Do eukaryotic carboxyl methyltransferase regulate protein function?

Protein carboxyl methylation in kidney brush-border membranes

Protein carboxyl methylation activity was detected in the cytosol and in purified brush-border membranes (BBM) from the kidney cortex. The protein carboxyl methyltransferase (PCMT) activity associated with the BBM was specific for endogenous membrane-bound protein substrates, while the cytosolic PCMT methylated exogenous substrates (ovalbumin and gelatin) as well as endogenous proteins. The apparent Km for S-adenosyl-L-methionine with endogenous proteins as substrates were 30 microM and 4 microM for the cytosolic and BBM enzymes, respectively. These activities were sensitive to S-adenosyl-L-homocysteine, a well known competitor of methyltransferase-catalyzed reactions, but were not affected by the presence of chymostatin and E-64, two protein methylesterase inhibitors. The activity of both cytosolic and BBM PCMT was maximal at pH 7.5, while BBM-phospholipid methylation was predominant at pH 10.0. Separation of the = methylated proteins by acidic gel electrophoresis in the presence of the cationic detergent benzyldimethyl-n-hexadecylammonium chloride revealed distinct methyl accepting proteins in the cytosol (14, 17, 21, 27, 31, 48, 61 and 168 kDa) and in the BBM (14, 60, 66, 82, and 105 kDa). Most of the labelling was lost following electrophoresis under moderately alkaline conditions, except for a 21 kDa protein in the cytosol and a 23 kDa protein in the BBM fraction. These results suggest the existence of two distinct PCMT in the kidney cortex: a cytosolic enzyme with low selectivity and affinity, methylating endogenous and exogenous protein substrates, and a high-affinity BBM-associated methylating activity.

Protein methylation inhibits Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles

We have examined the effect of membrane methylation on the Na+-Ca2+ exchange activity of canine cardiac sarcolemmal vesicles using S-adenosyl-L-methionine as methyl donor. Methylation leads to approximately 40% inhibition of the initial rate of Nai+-dependent Ca2+ uptake. The inhibition is due to a lowering of the Vmax for the reaction. The inhibition is not due to an effect on membrane permeability and is blocked by S-adenosyl-L-homocysteine, an inhibitor of methylation reactions. The following experiments indicated that inhibition of Na+-Ca2+ exchange was due to methylation of membrane protein and not due to methylated phosphatidylethanolamine (PE) compounds (i.e., phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N'-dimethylethanolamine (PDME]: (1) We solubilized sarcolemma and reconstituted activity into vesicles containing no PE. The inhibition by S-adenosyl-L-methionine was not diminished in this environment. (2) We reconstituted sarcolemma into vesicles containing PMME or PDME. These methylated lipid components had no effect on Na+-Ca2+ exchange activity. (3) We verified that many membrane proteins, probably including the exchanger, become methylated.

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.

Protein carboxyl methylation in synaptic membrane of rat brain: the possible presence of adenosine-bound S-adenosyl-L-homocysteine hydrolase in the membrane

The effects of some neurotransmitters, adenosine (Ad), and homocysteine (Hcys) on protein carboxyl methylation in synaptic plasma membranes from rat cerebral cortex were examined. Neither any of the neurotransmitters nor Ad had a detectable effect. Incubation of membrane with DL-Hcys alone (5 X 10(-5) M), the combination of both Ad (5 X 10(-5)) and DL-Hcys (5 X 10(-5)), or S-adenosyl-L-homocysteine (SAH) (1 X 10(-6)) strongly decreased the methyl ester formation. The inhibitory effect of the combination of both compounds may be interpreted in terms of the increased SAH concentration due to the presence of SAH hydrolase in the membrane. The inhibitory effect of Hcys alone was blocked by preincubation with Ad deaminase or Neplanocin A, a potent inhibitor of SAH hydrolase, suggesting the presence of Ad-bound SAH hydrolase in the synaptic membrane. Ad-bound SAH hydrolase activity estimated by the inhibition of methylation in the presence of Hcys was located in the membrane fractions including synaptosomes, myelin, and microsomes (about 70%), but the SAH hydrolase activity estimated on the basis of the inhibitory effect of the combination of both Ad and Hcys was localized exclusively in the soluble fraction (about 90%). The distribution of the latter activity is coincident with that of SAH hydrolase reported to date. Incubation of the synaptic membrane with Hcys markedly increased the SAH concentration. The stimulatory effect of Hcys alone was blocked by Ad deaminase.

Membrane methylation in isolated rat testis interstitial cells unmasks functional luteinizing hormone receptors

Treatment of intact isolated rat testis interstitial cells with S-adenosylmethionine as methyl donor, increases substantially the number of LH human CG receptors (100-200%) without modifying the equilibrium dissociation constant. The increase in binding capacity was associated with an augmentation in the sensitivity of the rat testis interstitial cells to produce testosterone in response to LH, suggesting a functional role of the unmasked receptors. The amount of S-adenosylmethionine necessary to obtain an increase in LH binding capacity and preserve cell viability was 25-50 micrograms/ml per 1.6 X 10(7) cells. 10 mM MgCl2 in addition to the Mg2+ present in the medium was necessary to maintain cell viability. 3H-labelled methyl groups were incorporated mainly into the lipid fraction (208 fmol/10(6) cells) when 3H-S-adenosylmethionine was incubated with the cells for 2 h at 30 degrees C. Our results are consistent with the conclusion that early action of LH may involve an activation of methyltransferase activity, phospholipid methylation, an increase in LH binding capacity and an increase in receptor function.

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.

Selective deamidation and enzymatic methylation of seminal ribonuclease

Isoenzymatic forms alpha 2, alpha beta, and beta 2 of bovine seminal ribonuclease are generated by the transformation of beta-type into alpha-type subunit through deamidation of a single amide group [Di Donato, A., & D'Alessio, G. (1981) Biochemistry 20, 7232-7237]. The residue involved in this selective deamidation has been identified as Asn67. Deamidation occurs by formation of a cyclic imide intermediate involving the Gly at position 68. Opening of the cyclic imide may occur on either side of the nitrogen, generating both the normal alpha-aspartyl and an isoaspartyl residue at position 67. The alpha-carboxyl of the isoaspartyl residue is effectively methylated by bovine brain protein carboxylmethyltransferase.

Protein carboxyl methyltransferase and methyl acceptor proteins in aging and cataractous tissue of the human eye lens

We have studied the enzymatic modification of proteins in human eye lens tissue where these molecules can be long-lived and can be exposed to non-enzymatic degradation processes for periods of time up to the age of the individual. We have detected a protein carboxyl methyltransferase that is similar to enzymes from other mammalian tissues which appear to catalyze the methyl esterification of altered aspartyl residues, including D-aspartyl and beta-isomerized L-aspartyl residues, but which have no activity on normal L-aspartyl sites. Upon gel filtration of human lens extracts, we find protein substrates for the lens methyltransferase in each of the major soluble classes of protein. In comparing individual lenses of various ages, protein carboxyl methyltransferase activity was present in tissue from all normal and yellow cataractous lenses tested, but was present only at very low apparent levels in brunescent lens tissue. We find that the methyltransferase is much more highly saturated by endogenous methyl acceptor substrates in lens extracts from older individuals, suggesting that the prolonged in vivo aging of lens protein leads to the accumulation and perhaps metabolism of altered aspartyl residues.

Enzymatic basis for the calcium-induced decrease of membrane protein methyl esterification in intact erythrocytes. Evidence for an impairment of S-adenosylmethionine synthesis

The effect of Ca2+ loading, induced by the ionophore A23187, on methyl esterification of membrane proteins (i.e. bands 2.1, 3, 4.1 and 4.5) has been investigated in intact human erythrocytes. When the cells were incubated with L-[methyl-3H]methionine, 40 microM CaCl2 and 10 microM A23187 induce a 50% inhibition of membrane protein methyl esterification. This effect is selectively due to the increased intracellular Ca2+ concentration, as it is antagonized by 10 mM EGTA, and other divalent cations such as Mn2+ do not exert any inhibition. In order to clarify the mechanism(s) of the reported inhibition, the various events involved in the methyl esterification process in vivo were analyzed. L-Methionine uptake as well as protein methylase II activity are not directly affected by altered intracellular Ca2+ concentrations. Conversely in the Ca2+-loaded erythrocytes the conversion of [3H]methionine into [3H]AdoMet, catalyzed by AdoMet synthetase, decreases up to 25%. When the undialyzed erythrocyte cytosolic fraction is assayed in vitro for AdoMet synthetase the activity of the enzyme from the CaCl2/A23187-treated erythrocytes is significantly lower than the control, up to 5 mM ATP. This result suggests that in the Ca2+-loaded erythrocytes the ATP intracellular concentration is significantly lowered. The direct evaluation of ATP intracellular concentration, by HPLC, confirms a significant drop of ATP level, as a consequence of the Ca2+ loading. The removal of Ca2+ from the cells quantitatively restores both the AdoMet synthesis and the methyl esterification levels. The possible role of altered ATP intracellular concentrations as a regulatory factor in the AdoMet-dependent reactions as well as in post-translational protein methylation related to the ageing process is also discussed.

Protein-O-carboxylmethyltransferase in the rat brain: high regional levels in the substantia nigra, locus coeruleus and paraventricular nucleus

Immunocytochemical techniques were used to localize protein-O-carboxylmethyltransferase in the rat brain. Particularly high levels of immunoreactive protein-O-carboxylmethyltransferase were found in the paraventricular and supraoptic nucleus, the substantia nigra and the locus coeruleus. The enhanced expression of the methyltransferase in these brain regions suggests that protein carboxylmethylation is of particular importance in these areas. These findings are consistent with previous biochemical studies which suggest that protein methylation plays a role in presynaptic monoaminergic neurons and in the release and/or processing of neurohypophyseal peptides.

Identification and topography of substrates for protein carboxyl methyltransferase in synaptic membrane and myelin-enriched fractions of bovine and rat brain

The major components of crude brain synaptosomes (synaptic membranes, mitochondria, and myelin) have been separated and analyzed by polyacrylamide gel electrophoresis for the presence of proteins that serve as substrates for protein carboxyl methyltransferase. Of the three fractions, synaptic membranes contain the largest number of individual methyl acceptors (at least seven), while mitochondria contain no well-defined methyl acceptors. Undisrupted myelin contains a single major methyl acceptor with a very low apparent molecular weight. The patterns of protein methylation in synaptic membranes prepared from cerebral cortex, hippocampus, striatum, thalamus, and tectum showed marked differences; however, these differences could largely be explained by differential degrees of myelin contamination in synaptic membranes from the different regions. The effect of trypsin pretreatment on the carboxyl methylation of intact and lysed synaptosomes was studied to estimate the sidedness of the major methylation sites on synaptic membranes. One of the methyl acceptors (Mr 48K) appears to be facing the intracellular surface of the synaptosome, but most sites appear to be outward facing.

Demethylation of protein carboxyl methyl esters: a nonenzymatic process in human erythrocytes?

We have compared the demethylation rate of protein carboxyl methyl esters from isolated human erythrocyte membranes with the corresponding rate of metabolic turnover of these same methyl groups in the intact erythrocyte. Surprisingly, the apparent spontaneous demethylation of these membrane protein methyl esters was significantly faster at physiological pH than the corresponding rate determined by pulse-chase analysis of intact cells incubated with L-[methyl-3H]methionine. Readdition of erythrocyte lysate to purified membranes did not increase the rate of demethylation, as might be expected if there were cytosolic or membrane-bound protein methylesterase activity, but resulted instead in an apparent stabilization of these methyl esters. Thus, the metabolic lability of these protein methyl esters in intact cells may be quantitatively explained by spontaneous, rather than enzymatic, demethylation reactions. A model is presented in which a rapid but nonenzymatic intramolecular demethylation reaction results in the formation of a polypeptide imide or anhydride intermediate. The metabolic fate of these hypothetical intermediates is unknown but may lead to the repair or degradation of protein D-aspartyl and L-isoaspartyl residues, which appear to be the substrates for the initial transmethylation reaction.

Unmasking of insulin receptors in rat submaxillary gland microsomes: effect of high ionic strength, phospholipase C and S-adenosyl-L-methionine

The specific binding of [125I]insulin to submaxillary gland microsomes was significantly enhanced by increasing the ionic strength of the incubation medium. This effect was neither related to changes in receptor or hormone degradation nor in the polymerization of the tracer. When equilibrium binding data from competition curves of unlabelled insulin versus [125I]insulin were analyzed, a marked increase in total binding capacity in high ionic strength was observed (from 890 to 2440 fmol/mg protein), with no change in binding affinity. Phospholipase C digestion was also able to increase specific [125I]insulin binding to microsomes. These results suggest the presence of masked receptors in submaxillary gland microsomes. Methylation of rat submaxillary gland microsomes by using S-adenosyl-L-methionine as the methyl donor significantly increased [125I]insulin binding. Scatchard analysis of the equilibrium binding data showed that addition of S-adenosyl-L-methionine (0.46 mM) to microsomes resulted in an enhancement of the total binding capacity (from 990 to 1520 fmol/mg protein) with no change in the affinity constants, which suggests the exposure of masked insulin receptors under such conditions. Both the methyl group incorporation into membrane phospholipids and the effect on insulin binding were dependent on the S-adenosyl-L-methionine concentration used and were partially suppressed in the presence of S-adenosyl-L-homocysteine, a specific competitive inhibitor of the methyltransferases activity. When microsomes were treated with S-adenosyl-L-[methyl-3H]methionine, the 3H-labelled methyl groups incorporated were found mainly in the lipid fraction associated to phosphatidylcholine, suggesting in this case that the unmasking of insulin receptors could be a consequence of alterations produced in membrane composition. The effects of phospholipase C, S-adenosyl-L-methionine and high ionic strength on insulin binding were not additive, suggesting that these procedures unmask receptors from the same pool.

Protein O-carboxylmethylation in relation to male gamete production and function

Protein O-carboxylmethyltransferase (PCM) activity of differentiating male germ cells in the testis and of spermatozoa is strikingly high. PCM catalyzes the methylesterification by S-adenosylmethionine of dicarboxylic amino acid residues in proteins. PCM appears to be the only type of protein methyltransferase present in mature spermatozoa. Mammalian sperms contain considerable amounts of S-adenosylmethionine and can apparently synthesize this nucleoside from L-methionine and ATP. Spermatozoa are rich in S-adenosylhomocysteine hydrolase. The characteristics of this enzyme in testicular germ cells and in sperms are very similar to those in other mammalian tissues; the very sub-stoichiometric extent of methylation of various pure protein substrates, and the rapid spontaneous hydrolysis of the protein methyl ester products at physiological and especially higher pH values, are particularly remarkable. From studies on processes related to protein O-carboxylmethylation in rat spermatozoa from different regions of the epididymis, and in ejaculated spermatozoa from normal and infertile men, unequivocal evidence could not be obtained for hypotheses of other investigators that PCM-catalyzed reactions are of regulatory importance for the acquisition of a potentiality for motility in sperms during their transit and maturation in the epididymis, or for the locomotion of ejaculated sperms. The findings are discussed in the light of the recent hypothesis of S. Clarke that PCM catalyzes methylesterification of D-aspartyl residues that accumulate in certain proteins as a result of slow spontaneous racemization of L-aspartyl residues, and that the methyl esterification of D-aspartyl residues may be related to disposal or repair of proteins damaged in this fashion.