terminal sequence regions (1-12 and 16-30) and the putative ATP-binding site region (496-506) were identified as epitopes of the native enzyme. These results confirm some aspects of the transmembrane folding models proposed by Shull et al. and Kawakami et al. for the membrane-bound (Na+ + K+)-ATPase.     

Subunits of succinyl-coenzyme A synthetase: coordination of production in Escherichia coli and discovery of a factor that precludes refolding.

Succinyl-coenzyme A synthetase of Escherichia coli has an alpha 2 beta 2 subunit structure. By measuring reconstituted enzyme activity present after addition of purified alpha or beta subunits to cell extracts followed by refolding, we have shown that extracts contain no significant excess of either subunit species. This equivalence suggests that the expression of the respective structural genes for the subunits is coordinately controlled. The presence of cell extract does not affect the rate or extent of reassembly of the subunits, pointing to a high degree of specificity of mutual recognition by the refolding subunits. In the course of these experiments, we have detected the presence in cell extracts of a low-molecular-weight factor that specifically inactivates unfolded alpha or beta subunits or prevents their reassembly into catalytically active enzyme. Under conditions where the subunits are completely inactivated, the factor has no detectable effect on native or refolded tetrameric enzyme, suggesting that the factor may react only with unfolded protein.     

Construction of a FRS1-FRS2 operon encoding the structural genes for the alpha and beta subunits of yeast phenylalanyl-tRNA synthetase and its use in deletion analysis.

FRS1 and FRS2, the structural genes encoding the large (alpha) and small (beta) subunits of yeast phenylalanyl-tRNA synthetase (PheRS) were placed under the control of the lacZ promoter by creating an artificial operon. The FRS2 gene was fused next to the promoter, followed by a 14 base pair intergenic sequence containing a translation reinitiation site in front of the FRS1 coding sequences. The engineered PheRS has 16 N-terminal amino acids from beta-galactosidase fused to the beta subunit. However, the purified protein shows a Km value for tRNA(Phe) that is indistinguishable from that of the the native enzyme. The product of the FRS2-FRS1 operon is not able to complement thermosensitive E. coli PheRS, indicating the lack of heterologous aminoacylation in vivo. We made a deletion in the FRS2 gene that removed about 150 amino terminal residues of the beta subunit. The truncated protein showed intact ATP-PPi exchange, whereas tRNA aminoacylation was lost. This result is similar to that of limited proteolysis performed on the native enzyme that yielded a tetrameric alpha 2 beta'2 structure, able to form aminoacyladenylate but unable to bind tRNA(Phe). A deletion of 50 amino acids from the carboxyl terminus of the beta chain resulted in the loss of both enzyme activities; this suggests the participation of the C-terminal end of the beta subunit in the active site or in subunit assembly to yield a tetrameric functional enzyme.     

Characterization of the alpha and beta subunits of casein kinase 2 by far-UV CD spectroscopy.

Although Chou-Fasman calculations of the secondary structure of recombinant casein kinase 2 subunits alpha and beta suggest they have a similar overall conformation, circular dichroism (CD) studies show that substantial differences in the conformation of the two subunits exist. In addition, comparison of the far-UV CD spectrum of reconstituted CK-2 with the spectra of the subunits indicates that conformational changes occur in the backbone region upon association. Such changes may explain the increased enzyme activity of the holoenzyme relative to that of the alpha subunit itself. In contrast, no changes in the far-UV CD spectrum of the alpha subunit are observed in the presence of casein or the synthetic decapeptide substrate RRRDDDSDDD. Furthermore, the alpha-helical structure of the alpha subunit (but not the beta subunit) can be increased in the presence of stoichiometric amounts of heparin, presumably by its binding to the polylysine stretch at amino acid positions 74-77. Heat denaturation experiments (25-90 degrees C) support the notion that heparin may provide a local protective function. A similar but much larger effect was also observed in the presence of the beta subunit only, which supports previous suggestions of a protective function for this subunit. These results indicate that the protection provided by the beta subunit and the increased enzyme activity of the holoenzyme may arise, in part, from a stabilization of the conformation of the enzyme complex and an increase in alpha-helical content.     

A new protein subunit k for RNA polymerase from Xanthomonas campestris pv. oryzae

During the purification of RNA polymerase from Xanthomonas campestris pv. oryzae, a new subunit named k was found to be associated with this enzyme. The removal of subunit k from holoenzyme by DEAE-cellulose column chromatography results in a decrease in specific activity of the enzyme. The readdition of subunit k to subunit k-depleted holoenzyme results in restoration of enzymatic activity. Subunit k increase the activity of RNA polymerase; the activation was in proportion to the concentration of subunit k added. Antiserum against holoenzyme devoid of subunit k was prepared. This antiserum did not react with purified subunit k; therefore, subunit k may not be the proteolytic fragment of the beta, beta', sigma, or alpha subunit. When this antiserum was used to precipitate RNA polymerase obtained from a crude extract of bacterial cells, subunit k was coprecipitated as determined by sodium dodecyl sulfate gel electrophoretic analysis. The molecular mass of subunit k is approximately 29 kDa, and the molar ratio of beta:beta':sigma:alpha:k was estimated to be 1:1:1:2:4. When native Xp10 DNA was used as template, subunit k stimulated subunit k-depleted holoenzyme, but not core enzyme. When the synthetic polynucleotide poly[d(A-T)] was used, subunit k activated both subunit k-depleted holoenzyme and core enzyme. Subunit k also activated the binding of RNA polymerase to template DNA.     

Immunochemical characterization of a functional site of (Na+,K+)-ATPase

Several hybridoma cell lines secreting antibodies specific to the membrane (Na+,K+)-dependent ATPase from lamb kidney medulla have been isolated by using the methods developed by Kohler and Milstein. One of these antibodies (designated M7-PB- E9 ) has been shown to be directed against a functional epitope or antigenic site of the catalytic (alpha) subunit of the enzyme. Although this antibody was raised to the "native" holoenzyme, it has a higher apparent affinity toward the isolated, delipidated, and inactive alpha subunit than toward the holoenzyme. This antibody shows a 10-fold faster initial rate of binding to the alpha subunit than to the holoenzyme. The antibody dissociation rates from both isolated alpha subunit and holoenzyme are similarly slow, and the binding can be considered a pseudoirreversible reaction. By binding at this site, the antibody, however, acts like a "partial competitive inhibitor" with respect to ATP and acts as an uncompetitive or mixed competitive inhibitor with respect to the Na+ and K+ dependence of ATPase hydrolysis. This antibody also does not alter the cooperativity at either the Na+ or the K+ sites. The antibody causes a partial inhibition of the Na+- and MgATP-dependent phosphoenzyme intermediate formation but has no effect on either ADP in equilibrium ATP exchange or the K+-stimulated dephosphorylation step. In addition, the K+-dependent p-nitrophenylphosphatase activity of the enzyme was not affected. In the presence of Mg2+, the antibody stimulates the rate of cardiac glycoside binding [( 3H]ouabain) to the (Na+,K+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of the major tRNA(Phe) binding domain in the tetrameric structure of cytoplasmic phenylalanyl-tRNA synthetase from baker's yeast

Native cytoplasmic phenylalanyl-tRNA synthetase from baker's yeast is a tetramer of the alpha 2 beta 2 type. On mild tryptic cleavage it gives rise to a modified alpha 2 beta 2 form that has lost the tRNA(Phe) binding capacity but is still able to activate phenylalanine. In this paper are presented data concerning peptides released by this limited proteolytic conversion as well as those arising from exhaustive tryptic digestion of the truncated beta subunit. Each purified peptide was unambiguously assigned to a unique stretch of the beta subunit amino acid sequence that was recently determined via gene cloning and DNA sequencing. Together with earlier results from affinity labelling studies the present data show that the Lys 172-Ile 173 bond is the unique target of trypsin under mild conditions and that the N-terminal domain of each beta subunit (residues 1-172) contains the major tRNA(Phe) binding sites.     

The effect of polylysine on casein-kinase-2 activity is influenced by both the structure of the protein/peptide substrates and the subunit composition of the enzyme.

The mechanism by which polybasic peptides stimulate the activity of casein kinase 2 (CK2) has been studied by comparing the effect of polylysine on the phosphorylation of a variety of protein and peptide substrates by the native CK2 holoenzyme and by its recombinant catalytic alpha subunit, either alone or in combination with the recombinant non-catalytic beta subunit. Calmodulin is not phosphorylated by the CK2 holoenzyme, in either the native or the reconstituted form, unless polylysine is added. In the presence of polylysine, it becomes a good substrate for CK2 (Km 14.2 microM, Kcat 4.6 mol.min-1.mol CK2-1). The recombinant alpha subunit, however, spontaneously phosphorylates calmodulin, this phosphorylation being actually inhibited rather than stimulated by polylysine. The calmodulin tridecapeptide, RKMKDTDSEEEIR, reproducing the phosphorylation site for CK2, is spontaneously phosphorylated by either CK2 holoenzyme or the recombinant alpha subunit with 5.8-fold and 2.8-fold stimulation by polylysine, respectively. The recombinant beta subunit of CK2 is itself a good exogenous substrate for the enzyme, its phosphorylation, however, is inhibited rather than enhanced by polylysine. On the contrary, the phosphorylation of the nonapeptide, MSSSEEVSW, reproducing the beta-subunit phosphoacceptor site, is dramatically stimulated by polylysine. Using a variety of small peptide substrates, it was shown that phosphorylation rate is diversely stimulated by polylysine. The observed stimulation, moreover, is variably accounted for by changes in Vmax and/or Km, depending on the structure of the peptide substrate. Maximum stimulation with all protein/peptide substrates tested requires the presence of the beta subunit, since the recombinant alpha subunit is much less responsive than CK2 holoenzyme, either native or reconstituted. While the phosphorylation of the peptide RRRDDDSDDD by CK2 is stimulated 2.8-fold, with 15 nM polylysine being required for half-maximal stimulation, a stimulation of only 1.9-fold, with 80 nM polylysine required for half-maximal stimulation, is attained with recombinant alpha subunit. The concentration of polylysine required for half-maximal stimulation is comparable to CK2 concentration and increases by increasing CK2 concentration, suggesting that polylysine primarily interacts with the enzyme, rather than with the peptide substrate.     

Characterization of the catalytic subunit of casein kinase II expressed in Escherichia coli and regulation of activity

The catalytic (alpha) subunit of casein kinase II from Drosophila, cloned and expressed in Escherichia coli (Saxena, A., Padmanabha, R., and Glover, C. V. C., (1987) Mol. Cell. Biol. 7, 3409-3417), has been purified and characterized, and the properties have been compared to those of the holoenzyme. The catalytic subunit exhibits protein kinase activity with casein as substrate and is autophosphorylated. The specific activity of the purified subunit is 6% of the activity of the holoenzyme from reticulocytes or from Drosophila. The alpha subunit is a monomer, eluting at Mr = 40,000 upon gel filtration in high salt, but as part of an aggregate in low salt. The alpha subunit has been purified to apparent homogeneity by sequential chromatography on DEAE-cellulose, Mono S, and Mono Q. A single band, Mr = 37,000, is detected by silver staining following polyacrylamide gel electrophoresis. The isolated alpha subunit displays apparent Km values for beta casein, ATP, and GTP similar to those of the holoenzyme. The activity of the alpha subunit is inhibited by heparin with an I50 of 0.1-0.3 micrograms/ml, a value similar to that observed for the holoenzyme; autophosphorylation is also inhibited by heparin. Polylysine has no stimulatory effect on the activity of the catalytic subunit, as measured with casein and by autophosphorylation, but stimulates both activities with the holoenzyme. When physiological substrates for casein kinase II are examined, glycogen synthase and eukaryotic initiation factor 3 (eIF-3) (p120) are phosphorylated by the alpha subunit at a rate equivalent to that of the holoenzyme, while phosphorylation of eIF-3 (p67) is reduced 9-fold and eIF-2 beta is not modified. From these data, it can be concluded that the alpha subunit of casein kinase II is sufficient for catalysis, is autophosphorylated, and can be directly inhibited by heparin, whereas the beta subunit mediates the effects of basic stimulatory compounds and is involved in recognition and/or binding to specific physiological substrates.     

Prokaryotic and eukaryotic tetrameric phenylalanyl-tRNA synthetases display conservation of the binding mode of the tRNA(Phe) CCA end

The interaction of human phenylalanyl-tRNA synthetase, a eukaryotic prototype with an unknown three-dimensional structure, with the tRNA(Phe) acceptor end was studied by s(4)U-induced affinity cross-linking with human tRNA(Phe) derivatives site-specifically substituted at the single-stranded 3' end. Two different subunits of the enzyme bind two adjacent nucleotides of the tRNA(Phe) 3' end: nucleotide 76 is associated with the catalytic alpha subunit, while nucleotide 75 is in contact with the beta subunit. The binding mode is similar to that revealed previously in structural and affinity cross-linking studies of the prokaryotic Thermus thermophilus phenylalanyl-tRNA synthetase. Our results suggest that the distinctive features of tRNA(Phe) acceptor end binding are conserved for the eukaryotic and prokaryotic tetrameric phenylalanyl-tRNA synthetases despite their significant differences in the domain composition of the beta subunits. The data from affinity cross-linking experiments with human phenylalanyl-tRNA synthetase complexed with small ligands (ATP and/or phenylalanine or a stable synthetic analogue of phenylalanyl adenylate) reveal that the location of the tRNA(Phe) acceptor end varies with the presence and nature of other substrates. The lack of substrate activity of human tRNA(Phe) substituted with s(4)U at the 3'-terminal position suggests that base-specific interactions of the terminal adenosine are critically important for a productive interaction. The conformational rearrangement of the tRNA 3' end induced by the other substrates and dictated by base-specific contacts of the terminal nucleotide is an additional means of ensuring the phenylalanylation specificity in both prokaryotic and eukaryotic systems.     

Interaction of the tRNA(Phe) acceptor end with the synthetase involves a sequence common to yeast and Escherichia coli phenylalanyl-tRNA synthetases

Modified lysines resulting from the cross-linking of the 3' end of tRNA(Phe) to yeast phenylalanyl-tRNA synthetase (an enzyme with an alpha 2 beta 2 structure) have been characterized by sequencing the labeled chymotryptic peptides that were isolated by means of gel filtration and reversed-phase chromatography. The analysis showed that Lys131 and Lys436 in the alpha subunit are the target sites of periodate-oxidized tRNA(Phe). Mutant protein with a Lys----Asn substitution established that each lysine contributes to the binding of the tRNA but is not essential for catalysis. The major labeled lysine (K131) belongs to the sequence IALQDKL (residues 126-132), which shares three identities with the peptide sequence ADKL found around the tRNAox-labeled Lys61 in the large subunit of Escherichia coli phenylalanyl-tRNA synthetase [Hountondji, C., Schmitter, J. M., Beauvallet, C., & Blanquet, S. (1987) Biochemistry 26, 5433-5439].     

Monoclonal antibodies to rabbit skeletal muscle phosphorylase kinase. Probes for studies of subunit function

Monoclonal antibodies to rabbit skeletal muscle phosphorylase kinase were produced by the conventional hybridoma cell technique. 90 out of 600 hybridomas were found to produce phosphorylase kinase binding antibodies from which only five secreted also phosphorylase kinase activity affecting antibodies. Three of them were cloned; two hybridomas resisted all cloning efforts. Employing immunoblot technique all monoclonal antibodies show cross-reactivity with the alpha, beta, and gamma subunits of phosphorylase kinase indicating that similar, if not identical, epitopes are present on these three subunits. No cross-reactivity with delta is observed. Monoclonal antibodies secreted by two clones which bind to the alpha subunit stimulate the Ca2+-independent A0 activity of phosphorylase kinase more than 30-fold, whereas all other monoclonal antibodies obtained are ineffective in this respect. Monoclonal antibodies binding to the beta subunit inhibit the Ca2+-dependent activities significantly. Antibody produced by one hybridoma binds to the alpha, beta, and gamma subunits with approximately the same affinity. Based on the dual function of calmodulin in phosphorylase kinase (Hessová, Z., Varsányi, M., and Heilmeyer, L.M.G., Jr. (1985) Eur. J. Biochem. 146, 107-115) we conclude that binding of anti-alpha monoclonal antibodies to a regulatory domain in the alpha subunit results in an uncoupling of the inhibitory function of the Ca2+-free delta from the holoenzyme which leads to a concomitant increase in A0 activity. Furthermore, binding of anti-beta monoclonal antibodies to the beta subunit prevents a signal transfer from the Ca2+-saturated delta to the catalytic site of the holoenzyme which inhibits the Ca2+-dependent activities.