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

PIMT Binding to C-Terminal Ala459 of CAIX Is Involved in Inside-Out Signaling Necessary for Its Catalytic Activity

Human carbonic anhydrase IX (CAIX), a unique member of the α carbonic anhydrase family, is a transmembrane glycoprotein with high enzymatic activity by which CAIX contributes to tumorigenesis through pH regulation. Due to its aberrant expression, CAIX is considered to be a marker of tumor hypoxia and a poor prognostic factor of several human cancers. Hypoxia-activated catalytic function of CAIX is dependent on posttranslational modification of its short intracellular domain. In this work, we have identified that C-terminal Ala459 residue, which is common across CAIX of various species as well as additional transmembrane isoforms, plays an important role in CAIX activation and in pH regulation. Moreover, structure prediction I-TASSER analysis revealed involvement of Ala459 in potential ligand binding. Using tandem mass spectrometry, Protein-L-isoaspartyl methyltransferase (PIMT) was identified as a novel interacting partner, further confirmed by an in vitro pulldown assay and an in situ proximity ligation assay. Indeed, suppression of PIMT led to increased alkalinization of culture media of C33a cells constitutively expressing CAIX in hypoxia. We suggest that binding of PIMT represents a novel intracellular signal required for enzymatic activity of CAIX with a potential unidentified downstream function.

Regulation of protein L-isoaspartyl methyltransferase by cell-matrix interactions: involvement of integrin alphavbeta3, PI 3-kinase, and the proteasome

The enzyme L-isoaspartyl methyltransferase (PIMT) is known to repair damaged proteins that have accumulated abnormal aspartyl residues during cell aging. However, little is known about the mechanisms involved in the regulation of PIMT expression. Here we report that PIMT expression in bovine aortic endothelial cells is regulated by cell detachment and readhesion to a substratum. During cell detachment, the PIMT level was rapidly and strongly increased and correlated with a stimulation of protein synthesis. Aside from endothelial cells, PIMT levels were also regulated by cell adhesion in various cancer cell lines. The upregulation of PIMT expression could be prevented by an anti-alphavbeta3 antibody (LM609) or by a cyclic RGD peptide (XJ735) specific to integrin alphavbeta3, indicating that this integrin was likely involved in PIMT regulation. Moreover, we found that PIMT expression returned to the basal level when cells were replated on a substratum after detachment, though downregulation of PIMT expression could be partly prevented by the PI3K inhibitors LY294002 and wortmannin, as well as by the proteasome inhibitors MG-132, lactacystin, and beta-lactone. These findings support the assumption that the PIMT level was downregulated by proteasomal degradation, involving the PI3K pathway, during cell attachment. This study reports new insights on the molecular mechanisms responsible for the regulation of PIMT expression in cells. The regulation of PIMT level upon cell-substratum contact suggests a potential role for PIMT in biological processes such as wound healing, cell migration, and tumor metastasis dissemination.

Identification of prenylcysteine carboxymethyltransferase in bovine adrenal chromaffin cells

Chromaffin cells from bovine adrenal medulla were examined for the presence of a specific prenylcysteine carboxymethyltransferase by using N-acetyl-S-farnesyl-L-cysteine and N-acetyl-S-geranylgeranyl-L-cysteine as artificial substrates and a crude cell homogenate as the enzyme source. From Michaelis-Menten kinetics the following constants were calculated: K(m) 90 microM and V(max) 3 pmol/min per mg proteins for N-acetyl-S-farnesyl-L-cysteine; K(m) 52 microM and V(max) 3 pmol/min per mg proteins for N-acetyl-S-geranylgeranyl-L-cysteine. Both substrates were methylated to an optimal extent at the pH range 7. 4-8.0. Methylation activity increased linearly up to 20 min incubation time and was dose dependent up to at least 160 microg of protein. Sinefungin and S-adenosylhomocysteine both caused pronounced inhibition, as also to a lesser extent did farnesylthioacetic acid, deoxymethylthioadenosine and 3-deaza-adenosine. Effector studies showed that the methyltransferase activity varied depending on the concentration and chemical nature of the cations present. Monovalent cations were slightly stimulatory, while divalent metallic ions displayed diverging inhibitory effects. The inhibition by cations was validated by the stimulatory effect of the chelators EDTA and EGTA. Sulphydryl reagents inhibited methylation but to different degrees: Hg(2+)-ions: 100%, N-ethylmaleimide: 30%, dithiothreitol: 0% and mono-iodoacetate: 20%. Due to the hydrophobicity of the substrates dimethyl sulfoxide had to be included in the incubation mixture (<4%; still moderate inhibition at more elevated concentrations). The detergents tested affected the methyltransferase activity to a varying degree. The membrane bound character of the methyltransferase was confirmed.

Isoprenylcysteine-O-carboxyl methyltransferase regulates aldosterone-sensitive Na(+) reabsorption

The Xenopus laevis distal tubule epithelial cell line A6 was used as a model epithelia to study the role of isoprenylcysteine-O-carboxyl methyltransferase (pcMTase) in aldosterone-mediated stimulation of Na(+) transport. Polyclonal antibodies raised against X. laevis pcMTase were immunoreactive with a 33-kDa protein in whole cell lysate. These antibodies were also reactive with a 33-kDa product from in vitro translation of the pcMTase cDNA. Aldosterone application increased pcMTase activity resulting in elevation of total protein methyl esterification in vivo, but pcMTase protein levels were not affected by steroid, suggesting that aldosterone increased activity independent of enzyme number. Inhibition of pcMTase resulted in a reduction of aldosterone-induced Na(+) transport demonstrating the necessity of pcMTase-mediated transmethylation for steroid induced Na(+) reabsorption. Transfection with an eukaryotic expression construct containing pcMTase cDNA increased pcMTase protein level and activity. This resulted in potentiation of the natriferic actions of aldosterone. However, overexpression did not change Na(+) reabsorption in the absence of steroid, suggesting that pcMTase activity is not limiting Na(+) transport in the absence of steroid, but that subsequent to aldosterone addition, pcMTase activity becomes limiting. These results suggest that a critical transmethylation is necessary for aldosterone-induction of Na(+) transport. It is likely that the protein catalyzing this methylation is isoprenylcysteine-O-carboxyl methyltransferase and that aldosterone activates pcMTase without affecting transferase expression.

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.