Functional size of C-terminal protein carboxyl methyltransferase from kidney basolateral plasma membranes

Mechanisms for auto-inhibition and forced product release in glycine N-methyltransferase: crystal structures of wild-type, mutant R175K and S-adenosylhomocysteine-bound R175K enzymes

Glycine N-methyltransferase (S-adenosyl-l-methionine: glycine methyltransferase, EC 2.1.1.20; GNMT) catalyzes the AdoMet-dependent methylation of glycine to form sarcosine (N-methylglycine). Unlike most methyltransferases, GNMT is a tetrameric protein showing a positive cooperativity in AdoMet binding and weak inhibition by S-adenosylhomocysteine (AdoHcy). The first crystal structure of GNMT complexed with AdoMet showed a unique "closed" molecular basket structure, in which the N-terminal section penetrates and corks the entrance of the adjacent subunit. Thus, the apparent entrance or exit of the active site is not recognizable in the subunit structure, suggesting that the enzyme must possess a second, enzymatically active, "open" structural conformation. A new crystalline form of the R175K enzyme has been grown in the presence of an excess of AdoHcy, and its crystal structure has been determined at 3.0 A resolution. In this structure, the N-terminal domain (40 amino acid residues) of each subunit has moved out of the active site of the adjacent subunit, and the entrances of the active sites are now opened widely. An AdoHcy molecule has entered the site occupied in the "closed" structure by Glu15 and Gly16 of the N-terminal domain of the adjacent subunit. An AdoHcy binds to the consensus AdoMet binding site observed in the other methyltransferase. This AdoHcy binding site supports the glycine binding site (Arg175) deduced from a chemical modification study and site-directed mutagenesis (R175K). The crystal structures of WT and R175K enzymes were also determined at 2.5 A resolution. These enzyme structures have a closed molecular basket structure and are isomorphous to the previously determined AdoMet-GNMT structure. By comparing the open structure to the closed structure, mechanisms for auto-inhibition and for the forced release of the product AdoHcy have been revealed in the GNMT structure. The N-terminal section of the adjacent subunit occupies the AdoMet binding site and thus inhibits the methyltransfer reaction, whereas the same N-terminal section forces the departure of the potentially potent inhibitor AdoHcy from the active site and thus facilitates the methyltransfer reaction. Consequently GNMT is less active at a low level of AdoMet concentration, and is only weakly inhibited by AdoHcy. These properties of GNMT are particularly suited for regulation of the cellular AdoMet/AdoHcy ratio.

The carboxyl methyltransferase modifying G proteins is a metalloenzyme

The prenylated protein carboxyl methyltransferase (PPMT) catalyzes the posttranslational methylation of isoprenylated C-terminal cysteine residues found in many signaling proteins such as the small monomeric G proteins and the gamma subunits of heterotrimeric G proteins. Here we report that both membrane-bound PPMT from rat kidney and the recombinant bacterially expressed form of the enzyme required divalent cations for catalytic activity. Unlike EDTA and EGTA, the metal chelator 1,10-phenanthroline strongly inhibited the PPMT activity of kidney intracellular membranes in a dose- and time-dependent manner. 1,10-Phenanthroline was found to inhibit the methylation of the prenylcysteine analog N-acetyl-S-all-trans-geranylgeranyl-l-cysteine, a synthetic substrate for PPMT, with an IC(50) of 2.2 mM. Gel electrophoretic analysis demonstrated that 1,10-phenanthroline almost totally abolished the labeling of methylated proteins in kidney intracellular membranes. Immunoblotting analysis showed that one of the two major peaks of (3)H-methylated proteins in intracellular membranes comigrated with the small G proteins Ras, Cdc42, RhoA, and Rab1. In addition, the methylation of immunoprecipitated Ras and RhoA from kidney intracellular membranes was strongly inhibited when 1,10-phenanthroline was present. Treatment of kidney intracellular membranes with 1,10-phenanthroline increased the proteolytic degradation of PPMT by exogenous trypsin, compared to untreated membranes. We conclude from these data that metal ions are essential for the activity and the stabilization of PPMT. The finding that PPMT is a metalloenzyme may provide new insights into the functions played by this methyltransferase in signal transduction processes.

Guanosine 5'-(3-O-Thio)triphosphate stimulates protein carboxyl methylation in cell membranes

Using guanosine 5'-(3-O-thio)triphosphate (GTPgammaS), we previously reported that protein carboxyl methyltransferase activities in kidney brush border membranes were increased by the GTP analog (Arch. Biochem. Biophys. 351, 149-158, 1998). Here, we investigated the distribution and characterized the effect of GTPgammaS on protein carboxyl methylation activity. The analysis of species distribution of carboxyl methylation in kidney brush border membranes showed that the GTPgammaS strongly stimulated this activity in rat (15.9-fold), mouse (14.7-fold), human (2.9-fold), and rabbit (2.7-fold). Analysis of GTPgammaS-dependent carboxyl methylation in rat tissues and cell fractions indicated that the activity was mainly localized in membranes of intestine, lung, and kidney, with the highest activity found in liver. To characterize the methyltransferase activity modulated by GTPgammaS in liver membranes, their sensitivity to the detergent 3-[(3-cholamido)dimethylammonio]-1-propanesulfonic acid (Chaps) was used. Methylation of N-acetyl-S-farnesyl cysteine, a prenylated protein methyltransferase (PPMT) substrate was strongly inhibited (86%) in the presence of Chaps, while the methylation of bovine calmodulin and ovalbumin, both of which are substrates for the protein L-isoaspartyl/d-aspartyl methyltransferase (PIMT), was slightly reduced by the detergent (0-12%). The GTPgammaS-dependent carboxyl methylation of endogenous substrates in liver membranes was decreased by 35% in the presence of Chaps, suggesting that PPMT was not the predominant methyltransferase involved in the methylation stimulated by GTPgammaS in liver membranes. Electrophoretic analysis showed that radioactive methylation of several substrates induced by GTPgammaS in liver membranes was reduced by adding calmodulin. Interestingly, addition of GTPgammaS partially inhibited the methylation of two PIMT substrates, ovalbumin (24%) and bovine calmodulin (19%), when incubated with liver membranes. Immunoprecipitation of PIMT from liver and lung membranes strongly inhibited (88-94%) the methylation stimulated by GTPgammaS. Altogether, these data support the hypothesis that GTPgammaS could regulate PIMT activity and may provide new insights into the function of the methyltransferase.

Regulation by GTPgammaS of protein carboxylmethyltransferase activity in kidney brush border membranes

The increase in carboxyl methylation induced by guanosine 5',3-O-(thio)triphosphate (GTPgammaS) in brush border membranes from rat kidney cortex was studied, and the methyltransferase activities affected by this nucleotide analog were identified. Addition of GTPgammaS to brush border membranes stimulated the carboxyl methylation in a time-dependent manner while adenosine and guanine nucleotides were ineffective. The GTPgammaS-dependent carboxyl methylation was inhibited by the chelating agents EDTA (63%) and 1,10-phenanthroline (68%), suggesting that this activity required divalent cations. The methyl ester groups induced by the addition of GTPgammaS to brush border membranes were unstable, with about 80% of them hydrolyzed following 1 h incubation at 37 degrees C. The GTPgammaS stimulation of the carboxyl methylation in brush border membranes was unaffected by the detergent 3-[(3cholamido)-dimethylammonio]-1-propanesulfonic acid up to a concentration of 0.4% (w/v). At this latter detergent concentration, the activity of prenylated protein methyltransferase (PPMT) was strongly inhibited and that of l-isoaspartyl/d-aspartylmethyltransferase (PIMT) was increased twofold, as measured with their respective exogenous substrates, N-acetyl-S-farnesyl cysteine and ovalbumin. GTPgammaS increased the methylation of several substrates in brush border membranes. The induced methylation in substrates migrating between 20 and 36 kDa was strongly decreased by the competitive inhibitor farnesylthioacetic acid, a synthetic farnesylated substrate for PPMT, while a delta-sleep-inducing peptide containing an L-isoaspartyl residue inhibited that of substrates with molecular weights above 36 kDa, suggesting that PIMT activity was also involved. This interpretation was strengthened by the observation that the increased methylation induced by GTPgammaS in these membrane substrates was completely lost following their analysis by gel electrophoresis under alkaline conditions. Taken together, these results indicate that both PPMT and PIMT activities are regulated by guanine nucleotides in brush border membranes of rat kidney.