Posttranslational covalent modification of proteins

High-sensitivity determination of tyrosine-phosphorylated peptides by on-line enzyme reactor and electrospray ionization mass spectrometry

We describe a simple, fast, sensitive, and nonisotopic bioanalytical technique for the detection of tyrosine-phosphorylated peptides and the determination of sites of protein tyrosine phosphorylation. The technique employs a protein tyrosine phosphatase micro enzyme reactor coupled on-line to either capillary electrophoresis or liquid chromatography and electrospray ionization mass spectrometry instruments. The micro enzyme reactor was constructed by immobilizing genetically engineered, metabolically biotinylated human protein tyrosine phosphatase beta onto the inner surface of a small piece of a 50-microns inner diameter, 360-microns outer diameter fused silica capillary or by immobilization of the phosphatase onto 40-90-microns avidin-activated resins. By coupling these reactors directly to either a capillary electrophoresis column or a liquid chromatography column, we were able to rapidly perform enzymatic dephosphorylation and separation of the reaction products. Detection and identification of the components of the reaction mixture exiting these reactors were done by mass analysis with an on-line electrospray ionization mass spectrometer. Tyrosine-phosphorylated peptides, even if present in a complex peptide mixture, were identified by subtractive analysis of peptide patterns generated with or without phosphatase treatment. Two criteria, namely a phosphatase-induced change in hydropathy and charge, respectively, and a change in molecular mass by 80 Da, were used jointly to identify phosphopeptides. We demonstrate that, with this technique, low picomole amounts of a tyrosine-phosphorylated peptide can be detected in a complex peptide mixture generated by proteolysis of a protein and that even higher sensitivities can be realized if more sensitive detection systems are applied.

Origin and significance of the heterogeneity of protein hormones

The heterogeneity of circulating protein hormone is the result of multiple steps including gene expression, mRNA maturation, post-translational processing and peripheral catabolism. As a consequence of these cumulative events, it seems difficult to evaluate the endocrine function by using specific radioimmunoassays for each circulating variant of a protein hormone. Moreover, the discovery of new molecular variants of protein hormones with unknown biological significance complicates the standardization and the clinical interpretation of immunoassays.

Analysis of protein modifications: recent advances in detection, characterization and mapping

The past year has seen several contributions, both in methods for determining and characterizing chemical modifications of proteins and in related technologies used to map peptides. These contributions mainly involve improvements in capillary zone electrophoresis and in various applications of mass spectrometry.

Determination of L-methionine-dl-sulphoxide in tissue extracts

The amino acid fraction from rat liver, heart and skeletal muscle was prepared by the separation of sulphosalicylic acid extract on Dowex 50 H+ form. The presence of L-methionine-dl-sulphoxide in these extracts was identified and compared by three independent chromatographic methods: ion-exchange, Pico-Tag and reversed-phase high-performance liquid chromatography after precolumn derivatisation with diethylethoxymethylenemalonate. Quantitative data indicate that L-methionine-dl-sulphoxide is present in the intracellular pool at the levels of free methionine.

High-level expression of biologically active chicken prolactin in E. coli

1. A large quantity of chicken prolactin (cPRL) was produced by manipulating the cPRL cDNA clone and an expression vector pKK223-3. To augment the production of the hormonal protein in E. coli, in addition to the potent tac promoter, a unique DNA linker containing a pair of Shine-Dalgarno sequences and a short preceding cistron sequence was inserted into adjacent 5'-region of the coding region. 2. In sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis, recombinant cPRL protein exhibited a molecular mass of 23 kDa. 3. The recombinant cPRL showed equivalent binding kinetics to an antiserum raised against turkey PRL. Also, this product increased the weight of pigeon crop sac mucosa to a degree comparable to that induced by turkey PRL. 4. These results indicate that this recombinant cPRL has immunological and biological activities identical to those of authentic avian PRL.

Protein damage and lipid peroxidation: effects of diethyl maleate, bromotrichloromethane and vitamin E on ammonia, urea and enzymes involved in ammonia metabolism

Changes in ammonia and urea were investigated as potential marker products of free radical damage to protein and subsequent metabolism of those damaged proteins in vivo. Both serum and liver lipid peroxidation products as measured by thiobarbituric-acid-reactive substances (TBARS) were increased by feeding rats a vitamin-E-deficient diet. The acute injection of diethyl maleate and bromotrichloromethane (DEM/BrCCl3) increased TBARS in liver of rats fed a vitamin-E-deficient diet. The concentrations of ammonia and urea in the serum and liver did not correlate with lipid peroxidation. The activities of liver glutaminase and arginase were decreased by DEM/BrCCl3 treatment in rats fed vitamin-E-deficient diet. Glutamate-ammonia ligase activity was decreased by vitamin-E-deficient diet but not by DEM/BrCCl3 treatment. Ornithine carbamoyltransferase, arginosuccinate synthase, argininosuccinate lyase and glutamate dehydrogenase (NAD(P)+) were not affected by dietary vitamin E or by DEM/BrCCl3. The data suggest that the concentrations of ammonia and urea, major by-products of nitrogen metabolism, are unchanged by the oxidant damage and lipid peroxidation, and that their control in vivo is a dynamic equilibrium of various metabolic pathways.

Origin of genetically encoded protein synthesis: a model based on selection for RNA peptidation

The difficulty in explaining the origin of genetic coding centres on the need to identify selective advantages that could account for the synthesis of peptidyl-tRNA, the essential intermediate in genetically programmed translation. It is resolved by a recognition of the functional advantages derivable from the post-transcriptional addition of peptide cofactors to RNA apo-catalysts. This enables the formulation of a theory for the origin of the genetic encoding of protein synthesis by RNA.

Isolation, Amino-Acid Sequence, Synthesis and Biological Properties of Urotensin I from Hippoglossoides elassodon

Abstract A 41-residue urotensin I neuropeptide (H-UI) was isolated from urophyses of the marine teleost Hippoglossoides elassodon (the flathead sole). The peptide was recognized by its partial cross-reactivity in a radioimmunoassay developed for Catostomus (sucker) Ul (S-UI), and was purified by reversed-phase high-performance liquid chromatography. The amino-acid sequence was shown to be H-Ser-Glu-Glu-Pro-Pro-Met-Ser-lle-Asp-Leu-Thr-Phe-His-Met-Leu-Arg-Asn-Met-lle-His-Arg-Ala-Lys-Met-Glu-Gly-Glu-Arg-Glu-Gln-Ala-Leu-lle-Asn-Arg-Asn-Leu-Leu-Asp-Glu-Val-NH(2). H-UI is 66% homologous with S-UI and 63% homologous with Cyprinus (carp) Ul (C-UI). Like S- and C-UI, H-UI is about 50% homologous with the frog skin peptide sauvagine and with Catostomus and mammalian corticotropin-releasing factors. H-UI had similar vasodilatory effects in mammals, and similar adrenocorticotropin-releasing effects (in rat and goldfish) to S-UI, C-UI, sauvagine and the corticotropin-releasing factors, but had relatively low potency (e.g. 10% to 30% of the vasodilatory potency of S- and C-UI) in all the bioassay systems studied.

Intracellular modification of human apolipoprotein AII (apoAII) and sites of apoAII mRNA synthesis: comparison of apoAII with apoCII and apoCIII isoproteins

We have studied the intracellular modifications of human apoAII by pulse-chase labeling of HepG2 cell cultures with [35S]methionine or [3H]arginine followed by two-dimensional analysis and autoradiography of the radiolabeled apoAII isoproteins. A short (5.0-min) pulse showed the presence of a precursor form of apoAII (pI = 5.75) designated proapoAII or apoAII3. A 5-10-min chase resulted in a decrease in the relative concentration of the proapoAII coupled with an increase in the relative concentration of a new form (pI = 5.3) designated modified proapoAII or apoAII1. Longer chase resulted in the appearance of the plasma apoAII form and at least five other acidic apoAII isoproteins in the cell lysate and the culture medium. Labeling with [3H]arginine showed that apoAII isoproteins designated 3, 1, -1, and -3 contained the prosegment whereas isoproteins designated 1a, 0, -1a, -2a, -3a, and -4a did not. Comparison of nascent apoAII, apoCII, and apoCIII isoproteins revealed that they were distinctly different on the two-dimensional gels. Neuraminidase treatment converted the acidic apoAII isoproteins to isoproteins 1a and 0 (modified and plasma apoAII forms). The combined data are consistent with the following intra- and/or extracellular modifications of apoAII: (a) modification of the apoAII which results in the net loss of two positive charges; (b) glycosylation of the modified proapoAII with carbohydrate chains containing sialic acid; (c) proteolytic removal of the prosegment and cyclization of the N-terminal glutamine. Analysis of apoAII mRNA distribution in 13 fetal human tissues as well as in cell lines of human origin showed abundance of apoAII mRNA in liver and HepG2 cells and only traces in the intestine.

An electromagnetic theory of life

For some 300 years the belief in "Life-as-Chemistry" has held sway as a singular theory understanding life. Despite its many successes, and in spite of decades of advanced research with highly sophisticated instrumentation, it has not solved any of the really basic mysteries of life (Science 181,583). Worse yet, every gain seems to be more than offset by additional serious problems and contradictions. We therefore offer a new "Life-as-Physics" paradigm for consideration.

Age-related changes in protein phosphorylation by rat hepatocytes

The qualitative and quantitative changes in the phosphorylation of specific proteins in hepatocyte suspension from 5-, 12- and 22-month-old male Fischer F344 rats were analyzed by two-dimensional polyacrylamide gel electrophoresis. No qualitative changes in phosphorylation of individual proteins were observed with age. In addition, very few quantitative changes (less than 2% of proteins studied) in protein phosphorylation were detected. The phosphorylation of two acidic proteins decreased (50%) with age while the phosphorylation of one basic protein increased (300%) with age. The two acidic proteins and one basic protein that showed quantitative changes with age were found predominately in the microsome and nuclear fractions of hepatocytes, respectively. The effect of dietary restriction on the phosphorylation of proteins in male Fischer F344 rats was also studied. Although, dietary restriction alters the age-related incident of disease and prolongs longevity, it did not have any significant effect on phosphorylation of individual proteins in the liver.

In vivo selection of populations of Plasmodium chabaudi chabaudi AS resistant to a monoclonal antibody that reacts with the precursor to the major merozoite surface antigen

Mice bearing a hybridoma secreting a monoclonal antibody (MAb), MAb-3, which significantly delays the onset of a Plasmodium chabaudi chabaudi AS, but not P. chabaudi chabaudi CB, challenge parasitemia in a passive transfer assay and which is specific for the precursor to the major merozoite surface antigen (PMMSA) of P. chabaudi chabaudi AS, were challenged intravenously with 10(3) P. chabaudi chabaudi AS-parasitized erythrocytes. The resultant parasitemia was very similar to that in normal mice except that initially the parasitemia was sometimes slightly delayed. Parasites derived from cryopreserved stabilates isolated from MAb-3 hybridoma mice with an unmodified parasitemia, or with a delayed parasitemia, were found to have lost their susceptibility to MAb-3 in the passive transfer assay. A number of anti-PMMSA MAb were used to immunoprecipitate lysates of parasite populations isolated directly from hybridoma-bearing mice. In some instances and with certain of the MAb, immunoprecipitation patterns were modified, but other isolates were not detectably different when compared with unselected P. chabaudi chabaudi AS parasites. Using a panel of MAb reacting with the PMMSA of P. chabaudi chabaudi AS, immunoprecipitation patterns of parasites derived from cryopreserved stabilates isolated from hybridoma-bearing mice were determined at 2-h intervals through the appropriate part of the parasite maturation cycle. In these derived populations, resistance to MAb-3 was not associated with a change in the immunoprecipitation reaction with the MAb used. These results are discussed in the context of current knowledge of genotypic and phenotypic antigenic diversity of malaria parasites and other protozoa.

Post-translational modifications of proteins: some problems left to solve

Three major questions regarding the post-translational modification of amino acid side chains in proteins are briefly considered: (1) What are the biological functions of the reactions, (2) what is the specificity of the processing reactions in selecting only a few or sometimes even only one residue for modification, and (3) how do we solve the uniqueness of the processing steps in the production of recombinant proteins? The answers to these questions are not obvious at this time.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

Sequence of one alpha- and two beta-tubulin genes of Tetrahymena pyriformis. Structural and functional relationships with other eukaryotic tubulin genes

Macronuclear DNA of the ciliate Tetrahymena pyriformis contains only one size class of fragments coding for alpha-tubulin, alpha TT. We have isolated alpha TT from a partial plasmid library, using Chlamydomonas reinhardtii alpha-tubulin gene as a probe. This gene as well as the two beta-tubulin genes, beta TT1 and beta TT2, have been sequenced. None of these genes contains introns and all use TGA as the stop codon. In the coding region of the two beta-tubulin genes, there are several TAA and TAG stop codons that probably code for glutamine. The codon usage is very biased. Regions flanking the tubulin coding sequences are A + T-rich (75%) and quite different among themselves. In these regions there are several putative transcription-regulatory sequences. Nuclear transcripts begin and terminate at multiple sites. The beta-tubulin proteins differ only in two amino acid residues. Primary structure of Tetrahymena tubulins as well as their hydropathy indexes show a high degree of homology with tubulins from other organisms. Two-dimensional electrophoretic analysis of the ciliary tubulins shows the presence of eight alpha-tubulins and four beta-tubulins. The alpha-tubulins migrate faster than the beta-tubulins, in contrast with what happens with brain tubulins. We suggest that there are several alpha- and beta-tubulin isoforms and the migratory inversion observed may be due to post-translational modifications.

Desmosine and isodesmosine as cross-links in the hinge-ligament protein of bivalves. 3,3'-Methylenebistyrosine as an artefact

Desmosine and isodesmosine were detected in an invertebrate molluscan species, i.e. in an insoluble protein in the hinge ligament of a bivalve species, Sakhalin surf clam (Pseudocardium sachalinensis, in family Mactridae). The protein is rich in glycine and methionine S-oxide but devoid of hydroxyproline and hydroxylysine. 3,3'-Methylenebistyrosine was also detected in the HCl hydrolysate of the hinge-ligament protein, but it was found to be an artefact produced from tyrosine and formaldehyde derived from methionine S-oxide during the HCl hydrolysis of the protein.

Current ideas on the significance of protein glycosylation

Carbohydrate has been removed from a number of glycoproteins without major effect on the structure or enzyme activity of the protein. Thus carbohydrate has been suggested to underly a non-primary function for proteins, such as in relatively non-specific interactions with other carbohydrates or macromolecules, stabilization of protein conformation, or protection from proteolysis. This non-specific concept is consistent with both the general similarity in carbohydrate structure on very diverse glycoproteins and the frequent structural microheterogeneity of carbohydrate chains at given sites. The concept is supported in a general sense by the viability of cells whose glycosylation processes have been globally disrupted by mutation or pharmacological inhibitors. In contrast to the above observations, other studies have revealed the existence of specific, selective receptors for discrete oligosaccharide structures on glycoproteins which seem to be important for compartmentalization of the glycoprotein, or the positioning of cells on which the glycoprotein is concentrated. Sometimes multivalency in the carbohydrate-receptor interaction is crucial. There are additional possible roles for carbohydrate in the transduction of information upon binding to a receptor. The possibility of specific roles for carbohydrate is supported by the existence of numerous unique carbohydrate structures, many of which have been detected as glycoantigens by monoclonal antibodies, with unique distributions in developing and differentiated cells. This article attempts to summarize and rationalize the contradictory results. It appears that in general carbohydrate does in fact underlie only roles secondary to a protein's primary function. These secondary roles are simple non-specific ones of protection and stabilization, but often also satisfy the more sophisticated needs of spatial position control and compartmentalization in multicellular eukaryotic organisms. It is suggested that there are advantages, evolutionarily speaking, for the shared use of carbohydrate for non-specific roles and for specific roles primarily as luxury functions to be executed during the processes of cell differentiation and morphogenesis.

A novel sulfotransferase sulfates tyrosine-containing peptides and proteins

The novel sulfotransferase (M.W. 315 kDa) obtained from Eubacterium A-44 catalyzed the sulfation of tyrosine residues of peptides and proteins such as kyotorphin, enkephalin, cholecystokinin-8 (non-sulfated form), trypsin inhibitor and insulin. Also, the enzyme sulfated tyrosine residues of protein fractions purified from Eubacterium A-44.

The role of glucose, insulin and glucagon in the regulation of food intake and body weight

Glucose and related pancreatic hormones play a major role in the metabolism of monogastric mammals yet their influence on hunger and/or satiety is, as yet, poorly understood. Glucose, insulin and glucagon rise during a meal and gradually decline to baseline levels shortly after a meal. A sudden drop in plasma glucose as well as insulin have been reported just prior to the onset of a meal but the functional significance of this is not yet clear. Systemic injections of glucose have no acute satiety effects but intraduodenal and intrahepatic infusions reduce food intake and free-feeding and deprived animals respectively. Treatments which decrease cellular glucose utilization directly (2-DG) or indirectly (insulin) increase food intake while exogenous glucagon (which produces hyperglycemia) decreases it. There is considerable evidence that some or all of these effects may be due to a direct central action of glucose, 2-DG, insulin, and glucagon on brain mechanisms concerned with the regulation of hunger and satiety although influences on peripheral "glucoreceptors" have been demonstrated as well. The functional significance of glucoprivic feeding is, however, questioned. The feeding response to 2-DG and related compounds is capricious, and its temporal course does not parallel the hyperglycemic reaction which presumably reflects cellular glucopenia. Moreover, numerous brain lesions which increase, decrease, or have no effect on ad lib intake and often have no effect on the response to deprivation have been shown to severely impair or abolish feeding responses to systemic injections of 2-DG that produce severe central as well as peripheral glucopenia. Feeding responses to insulin are intact after most of these lesions, suggesting that this hormone may influence food intake in a fundamentally different fashion. The mechanism of insulin action is not understood--the classic feeding response is obtained only with doses that are pharmacological when compared to normal plasma levels and there is increasing evidence that lower doses may have opposite, inhibitory effects on food intake and body weight. Relatively small doses of glucagon decrease food intake (although opposite facilitatory effects have been reported after even smaller doses) but the effect does not appear to be due to hepatic mobilization of glucose as initially assumed. Decreases in food intake after intracranial injections of very small doses suggest a direct central action.

Hominoid triosephosphate isomerase: characterization of the major cell proliferation specific isozyme

Proliferating cells derived from hominoid species contain electrophoretically separable forms of triosephosphate isomerase (TPI), including a constitutive isozyme and major and minor cell proliferation specific isozymes. Genetic studies have shown that the constitutive and inducible isozymes are products of the same structural gene. A procedure has been developed for the rapid isolation of the constitutive and major proliferation specific TPI isozymes from human lymphoblastoid B cells. [35S]methionine labeled isozymes were purified through several steps of polyacrylamide gel electrophoresis in sufficient quantities for turnover studies and preliminary structural analysis. The intact isozymes were subjected to 23 steps of automated Edman degradation; both preparations yield a [35S] PTH-methionine only at cycle 14, as expected if the protein is TPI. Neither isozyme contains a blocked NH2-terminus and length heterogeneity at the amino terminal does not exist. A comparison of the two purified isozymes on 2-D PAGE confirms that the constitutive isozyme consists of only type 1 subunits while the major proliferation specific isozyme is composed of a type 1 subunit and a unique type 2 subunit. The type 1 and type 2 subunits differ by at least four charge units under native, nondenaturing conditions of electrophoresis but do not differ in molecular mass. The difference between the type 1 and type 2 subunits is covalent, as the difference in isoelectric point between the two subunits is stable to both 2% SDS and 8 M urea. The expression of TPI-2 does not correlate with the existence of the labile asparagine residues. Turnover studies indicate that the level of each subunit is regulated by differences in rates of synthesis rather than degradation but a precursor-product relationship between the subunits was not observed. Thus the mechanism for synthesis of TPI-2 must operate either during mRNA processing or nascent peptide synthesis and then only in cells from hominoid species.

Analysis of ras oncogene products by two-dimensional gel electrophoresis: evidence for protein families with distinctive molecular forms

Protein products of the ras family of oncogenes were immunoprecipitated by an anti-p21 monoclonal antibody, separated by two-dimensional gel electrophoresis and subsequently detected by western immunoblot analysis using the same anti-p21 monoclonal antibody as a probe. Using this method, a 21 kDa oncogene protein (p21) was detected and characterized in cell lines containing Harvey (Ha), Kirsten (Ki), neuroblastoma (N), or cellular (proto) ras genes. The ras gene products from all cell types occurred with multiple forms differing in size, charge or in both parameters. Transforming ras oncogene proteins occurred in easily identifiable groups that were different from each other in molecular weight and charge, were distinctive for each ras gene type and were different from cellular ras equivalents. Similar, but not identical, family groups occurred in different cell types containing the same oncogene. The reproducible occurrence of unpredicted p21 forms suggests that previously unreported post-translational processing steps may be associated with the synthesis of certain oncogene products. This immunoprecipitation/two-dimensional gel/western blot technique is a simple method for the identification and characterization of p21 gene products.

Characterization of a tyrosine sulfotransferase in rat brain using cholecystokinin derivatives as acceptors

An apparently novel tyrosyl sulfotransferase activity was detected in a crude microsomal fraction from rat cerebral cortex by using 3'-phosphoadenosine 5'-phospho[35S]sulfate [( 35S]PAPS) as the sulfate donor and various cholecystokinin (CCK) fragments or derivatives as acceptors. Among the latter, the shortest substrate was tert-butoxycarbonylaspartyltyrosine (Boc-Asp-Tyr), but the reaction was optimized by increasing the length of the peptide sequence on the C-terminal side up to tert-butoxycarbonylcholecystokinin octapeptide (Boc-CCK-8) as well as by the presence of acidic amino acid residues at the N-terminal side. Peptides with an N-terminal Tyr residue (e.g., CCK-7 or enkephalins) were not sulfated. With Boc-CCK-8 the optimum pH was 5.8, and apparent KM values were 0.14 +/- 0.02 mM for the peptide (0.5 microM PAPS) and 0.12 +/- 0.01 microM for PAPS (0.25 mM Boc-CCK-8). In the presence of 0.2 mM MnCl2 the Vmax of the reaction was enhanced without change of apparent affinities of the two substrates. The possible role of this sulfotransferase activity in posttranslational modification of CCK and other secretory proteins is suggested.

Sulfation and desulfation of cerebral cholecystokinin

An apparently novel sulfotransferase present in microsomal and vesicular fractions from rat brain is able to transfer [35S]sulfate groups from [35S]PAPS to CCK derivatives. Its optimum pH (approximately 6), its substrate specificity, and its subcellular localization are all consistent with its function as post-translational processing enzyme. However, its presence in tissues devoid of CCK argues against this idea, or could simply mean that it is involved in the processing of other peptides besides CCK. No evidence could be obtained for extensive desulfation of endogenous CCK-8 released by depolarization of brain slices.

An amino-terminal tryptophan derivative which is refractory to Edman degradation

A new derivative of tryptophan is proposed to account for the observation that some peptides having amino-terminal tryptophan residues become refractory to Edman degradation. An acid-catalyzed oxidation of the indole nucleus and subsequent cyclization to a unique 3-anilinopyrrolidin-2-one derivative with cleavage of the peptide chain is the most likely chemical explanation. This amino acid is reactive with ninhydrin and contains an aryl amine. However, the amine does not bond to the alpha carbon so while reactive to phenyl isothiocyanate, distances are too great for the residue to be cleaved from the peptide during Edman degradation.

The mechanism of N-terminal acetylation of proteins

N alpha-acetylation is almost exclusively restricted to eukaryotic structural proteins. As a rule it is a post-initiational process, requiring the presence of the enzyme N alpha-acetyltransferase and the acetyl donor acetylcoenzyme A. N alpha-acetyltransferases appear to have a narrow substrate specificity, which is very similar for enzymes from different tissues and species. Amino acids predominantly present at the N terminus of N alpha-acetylated proteins are alanine, serine, and methionine. The occurrence of these residues is apparently a prerequisite for acetylation. The region following these amino acids is also important. If methionine is at the N terminus, the second position is always occupied by a strongly hydrophilic amino acid. Two- and three-dimensional structural characteristics of the protein do not seem to play a major role in N alpha-acetylation. Up to now the exact function for N alpha-acetylation is not known.

Specific protein modifications are altered in a temperature-sensitive Drosophila developmental mutant

The temperature-sensitive Drosophila mutation l(3)c21RRW630 disturbs oogenesis and imaginal disc development and has a maternal effect on embryogenesis. Two-dimensional gel electrophoretic analysis of protein synthesis in mutant tissue at a restrictive temperature shows that the synthesis of three proteins is elevated and the synthesis of three other proteins is reduced, when compared to wild type. Each protein with increased synthesis is similar to a protein whose synthesis is reduced, as judged by comparison of partial proteolytic digests of these proteins. To explain these findings, we propose that the wild-type c21R gene codes for a protein-modifying enzyme. This enzyme catalyzes the acidic modification of three abundant proteins. The correct modification of these proteins is required for cell division, cell motility, and the formation of adult hairs and bristles. In the mutant at restrictive temperature, the enzyme does not function properly and so the unmodified substrate proteins accumulate. This study correlates the morphological defects in a Drosophila developmental mutant with an altered molecular process.

Catalytic properties of alcohol dehydrogenase isozymes specified by duplicate genes in the diploid plant Stephanomeria exigua

The alcohol dehydrogenase (ADH) isozymes induced in flooded roots of the diploid plant Stephanomeria exigua are specified by tightly linked genes comprising a complex locus, Adh1. Individuals homozygous for a complex with two active genes which specify electrophoretically different subunits have three ADH-1 isozymes, two intragenic homodimers and an intergenic heterodimer. Individual isozymes were partially purified from plants homozygous for several different Adh1 complexes and apparent Km values for acetaldehyde, ethanol, NAD, and NADH and responses to temperature, pH, and two different alcohols were determined. The two homodimeric enzymes specified by a particular Adh1 complex generally differed in one or more of the properties studied, and in three of four cases, intergenic heterodimers differed significantly from intermediacy, often having lower Km values than either homodimer. None of the isozymes studied could be considered greatly divergent or defective. Constraints on evolution of duplicate genes which form intergenic heterodimers are considered.