Isolation and characterization of complementation products of Escherichia coli alkaline phosphatase

Elucidation and control of low and high active populations of alkaline phosphatase molecules for quantitative digital bioassay

Alkaline phosphatase (ALP), a homo‐dimeric enzyme has been widely used in various bioassays as disease markers and enzyme probes. Recent advancements of digital bioassay revolutionized ALP‐based diagnostic assays as seen in rapid growth of digital ELISA and the emerging multiplex profiling of single‐molecule ALP isomers. However, the intrinsic heterogeneity found among ALP molecules hampers the ALP‐based quantitative digital bioassays. This study aims quantitative analysis of single‐molecule activities of ALP from Escherichia coli and reveals the static heterogeneity in catalytic activity of ALP with two distinct populations: half‐active and fully‐active portions. Digital assays with serial buffer exchange uncovered single‐molecule Michaelis–Menten kinetics of ALP; half‐active molecules have halved values of the catalytic turnover rate, k_cat, and the rate constant of productive binding, k_on, of the fully active molecules. These findings suggest that half‐active ALP molecules are heterogenic dimers composed of inactive and active monomer units, while fully active ALP molecules comprise two active units. Static heterogeneity was also observed for ALP with other origins: calf intestine or shrimp, showing how the findings can be generalized across species. Cell‐free expression of ALP with disulfide bond enhancer and spiked zinc ion resulted in homogenous population of ALP of full activity, implying that inactive monomer units of ALP are deficient in correct disulfide bond formation and zinc ion coordination. These findings provide basis for further study on molecular mechanism and biogenesis of ALP, and also offer the way to prepare homogenous and active populations of ALP for highly quantitative and sensitive bioassays with ALP.

Characterization of a monomeric Escherichia coli alkaline phosphatase formed upon a single amino acid substitution

Alkaline phosphatase (AP) from Escherichia coli as well as APs from many other organisms exist in a dimeric quaternary structure. Each monomer contains an active site located 32 A away from the active site in the second subunit. Indirect evidence has previously suggested that the monomeric form of AP is inactive. Molecular modeling studies indicated that destabilization of the dimeric interface should occur if Thr-59, located near the 2-fold axis of symmetry, were replaced by a sterically large and charged residue such as arginine. The T59R enzyme was constructed and characterized by sucrose-density gradient sedimentation, size-exclusion chromatography, and circular dichroism (CD) and compared with the previously constructed T59A enzyme. The T59A enzyme was found to exist as a dimer, whereas the T59R enzyme was found to exist as a monomer. The T59A, T59R, and wild-type APs exhibited almost identical secondary structures as judged by CD. The T59R monomeric AP has a melting temperature (Tm) of 43 degrees C, whereas the wild-type AP dimer has a Tm of 97 degrees C. The catalytic activity of the T59R enzyme was reduced by 104-fold, whereas the T59A enzyme exhibited an activity similar to that of the wild-type enzyme. The T59A and wild-type enzymes contained similar levels of zinc and magnesium, whereas the T59R enzyme has almost undetectable amounts of tightly bound metals. These results suggest that a significant conformational change occurs upon dimerization, which enhances thermal stability, metal binding, and catalysis.

Characterization of heterodimeric alkaline phosphatases from Escherichia coli: an investigation of intragenic complementation

Escherichia coli alkaline phosphatase (EC 3.1.3.1) belongs to a rare group of enzymes that exhibit intragenic complementation. When certain mutant versions of alkaline phosphatase are combined, the resulting heterodimeric enzymes exhibit a higher level of activity than would be expected based upon the relative activities of the parental enzymes. Nine previously identified alkaline phosphatase complementation mutants were re-examined in this work in order to determine a molecular explanation of intragenic complementation in this experimental system. The locations of these mutations were determined by DNA sequence analysis after PCR amplification of the phosphatase-negative phoA gene. Most of the mutations involved ligands to metal-binding sites. Each of the mutant enzymes was re-created by site-specific mutagenesis, expressed, purified, and kinetically characterized. To investigate cooperativity between the two subunits, we analyzed heterodimeric forms of some of the site-specific mutant enzymes. To enable the isolation of the heterodimeric alkaline phosphatase in pure form, the overall charge of one subunit was altered by replacing the C-terminal Lys residue with three Asp residues. This modification had no effect on the kinetic properties of the enzyme. Heterodimeric alkaline phosphatases were created using two methods: (1) in vitro formation by dissociation at acid pH followed by reassociation at slightly alkaline pH conditions in the presence of zinc and magnesium ions; and (2) in vivo expression from a plasmid carrying two different phoA genes. Increases in k(cat), as well as a large reduction in the p-nitrophenyl phosphate K(m) were observed for certain combinations of mutant enzymes. These results suggest that the structural assembly of E. coli alkaline phosphatase into the dimer induces cooperative interactions between the monomers necessary for the formation of the functional form of the holoenzyme.

Purification and in vitro complementation of mutant histidinol dehydrogenases

The biochemistry of interallelic complementation within the Salmonella typhimurium hisD gene was investigated by in vitro protein complementation of mutant histidinol dehydrogenases (EC 1.1.1.23). Double-mutant strains were constructed containing the hisO1242 (constitutive overproducer) attenuator mutation and selected hisDa or hisDb mutations. Extracts from such hisDa986 and hisDb1799 mutant cells failed to show histidinol dehydrogenase activity but complemented to produce active enzyme. Inactive mutant histidinol dehydrogenases were purified from each of the two mutants by ion-exchange chromatography. Complementation by the purified mutant proteins required the presence of 2-mercaptoethanol and MnCl2, and protein-protein titrations indicated that heterodimers were strongly preferred in mixtures of the complementary mutant enzymes. Neither mutant protein showed negative complementation with wild-type enzyme. The Vmax for hybrid histidinol dehydrogenase was 11% of that for native enzyme, with only minor changes in Km values for substrate or coenzyme. Both purified mutant proteins failed to catalyze NAD-NADH exchange reactions reflective of the first catalytic step of the two-step reaction. The inactive enzymes bound 54Mn2+ weakly or not at all in the presence of 2-mercaptoethanol, in contrast to wild-type enzyme which bound 54Mn2+ to 0.6 sites per monomer under the same conditions. The mutant proteins, like wild-type histidinol dehydrogenase, behaved as dimers on analytical gel filtration chromatography, but dissociated to form monomers in the presence of 2-mercaptoethanol. This effect of 2-mercaptoethanol was prevented by low levels of MnCl2. It thus appears that mutant histidinol dehydrogenase molecules bind metal ion poorly. The complementation procedure may allow for formation of a functional Mn2+-binding site, perhaps at the subunit interface.

Effect of silver ions on transport and retention of phosphate by Escherichia coli

Silver ions inhibited phosphate uptake and exchange in Escherichia coli and caused efflux of accumulated phosphate as well as of mannitol, succinate, glutamine, and proline. The effects of Ag+ were reversed by thiols and, to a lesser extent, by bromide. In the presence of N-ethylmaleimide and several uncouplers, Ag+ failed to cause phosphate efflux, but still inhibited exchange of intracellular and extracellular phosphate, indicating an interaction at more than one site. It is unlikely that Ag+ caused metabolite efflux by acting solely as an uncoupler, as an inhibitor of the respiratory chain, or as a thiol reagent.

Developmental modification and hybridization of allelic acid phosphatase isozymes in homo- and heterozygotes for the Acp- 1 locus in rice

A number of alleles each specified a set of three major and three minor bands of acid phosphatase (E.C. 3.1.3.2) in wild and cultivated rice strains. Relative intensity of the major bands was found to differ significantly according to the developmental stages of the leaves, suggesting the presence of protein modification genes. In heterozygotes, six parental and three hybrid bands were clearly observed in most of the heterozygotes, but the intensities of the hybrid bands were found to be generally lower than those theoretically expected due to random association of enzyme subunits. The cause of this phenomenon is discussed.

The nature of the link between potassium transport and phosphate transport in Escherichia coli

A series of mutants of Escherichia coli, combining defects in either of the two phosphate transport systems with defects in one or more of the potassium transport systems, was used to study the nature of the previously observed obligatory requirement for each one of these ions in the transport of the other. The results show that no pair of systems is obligatorily linked, and that either ion can be transported by any one of its systems, provided that a means of entry for the other ion is available. Furthermore, in the total absence of Pi, K+ entry accompanies the transport of other anions, such as aspartate, glutamate, sn-glycero-3-phosphate and glucose 6-phosphate. The results indicate that Pi and the other anions enter by symport with protons, and that a simultaneous K+/H+ exchange, which would serve to maintain the intracellular pH, is responsible for the observed K+ 'symport' with these anions.

Isolation of a trans-dominant histidase-negative mutant of Salmonella typhimurium

A mutation of Salmonella typhimurium was obtained that results in the failure of cells to synthesize the enzyme l-histidine ammonia-lyase (histidase). The mutation mapped within the hutH gene and in merodiploid strains was dominant over the wild-type allele. Extracts from cells bearing the trans-dominant histidase-negative allele were shown to contain material that reacts immunologically with antiserum against purified wild-type histidase. It is proposed that the trans-dominant allele results in the synthesis of defective histidase subunits that can combine with, and partially inactivate, wild-type histidase subunits. This subunit mixing presumably does occur, as the enzyme synthesized in a hybrid merodiploid strain is abnormally heat sensitive.

Mutationally altered rate constants in the mechanism of alkaline phosphatase

The hydrolysis of phosphate esters by a mutationally altered alkaline phosphatase from Escherichia coli was studied by both steady-state and transient-kinetic methods. The difference between the catalytic-centre activities of the mutationally altered and the wild-type alkaline phosphatases was found to vary with pH and at optimal pH values the modified enzyme had the higher activity. Stopped-flow experiments at acidic pH values showed that transient product formation by the mutationally altered enzyme was faster than that with the wild-type enzyme whereas the rate of the steady state was slower. In the alkaline pH region, the transient was observed in the reaction of only the modified enzyme and not the wild type. These observations permit a fuller characterization of the individual steps in the catalytic mechanism of alkaline phosphatase than is possible by study of only the wild-type enzyme.

Complementation in vitro between mutationally altered beta2 subunits of Escherichia coli tryptophan synthetase

Cross-reacting beta(2) subunits (CRMs) were purified from eight trpB missense mutants to test for complementation in vitro after urea dissociation and reaggregation. One CRM (B290, demonstrating "repairability," i.e., the appearance of enzymatic activity on combination with alpha subunits) was clearly positive with four others, all "non-repairable" CRMs resulting from mutations at three different but neighboring sites. One complementing pair, B290-B248, was studied in more detail and found, upon mixing purified proteins, to give complementation in the absence of denaturants. Complementation activity was low in each case. To study the mechanism of the modest increases in activity, we used a reduced beta(2) subunit as an artificial CRM to form hybrids where both the amount of activity due to complementation and the amount of hybrid could be measured. (In a reduced beta(2) subunit, the two pyridoxal phosphate cofactors have been chemically reduced by sodium borohydride and are covalently attached to lysine residues. This abolishes activity in the tryptophan synthetic reaction and causes the protein to migrate much faster than normal in acrylamide gel electrophoresis.) Reduced beta(2) subunit formed hybrid dimers with the non-repairable CRMs B244 and B248 at pH 6.0, but no enzymatic activity appeared. On the other hand, when reduced beta(2) subunit was mixed with B290 CRM at pH 6.0 to 6.6, an activity increase was seen that was proportional to the amount of hybrid. We conclude that hybrid formation is essential for complementation and that the mechanism of complementation in this system is the correction of a repairable active site on the B290 beta chain by a conformational change occuring when hybrid dimer is formed. This type of complementation must be restricted to a small class of CRMs having a conformationally deformed active site. From the amount of hybrid present and the increase in activity, a specific activity of 50 U/mg was calculated for the hybrid containing reduced and B290 beta chains. This value is slightly less than but close to the activity of the hybrid formed between reduced and normal beta chains, shown earlier to have half the specific activity of the normal dimer.

Human-mouse cell hybrids: a suggestion of structural mutation for dipeptidase-2 deficiency in mouse cells

The dipeptidase-2 enzyme is inactive in certain cultured cell lines from the mouse. In somatic cell hybrids between such deficient cells and diploid human fibroblasts, the mouse deficiency was complemented when the homologous human peptidase-A was retained. The results suggested that the murine peptidase deficiency was the result of a structural mutation, rather than a regulatory one.

Location and orientation of the phoA locus on the Escherichia coli K-12 linkage map

The order of gene loci in the phoA-phoR region of the Escherichia coli K-12 linkage map was demonstrated to be lac phoA proC phoR. The end of the phoA locus corresponding to the amino terminus of alkaline phosphatase was shown to be the end nearer proC. Translation (and transcription) of phoA is therefore in the anticlockwise direction relative to the conventional E. coli linkage map.

Direct evidence for models of heterosis provided by mutants of Arabidopsis blocked in the thiamine pathway

Auxotrophic mutants genetically blocked at different steps of the thiamine pathway dramatically demonstrate the biochemical mechanism of hybrid vigor due to simple and perfect dominance at two unlinked loci. Heteroallelic hybrids of mutants requiring the pyrimidine moiety of thiamine display allelic complementation and thus furnish clear biochemical and genetic evidence for the superdominance hypothesis. Hybrids of low- and high-temperature-requiring leaky mutants demonstrate that heterozygosity at a single gene locus may confer developmental homeostasis on the heteroallelic combinations superior to that of the homoallelic parents. The results of this study on the autogamous plant, Arabidopsis, and of recent reports on the outbreeding species, Drosophila, render untenable the generalization that high versus low temperature dependent heterosis is determined by the breeding system.ZUSAMMENFASSUNG: Auxotrophe Arabidopsis-Mutanten, bei denen verschiedene Schritte der Thiaminsynthese genetisch blockiert sind, lassen deutlich den biochemischen Mechanismus der Heterosis erkennen, der auf einfacher und völliger Dominanz in zwei ungekoppelten Loci beruht. Heteroallele Hybriden von Mutanten, die den Pyrimidinanteil des Thiamins benötigen, zeigen allele Komplementation und liefern damit den klaren biochemischen und genetischen Beweis für die Superdominanz-Hypothese. Hybriden von leaky-Mutanten, die einen niedrigen bzw. hohen Temperaturbedarf haben, zeigen, daß Heterozygotie in einem einzelnen Genlocus den heteroallelen Kombinationen eine Entwicklungshomeostasie verleihen kann, die größer als die der homoallelen Eltern ist. Die Ergebnisse unserer Untersuchungen an der autogamen Pflanze Arabidopsis und neuere Ergebnisse bei Drosophila lassen die Verallgemeinerung, daß die Art der Temperaturabhängigkeit der Heterosis durch das Zuchtsystem bestimmt wird, nicht zu.

Secretion of alkaline phosphatase subunits by spheroplasts of Escherichia coli

Under conditions that permitted continued protein synthesis, spheroplasts of Escherichia coli were unable to form active alkaline phosphatase, although they synthesized protein that was antigenically related to alkaline phosphatase subunits. This cross-reacting protein was primarily detected in the medium of the spheroplast culture, and it had properties that closely resembled those of the alkaline phosphatase subunit. These results suggest that formation of the active alkaline phosphatase dimer by intact E. coli cells proceeds by a pathway in which inactive subunits released from polyribosomes diffuse through the bacterial cell membrane to a periplasmic space where subsequent dimerization to active enzyme occurs. This pathway provides a possible mechanism for the specific localization of this enzyme to the E. coli periplasmic space.