Protein carboxymethylation in rat islets of Langerhans

Specificity of the CheR2 methyltransferase in Pseudomonas aeruginosa is directed by a C-terminal pentapeptide in the McpB chemoreceptor

Methyltransferases of the CheR family and methylesterases of the CheB family control chemoreceptor methylation, and this dynamic posttranslational modification is necessary for proper chemotaxis of bacteria. Studies with enterobacteria that contain a single CheR or CheB show that, in addition to binding at the methylation site, some chemoreceptors bind CheR or CheB through additional high-affinity sites at distinct pentapeptide sequences in the chemoreceptors. We investigated the recognition of chemoreceptors by CheR proteins in the human pathogen Pseudomonas aeruginosa PAO1. Of the four methyltransferases in PAO1, we detected an interaction only between CheR2 and the chemoreceptor methyl-accepting chemotaxis protein B (McpB), which contains the pentapeptide GWEEF at its carboxyl terminus. Furthermore, CheR2 was also the only paralog that methylated McpB in vitro, and deletion of the pentapeptide sequence abolished both the CheR2-McpB interaction and the methylation of McpB. When clustered according to protein sequence, bacterial CheR proteins form two distinct families-those that bind pentapeptide-containing chemoreceptors and those that do not. These two families are distinguished by an insertion of three amino acids in the β-subdomain of CheR. Deletion of this insertion in CheR2 prevented its interaction with and methylation of McpB. Pentapeptide-containing chemoreceptors are common to many bacteria species; thus, these short, distinct motifs may enable the specific assembly of signaling complexes that mediate different responses.

High specificity in CheR methyltransferase function: CheR2 of Pseudomonas putida is essential for chemotaxis, whereas CheR1 is involved in biofilm formation

Chemosensory pathways are a major signal transduction mechanism in bacteria. CheR methyltransferases catalyze the methylation of the cytosolic signaling domain of chemoreceptors and are among the core proteins of chemosensory cascades. These enzymes have primarily been studied Escherichia coli and Salmonella typhimurium, which possess a single CheR involved in chemotaxis. Many other bacteria possess multiple cheR genes. Because the sequences of chemoreceptor signaling domains are highly conserved, it remains to be established with what degree of specificity CheR paralogues exert their activity. We report here a comparative analysis of the three CheR paralogues of Pseudomonas putida. Isothermal titration calorimetry studies show that these paralogues bind the product of the methylation reaction, S-adenosylhomocysteine, with much higher affinity (KD of 0.14-2.2 μM) than the substrate S-adenosylmethionine (KD of 22-43 μM), which indicates product feedback inhibition. Product binding was particularly tight for CheR2. Analytical ultracentrifugation experiments demonstrate that CheR2 is monomeric in the absence and presence of S-adenosylmethionine or S-adenosylhomocysteine. Methylation assays show that CheR2, but not the other paralogues, methylates the McpS and McpT chemotaxis receptors. The mutant in CheR2 was deficient in chemotaxis, whereas mutation of CheR1 and CheR3 had either no or little effect on chemotaxis. In contrast, biofilm formation of the CheR1 mutant was largely impaired but not affected in the other mutants. We conclude that CheR2 forms part of a chemotaxis pathway, and CheR1 forms part of a chemosensory route that controls biofilm formation. Data suggest that CheR methyltransferases act with high specificity on their cognate chemoreceptors.

Glucose activates the carboxyl methylation of gamma subunits of trimeric GTP-binding proteins in pancreatic beta cells. Modulation in vivo by calcium, GTP, and pertussis toxin

The gamma subunits of trimeric G-proteins (gamma1, gamma2, gamma5, and gamma7 isoforms) were found to be methylated at their carboxyl termini in normal rat islets, human islets and pure beta [HIT-T15] cells. Of these, GTPgammaS significantly stimulated the carboxyl methylation selectively of gamma2 and gamma5 isoforms. Exposure of intact HIT cells to either of two receptor-independent agonists--a stimulatory concentration of glucose or a depolarizing concentration of K+--resulted in a rapid (within 30 s) and sustained (at least up to 60 min) stimulation of gamma subunit carboxyl methylation. Mastoparan, which directly activates G-proteins (and insulin secretion from beta cells), also stimulated the carboxyl methylation of gamma subunits in intact HIT cells. Stimulatory effects of glucose or K+ were not demonstrable after removal of extracellular Ca2+ or depletion of intracellular GTP, implying regulatory roles for calcium fluxes and GTP; however, the methyl transferase itself was not directly activated by either. The stimulatory effects of mastoparan were resistant to removal of extracellular Ca2+, implying a mechanism of action that is different from glucose or K+ but also suggesting that dissociation of the alphabetagamma trimer is conducive to gamma subunit carboxyl methylation. Indeed, pertussis toxin also markedly attenuated the stimulatory effects of glucose, K+ or mastoparan without altering the rise in intracellular calcium induced by glucose or K+. Glucose-induced carboxyl methylation of gamma2 and gamma5 isoforms was vitiated by coprovision of any of three structurally different cyclooxygenase inhibitors. Conversely, exogenous PGE2, which activates Gi and Go in HIT cells and which thereby would dissociate alpha from beta(gamma), stimulated the carboxyl methylation of gamma2 and gamma5 isoforms and reversed the inhibition of glucose-stimulated carboxyl methylation of gamma subunits elicited by cyclooxygenase inhibitors. These data indicate that gamma subunits of trimeric G-proteins undergo a glucose- and calcium-regulated methylation-demethylation cycle in insulin-secreting cells, findings that may imply an important role in beta cell function. Furthermore, this is the first example of the regulation of the posttranslational modification of G-protein gamma subunits via nonreceptor-mediated activation mechanisms, which are apparently dependent on calcium influx and the consequent activation of phospholipases releasing arachidonic acid.

Modulation of insulin secretion from normal rat islets by inhibitors of the post-translational modifications of GTP-binding proteins

Many GTP-binding proteins (GBPs) are modified by mevalonic acid (MVA)-dependent isoprenylation, carboxyl methylation or palmitoylation. The effects of inhibitors of these processes on insulin release were studied. Intact pancreatic islets were shown to synthesize and metabolize MVA and to prenylate several candidate proteins. Culture with lovastatin (to inhibit synthesis of endogenous MVA) caused the accumulation in the cytosol of low-M(r) GBPs (labelled by the [alpha-32P]GTP overlay technique), suggesting a disturbance of membrane association. Concomitantly, lovastatin pretreatment reduced glucose-induced insulin release by about 50%; co-provision of 100-200 microM MVA totally prevented this effect. Perillic acid, a purported inhibitor of the prenylation of small GBPs, also markedly reduced glucose-induced insulin secretion. Furthermore, both N-acetyl-S-trans,trans-farnesyl-L-cysteine (AFC), which inhibited the base-labile carboxyl methylation of GBPs in islets or in transformed beta-cells, and cerulenic acid, an inhibitor of protein palmitoylation, also reduced nutrient-induced secretion; an inactive analogue of AFC (which did not inhibit carboxyl methylation in islets) had no effect on secretion. In contrast with nutrients, the effects of agonists that induce secretion by directly activating distal components in signal transduction (such as a phorbol ester or mastoparan) were either unaffected or enhanced by lovastatin or AFC. These data are compatible with the hypothesis that post-translational modifications are required for one or more stimulatory GBPs to promote proximal step(s) in fuel-induced insulin secretion, whereas one or more inhibitory GBPs might reduce secretion at a more distal locus.

Mechanisms of action involved in the chemoattractant activity of three beta-lactamic antibiotics upon human neutrophils

The effects produced by three beta-lactamic antibiotics (N-formimidoyl thienamycin or imipenem, cefmetazole and cefoxitin) in vitro on protein carboxylmethylation, cAMP and cGMP levels in human polymorphonuclear neutrophils were studied. These antibiotics (50 micrograms/mL) exhibited chemoattractant activity for phagocytic cells and produced a fast (0.5 min) and significant stimulation of protein carboxylmethylation. They also increased intraphagocytic cGMP levels although no changes in cAMP levels were observed. Since the involvement of the above-mentioned mechanisms in leukotaxis have been established, the stimulation of neutrophil chemotaxis by the three antibiotics studied could possibly be mediated by one or more of these mechanisms.

Carboxylmethylation of insulin and glucagon in vitro

We studied the methyl acceptor capacity of insulin and glucagon in vitro. The levels of carboxylmethylation of pancreatic hormones (dpm x 10(3], when incubated with S-adenosyl-L-(3H-methyl)-methionine as methyl donor and purified protein carboxylmethylase, were: insulin (n = 6) 8.1 +/- 0.2 and 11.1 +/- 1.5 (mean +/- SEM) for 0.25 and 1.0 mg/ml, respectively; glucagon (n = 6) 17.0 +/- 3.2 and 40.2 +/- 2.5 (mean +/- SEM) for 0.5 and 1.0 mg/ml, respectively. On a molar basis, the methyl acceptor capacity was 1.0 dpm/pmol for insulin and 9.5 dpm/pmol for glucagon. Polyacrylamide gel electrophoresis of carboxylmethylated hormones showed a radioactivity (3H-methyl) peak that co-migrated with the corresponding 125I-hormone. Glucagon, but not insulin, seems to be a relatively good substrate for carboxylmethylation.

Correlation of sinefungin susceptibility and drug-affinity for protein carboxymethyltransferase activity in American Leishmania species

Among promastigotes of 22 different American Leishmania strains, a 5000-fold variation in sinefungin susceptibility was found, apparently independent of their taxonomic classification, although L. mexicana strains did tend to be more resistant than L. braziliensis. Protein carboxymethyltransferase (EC 2.1.1.24) and glycine N-methyltransferase (EC 2.1.1.20) activities were not substantially different in sinefungin-susceptible and -resistant American Leishmania strains. However, when [methyl-3H]methionine incorporation into total protein or gamma-glutamyl residues of leishmanial proteins was carried out in the presence or absence of sinefungin, protein carboxymethylating activity was significantly inhibited only in sinefungin-susceptible Leishmania strains. Furthermore, when protein carboxymethyltransferase activity was purified from several leishmanial strains to a state of electrophoretic homogeneity (sp. act. = 240 nmol h-1 (mg protein)-1), the enzyme from the resistant cells showed a higher inhibition constant (mean Ki 55 microM against 2 microM in susceptible cells) for sinefungin. This 28-times stronger affinity of the susceptible cell enzyme towards sinefungin despite normal protein carboxymethyltransferase specific activity seems to be a key element of the resistance mechanism of certain American Leishmania strains.

What is the function of protein carboxyl methylation?

The following functions of protein carboxyl methylation seem to be reasonably well established: Multiple, stoichiometric methylation of chemotactic receptors in bacteria at glutamyl residues serves as one (but not the only) adaptation mechanism of the transduction chain to constant background levels of chemotactic stimuli. Stoichiometric methylation of hormones and hormone carrier proteins plays a role in hormone storage and secretion by the pituitary gland. Substoichiometric methylation at D-aspartyl residues is involved in a repair mechanism of aged proteins. Stoichiometric methylation of calmodulin modulates the sensitivity of calmodulin-dependent processes to calcium. Research of the past 3 years has indicated that in order to demonstrate an involvement of methylation in the coupling of surface receptors to intracellular events three new criteria have to be met: (a) the cell should possess a protein carboxyl methylase with relatively narrow substrate specificity; (b) methylation should take place at L-amino acid residues; (c) the methyl accepting proteins should be methylated in a stoichiometric fashion.

Stimulation of protein methylase II from Torpedo marmorata by cholinergic effectors

The enzymatic transfer of methyl groups mediated by protein methylase II onto proteins of the electroplaque tissue of Torpedo marmorata is described. The protein methylase II resides to the extent of 80% in the cytosol and 20% in the acetylcholine receptor-rich membrane. The kinetics of the methyl-group transfer are characteristically different in the cytosol and membrane fractions. The reaction is inhibited by phosphate with IC50 = 450 microM. The cholinergic effectors carbamoylcholine, flaxedil and alpha-bungarotoxin applied to the outside of the acetylcholine receptor-rich membrane vesicles stimulated the protein methylase II which is exclusively located inside the vesicles. The stimulation is biphasic and transient, yielding an increased initial velocity and a peak of activity at 2 min after the addition of the effector. The stimulation by carbamoylcholine is qualitatively similar to that elicited by the antagonist. In addition, the protein methylase II is stimulated transiently by phospholipase A2 with a time-course clearly different from that of the cholinergic effectors. We conclude that the conformational change in the receptor-protein elicited by cholinergic effectors is efficiently transduced to the cytoplasmic methylation sites.

A diffusion assay for detection and quantitation of methyl-esterified proteins on polyacrylamide gels

The methyl esterification of bacterial and mammalian proteins is a subject of increasing interest and effort. Such studies in intact cells typically involve the use of [methyl-3H]methionine which is taken up and incorporated into S-adenosyl-L-methionine, the methyl donor. The level of methylation, however, is much less than the incorporation of labeled methionine directly into protein. A diffusion assay which distinguishes [3H]methionine from the base-labile [3H]methyl esters is described here. The ester linkage is hydrolyzed at high pH to release [3H]methanol from the sample which diffuses into an adjacent pool of scintillation fluid. The assay is contained in a scintillation vial which can be counted directly.

Impairment of insulin release by methylation inhibitors

The possible participation of enzymatic methylation reactions in the process of insulin release was investigated in rat pancreatic islets. The combination of 3-deazaadenosine and DL-homocysteine impaired the incorporation of 3H-methyl from L-[methyl-3H]methionine into endogenous islet proteins and phospholipids, but failed to affect turnover in the phosphatidylinositol cycle. The inhibitors of methylation decreased insulin release evoked by D-glucose or the combinations of D-glucose and gliclazide, L-leucine and L-glutamine, or Ba2+ and theophylline. The inhibitors of methylation did not impair either the oxidation of D-glucose or affect its capacity to decrease K+ conductance, stimulate Ca2+ inflow and provoke 45Ca accumulation in pancreatic islets. It is proposed that, in the process of insulin secretion, a methyl acceptor protein and/or phospholipid play(s) a limited modulatory role in the coupling of cytosolic Ca2+ accumulation to exocytosis.

Separation of signal transduction and adaptation functions of the aspartate receptor in bacterial sensing

In order to investigate the functions of stimulus recognition, signal transduction, and adaptation, the aspartate receptor gene for bacterial chemotaxis in Salmonella typhimurium has been sequenced and modified. A carboxyl-terminal truncated receptor was shown to bind aspartate and to transmit a signal to change motility behavior. However, the truncated receptor showed greatly reduced methyl-accepting capacity, and did not allow adaptation to the sensory stimulation. The separation of receptor functions by alteration of primary structure emphasizes that the receptor is directly involved in adaptation and is not solely a device for transmitting a signal across a membrane.