Posttranslational covalent modification of proteins

Allelic forms of beta 2-microglobulin in the mouse

Spleen cells from BALB/c and C57BL/6 mice were cultured separately or together, and the biosynthetically labeled supernates were examined by two-dimensional polyacrylamide gel electrophoresis. Although there were no major labeled proteins in the mixed group that were not present in the separate cultures, there was a major low-molecular-weight protein that differed in charge in the two strains. This protein was identified as beta 2-microglobulin; it could be labeled with 125I on the cell surface by using the lactoperoxidase technique, was noncovalently attached to the H-2K molecule, and had the expected size and charge when compared with human beta 2-microglobulin. Both acidic and basic forms were present in (BALB/c X C57BL/6) F1 hybrids, suggesting codominant expression, although allelic exclusion was not ruled out. Either parental form could combine with one parental form of the H-2K molecule. The beta 2-microglobulin gene does not appear to be closely linked to either the H-2 or th immunoglobulin heavy-chain complexes. It is proposed that beta 2-microglobulin is an "effector subunit" of histocompatibility antigens and that its physiological role is to interact with a specific killing structure on the surface of cytolytic T lymphocytes and thereby initiate cell destruction.

Heterogeneity of human serum amyloid A proteins

Serum amyloid A proteins (SAA), presumed precursors of the tissue amyloid A proteins (AA) characteristic of secondary amyloidosis, have been isolated from the plasma high-density lipoproteins (HDL) of normals after etiocholanolone-induced inflammation and from patients with Wegener's granulomatosis, systemic lupus erythematosis, juvenile rheumatoid arthritis, Waldenström's macroglobulinemia, and Goodpasture's syndrome. At least six polymorphic forms of SAA wer identified among the low molecular weight proteins of HDL, and these comprosed up to 27% of the total HDL protein. Gel and ion-exchange chromatography permitted isolation of the SAA polymorphs in homogeneous form. Their amino acid compositions were very similar, they were indistinguishable in cationic and sodium dodecyl sulfate-polyacrylamide gel electrophoresis systems, and each had the terminal sequency COOH-Tyr-Lys-Phe-. Charge heterogeneity in anionic-urea polyacrylamide gel electropherograms was unaffected by neuaminidase treatment, and none of the SAA protein bands stained with the periodate-Schiff reagent. The two major SAA polymorphs, designated SAA4 and SAA5 according to their order of elution from DEAE-cellulose, had different NH2-terminal sequences. Manual Edman degradation demonstrated NH2-arg-ser-phe-phe- for SAA4 and NH2-ser-phe-phe- for SAA5. This NH2-terminal heterogeneity corresponds to that most frequently reported for AA and suggests that microheterogeneity in SAA may underlie that already documented in AA. Sufficient quantitites of the other SAA polymorphs were not available for similar analyses, but the amino acid compositions do not indicate that NH2-terminal heterogeneity accounts for all of the observed polymorphism. Artifactual polymorphism also appears unlikely, and the heterogeneiy of SAA may reflect origin from more than one cell type with or without posttranslational modificaton. We calculate from quantitative COOH-terminal analyses that SAA is of 11,000-11,900 mol wt. Primary structure studies have shown AA t be a single chain protein of 76 residues, and SAA, therefore, appears to contain a peptide of 33 amino acids that is missing from AA.

Synthesis and degradation of retinal proteins in darkness and during photic stimulation

Rabbit retinas were maintained in a physiological state in vitro and exposed to 0.5-h pulses of labeled leucine. Protein synthesis was determined from incorporation of the label, and degradation from its subsequent release. The retinas were treated as test-control pairs. The control retina remained in darkness while the test retina was subjected to photic stimulation, either during labeling to determine the effect on synthesis or after labeling to determine the effect on degradation. 3H and 14C alternated as test and control labels. The two retinas were combined for solubilization. Their proteins were separated according to size by gel electrophoresis. Each gel was cut into 95 slices and each slice was differentially counted. The isolated retinas synthesized new protein rapidly and reproducibly. The average S.D. of the isotope ratios measured on gel slices from replicate experiments was 2.2% of the mean. Retinas driven by flashing light of constant intensity exhibited a marked increase in ganglion cell firing and accumulated 38% more (p = 0.005) 2-deoxyglucose than their controls kept in darkness. However, they did not differ from the controls in the incorporation of labeled leucine into total protein or into any of the protein fractions separated on the gel, the largest deviation from unity in the 95 test-control ratios being 1.7%. Continuous light and flashing light of increasing intensity also markedly affected function and energy metabolism, but had no significant effects on leucine incorporation. None of the stimuli affected degradation, though the experiments would have been less sensitive to a change in degradation that to a change in synthesis. These results indicate that the synthesis and degradation of proteins are little affected by a marked increase in functional activity.

Calmodulin--an intracellular calcium receptor

Calmodulin, a protein that binds calcium with high affinity and specificity, is structurally conserved and functionally preserved throughout the animal and plant kingdoms. It serves as an intracellular Ca2+-receptor and mediates the Ca2+ regulation of cyclic nucleotide and glycogen metabolism, secretion, motility and Ca2+ transport. Calmodulin is also a dynamic component of the mitotic apparatus.

Multiple methylation in processing of sensory signals during bacterial chemotaxis

The multiple banding pattern on NaDodSO4/polyacrylamide electrophoresis of a subset of the signal transduction proteins involved in bacterial chemotaxis has been shown to be caused by multiple methylation of a single gene product. At least four methyl groups are introduced per molecule of the sensing membrane protein to give a total of five bands. The separation of the bands appears to correspond to a Mr difference of 1500, probably caused by the binding of different amounts of DodSO4 molecules as carboxyl groups are modified. Multiple methylation, similar to multiple phosphorylation in other systems, appears to offer advantages in control and sensory processing.

Transforming gene product of Rous sarcoma virus phosphorylates tyrosine

The protein kinase activity associated with pp60src, the transforming protein of Rous sarcoma virus, was found to phosphorylate tyrosine when assayed in an immunoprecipitate. Despite the fact that a protein kinase with this activity has not been described before, several observations suggest that pp60src also phosphorylates tyrosine in vivo. First, chicken cells transformed by Rous sarcoma virus contain as much as 8-fold more phosphotyrosine than do uninfected cells. Second, phosphotyrosine is present in pp60src itself, at one of the two sites of phosphorylation. Third, phosphotyrosine is present in the 50,000-dalton phosphoprotein that coprecipitates with pp60src extracted from transformed chicken cells. We infer from these observations that pp60src is a novel protein kinase and that the modification of proteins via the phosphorylation of tyrosine is essential to the malignant transformation of cells by Rous sarcoma virus. pp60sarc, the closely related cellular homologue of viral pp60src, is present in all vertebrate cells. This normal cellular protein, obtained from both chicken and human cells, also phosphorylated tyrosine when assayed in an immunoprecipitate. This is additional evidence of the functional similarity of these structurally related proteins and demonstrates that all uninfected vertebrate cells contain at least one protein kinase that phosphorylates tyrosine.

Characterization of the subunit composition of HGPRTase from human erythrocytes and cultured fibroblasts

Hypoxanthine-guanine phosphoribosyltransferase is a ubiquitous human enzyme, the inherited deficiency of which leads to a specific metabolic-neurological syndrome. Native acrylamide isoelectric focusing revealed that the human enzyme consists of different numbers of isoenzymes depending on the tissue of origin. The erythrocytic enzyme has the most isoenzymes while the enzyme from cultured fibroblasts has only a single isoenzyme. The isoenzyme pattern of the erythrocytic enzyme changes on storage of the crude hemolysate at 4 C. Treatment of the stored crude hemolysate with 4.5 M urea and 0.35 mM beta-mercaptoethanol results in an isoenzyme pattern similar to that of the fresh crude extract. Thus the additional isoenzymes are generated on storage not by covalent modification of the enzyme but probably by binding of small molecules to the enzyme or to association of the enzyme molecules. Hypoxanthine-guanine phosphoribosyltransferase has been purified to 80% homogeneity in three steps, DEAE Sephadex chromatography, heat treatment at 85 C for 5 min, and hydroxylapatite chromatography. Denaturing two-dimensional gel electrophoresis of the erythrocytic enzyme revealed that the erythrocytic enzyme is composed of three major types of subunits (1-3) with the same molecular weight but different isoelectric points. In contrast, the fibroblast enzyme is composed of only a single type of subunit, which comigrates with subunit 1 of the erythrocytic enzyme. Since there is a single genetic locus in humans for HGPRTase (the enzyme is X linked) (Nyhan et al., 1967), the observed subunit modification of the erythrocyte enzyme appears to be the result of posttranslational modification. These findings provide a simple explanation for the observed electrophoretic properties of human HGPRTase. A patient with 0.5% of HGPRTase activity in his erythrocytes was found to have small amounts (greater than 0.5% but less than 5% of normal) of the erythrocytic HGPRTase subunits.

Posttranslational modification of elongation factor 2 in diphtheria-toxin-resistant mutants of CHO-K1 cells

We have identified two types of mutants of Chinese hamster ovary cells in which the unique ADP-ribose attachment site in elongation factor 2 (EF-2) is altered, thereby rendering them resistant to diphtheria and Pseudomonas toxins (TOXR). The first is mutant in the gene for EF-2 and possesses a permanently altered, TOXR gene product. The second lacks a component of a posttranslational modification system that converts TOXR EF-2 to the toxin-sensitive (TOXS) state. We postulate that this modification system is involved in the conversion of a single histidine residue in EF-2 to the specific target of toxin-catalyzed ADP-ribosylation, the novel amino acid X. We have designated the second type MOD- mutants. The missing of nonfunctional component in the MOD- mutants can be restored by hybridizing them with either normal TOXS cells or with EF-2 structural gene mutants. The TOXR EF-2 from MOD- mutants is also converted to toxin sensitivity in vitro by incubation with extracts of TOXS or EF-2 gene mutant cells in the presence of an energy-generating system. Our results demonstrate that EF-2 can be synthesized and released from ribosomes in a toxin-resistant form and then converted to toxin sensitivity by posttranslational modification.

The nuclear envelope lamina is reversibly depolymerized during mitosis

The nuclear envelope lamina is a supramolecular protein assembly associated with the nucleoplasmic surface of the inner nuclear membrane, which contains three predominant polypeptide components in mammalian cells (lamins A, B and C). We previously demonstrated by immunofluorescence microscopy that the lamina is reversibly disassembled during cell division, coincident with the disassembly and reconstruction of the mitotic nuclear envelope architecture. In this paper, these immunocytochemical observations are extended with cell fractionation and immunoprecipitation studies performed on synchronized populations of tissue culture cells. With these techniques, we have established that during mitosis, lamina A and C occur in a soluble and nonmembrane-associated state. In contrast, the mitotic lamin B may be associated with membrane fragments derived from the disassembled interphase nuclear envelope. From sedimentation analysis on sucrose gradients, we have determined that all three lamins are monomeric at periods of mitotic lamina disassembly. These results, together with quantitative immunoprecipitation studies, demonstrate that the lamina is reversibly depolymerized during cell division. Attendant with the depolymerized state of the lamina, the mitotic lamins (which are phosphoproteins) have a distinctly more acidic isoelectric point and a substantially higher level of phosphorylation compared to their interphase counterparts. This indicates that reversible enzymatic phosphorylations of the lamins may be involved in modulating the state of polymerization of the lamina and its reversible mitotic disassembly.

Reverse turns in peptides and proteins

The evidence that reverse turns frequently occur as structural components of proteins, as well as of linear and cyclic peptides, is overwhelming. This review summarizes and examines critically the experimental evidence derived from physical methods such as 1H and 13C nuclear magnetic resonance spectroscopy, spin-lattice relaxation time, circular dichroism, IR spectroscopy, and X-ray crystallography. Secondly, theoretical evidence obtained from energy calculations, which rely on empirical energy functions, and correlative methods, which rely on algorithms based on the frequency of occurrence of amino acids, is evaluated. In particular, those theoretical studies for which complementary physical studies have been completed are emphasized. Finally, examples of structure-function relationships involving reverse turns and their biological recognition are demonstrated.

The halophilic properties of pyruvate kinase from Vibrio costicola, a moderate halophile

Pyruvate kinase from Vibrio costicola, a moderate halophile, appears to be adapted to functioning in the presence of salt. Its stability depends on the ionic strength of the medium. The amino acid composition resembles that of other halophilic enzymes. It is proposed that the halophilic pyruvate kinase utilizes preferentially the Mn++ cofactor which forms more stable complexes in the presence of physiological concentrations of salt.

Two-dimensional gel electrophoresis of 125I-labeled surface proteins of human fibroblasts

Human fibroblasts were labeled by the 125I-lactoperoxidase technique and the sodium dodecyl sulfate soluble proteins examined by two-dimensional gel electrophoresis to determine the charge heterogeneity of the surface proteins and test for differences in surface proteins in several hereditary disorders. Approximately 80 polypeptides were observed. Those below 65 000 daltons tended to occur as single spots, while those of higher molecular weight were often present as a series of polypeptides of similar molecular weight (charge isomers). The possible role of these proteins in cell-cell recognition is discussed.

Gene expression in Drosophila: post-translational modification of aldehyde oxidase and xanthine dehydrogenase

Maroon-like homozygotes are completely deficient for xanthine dehydrogenase (XDH) and aldehyde oxidase (AO), however, ma-l is not a structural locus for either enzyme. Quantitative immunoelectrophoresis of ma-l and wild type extracts suggests that the ma-l function must be post-translational. To determine whether the ma-l function involves some direct physical changes in XDH and/or AO the enzymes were characterized with respect to temperature sensitivity and behavior in gel sieving electrophoresis. Since the XDH and AO from complementary ma-l heterozygotes is more thermolabile and different in shape from wild type XDH and AO, we conclude that ma-l is involved in a post-translational modification of these enzymes.

Comparative aspects of milk proteins

The current status of knowledge of the major proteins of milks of various species is evaluated. Most of the non-bovine milk proteins are homologous with the recognized families of those of Bos taurus, alpha S1-caseins, alpha S2-caseins, beta-caseins, kappa-caseins, beta-lactoglobulins, and alpha-lactalbumins, each family representing a separate genetic locus specific to the mammary gland. No prominent milk protein not homologous to one of these families has yet been discovered in milk of any species. Genetic polymorphism resulting from substitutions in the polypeptide chains and various degrees of post-translational phosphorylation, glycosylation, and proteolysis have been identified in milk proteins of several species. Total protein production ranges among species from about 0.5 to 10 g/d per kg0.75 maternal weight. Proportions of the several proteins vary greatly among species, but few accurate analytical data are available except for total casein and total whey protein contents.

Studies of esterase 6 in Drosophila melanogaster. I. The genetics of a posttranslational modification

A locus has been found, an allele of which causes a modification of some allozymes of the enzyme esterase 6 in Drosophila melanogaster. There are two alleles of this locus, one of which is dominant to the other and results in increased electrophoretic mobility of affected allozymes. The locus responsible has been mapped to 3-56.7 on the standard genetic map (Est-6 is at 3-36.8). Of 13 other enzyme systems analyzed, only leucine aminopeptidase is affected by the modifier locus. Neuraminidase incubations of homogenates altered the electrophoretic mobility of esterase 6 allozymes, but the mobility differences found are not large enough to conclude that esterase 6 is sialylated.

The glycosylation of hemoglobin: relevance to diabetes mellitus

Glucose reacts nonenzymatically with the NH2-terminal amino acid of the beta chain of human hemoglobin by way of a ketoamine linkage, resulting in the formation of hemoglobin AIc. Other minor components appear to be adducts of glucose 6-phosphate and fructose 1,6-diphosphate. These hemoglobins are formed slowly and continuously throughout the 120-day life-span of the red cell. There is a two- to threefold increase in hemoglobin AIc in the red cells of patients with diabetes mellitus. By providing an integrated measurement of blood glucose, hemoglobin AIc is useful in assessing the degree of diabetic control. Furthermore, this hemoglobin is a useful model of nonenzymatic glycosylation of other proteins that may be involved in the long-term complications of the disease.