Phase separation of 53BP1 determines liquid-like behavior of DNA repair compartments

The DNA damage response (DDR) generates transient repair compartments to concentrate repair proteins and activate signaling factors. The physicochemical properties of these spatially confined compartments and their function remain poorly understood. Here, we establish, based on live cell microscopy and CRISPR/Cas9-mediated endogenous protein tagging, that 53BP1-marked repair compartments are dynamic, show droplet-like behavior, and undergo frequent fusion and fission events. 53BP1 assembly, but not the upstream accumulation of γH2AX and MDC1, is highly sensitive to changes in osmotic pressure, temperature, salt concentration and to disruption of hydrophobic interactions. Phase separation of 53BP1 is substantiated by optoDroplet experiments, which further allowed dissection of the 53BP1 sequence elements that cooperate for light-induced clustering. Moreover, we found the tumor suppressor protein p53 to be enriched within 53BP1 optoDroplets, and conditions that disrupt 53BP1 phase separation impair 53BP1-dependent induction of p53 and diminish p53 target gene expression. We thus suggest that 53BP1 phase separation integrates localized DNA damage recognition and repair factor assembly with global p53-dependent gene activation and cell fate decisions.

Macrodomain-containing proteins are new mono-ADP-ribosylhydrolases

ADP-ribosylation is an important post-translational protein modification (PTM) that regulates diverse biological processes. ADP-ribosyltransferase diphtheria toxin-like 10 (ARTD10, also known as PARP10) mono-ADP-ribosylates acidic side chains and is one of eighteen ADP-ribosyltransferases that catalyze mono- or poly-ADP-ribosylation of target proteins. Currently, no enzyme is known that reverses ARTD10-catalyzed mono-ADP-ribosylation. Here we report that ARTD10-modified targets are substrates for the macrodomain proteins MacroD1, MacroD2 and C6orf130 from Homo sapiens as well as for the macrodomain protein Af1521 from archaebacteria. Structural modeling and mutagenesis of MacroD1 and MacroD2 revealed a common core structure with Asp102 and His106 of MacroD2 implicated in the hydrolytic reaction. Notably, MacroD2 reversed the ARTD10-catalyzed, mono-ADP-ribose-mediated inhibition of glycogen synthase kinase 3β (GSK3β) in vitro and in cells, thus underlining the physiological and regulatory importance of mono-ADP-ribosylhydrolase activity. Our results establish macrodomain-containing proteins as mono-ADP-ribosylhydrolases and define a class of enzymes that renders mono-ADP-ribosylation a reversible modification.

Acetylation of poly(ADP-ribose) polymerase-1 by p300/CREB-binding protein regulates coactivation of NF-kappaB-dependent transcription

Poly(ADP-ribose) polymerase-1 (PARP-1) and nuclear factor kappaB (NF-kappaB) have both been demonstrated to play a pathophysiological role in a number of inflammatory disorders. We recently presented evidence that PARP-1 can act as a promoter-specific coactivator of NF-kappaB in vivo independent of its enzymatic activity. PARP-1 directly interacts with p300 and both subunits of NF-kappaB (p65 and p50) and synergistically coactivates NF-kappaB-dependent transcription. Here we show that PARP-1 is acetylated in vivo at specific lysine residues by p300/CREB-binding protein upon stimulation. Furthermore, acetylation of PARP-1 at these residues is required for the interaction of PARP-1 with p50 and synergistic coactivation of NF-kappaB by p300 and the Mediator complex in response to inflammatory stimuli. PARP-1 physically interacts with the Mediator. Interestingly, PARP-1 interacts in vivo with histone deacetylases (HDACs) 1-3 but not with HDACs 4-6 and might be deacetylated in vivo by HDACs 1-3. Thus, acetylation of PARP-1 by p300/CREB-binding protein plays an important regulatory role in NF-kappaB-dependent gene activation by enhancing its functional interaction with p300 and the Mediator complex.

The enzymatic and DNA binding activity of PARP-1 are not required for NF-kappa B coactivator function

Poly(ADP-ribose) polymerase 1 (PARP-1)-deficient mice are protected against septic shock, diabetes type I, stroke, and inflammation. We report that primary cells from PARP-1(-/-) animals are impaired in kappa B-dependent transcriptional activation induced by different stimuli involved in inflammatory and genotoxic stress signaling. PARP-1 was also required for p65-mediated transcriptional activation. PARP-1 enzymatic inhibitors did not inhibit the transcriptional activation of a kappa B-dependent reporter gene in wild type cells. Remarkably, neither the enzymatic activity nor the DNA binding activity of PARP-1 was required for kappa B-dependent transcriptional activation in PARP-1(-/-) cells complemented with different PARP-1 mutants. However, PARP-1 interacted in vitro directly with both subunits of NF-kappa B (p50 and p65), and mapping of the interaction domains revealed that both subunits bind to different PARP-1 domains. Furthermore, a PARP-1 mutant lacking the enzymatic and DNA binding activity interacted comparably to the wild type PARP-1 with p65 or p50. Finally, we showed that PARP-1 is activating the natural inducible nitric-oxide synthase and P-selectin promoter in a kappa B-dependent manner upon stimulation of the cells with inflammatory stimuli or cotransfection of p65. Our results provide evidence that neither the DNA binding nor the enzymatic activity of PARP-1 but its direct protein-protein interaction with both subunits of NF-kappa B is required for its coactivator function, thus expanding the role of PARP-1 as an essential and novel classical transcriptional coactivator for kappa B-dependent gene expression in vivo.

Pharmacological Inhibition of poly(ADP-ribose) polymerases improves fitness and mitochondrial function in skeletal muscle

We previously demonstrated that the deletion of the poly(ADP-ribose)polymerase (Parp)-1 gene in mice enhances oxidative metabolism, thereby protecting against diet-induced obesity. However, the therapeutic use of PARP inhibitors to enhance mitochondrial function remains to be explored. Here, we show tight negative correlation between Parp-1 expression and energy expenditure in heterogeneous mouse populations, indicating that variations in PARP-1 activity have an impact on metabolic homeostasis. Notably, these genetic correlations can be translated into pharmacological applications. Long-term treatment with PARP inhibitors enhances fitness in mice by increasing the abundance of mitochondrial respiratory complexes and boosting mitochondrial respiratory capacity. Furthermore, PARP inhibitors reverse mitochondrial defects in primary myotubes of obese humans and attenuate genetic defects of mitochondrial metabolism in human fibroblasts and C. elegans. Overall, our work validates in worm, mouse, and human models that PARP inhibition may be used to treat both genetic and acquired muscle dysfunction linked to defective mitochondrial function.

Transcriptional coactivation of nuclear factor-kappaB-dependent gene expression by p300 is regulated by poly(ADP)-ribose polymerase-1

Nuclear factor kappaB (NF-kappaB) plays an important role in the transcriptional regulation of genes involved in inflammation and cell survival. In this study, we demonstrated that NF-kappaB-dependent gene expression was inhibited by E1A in poly(ADP)-ribose polymerase-1 knock out (PARP-1 (-/-)) cells complemented with wild type PARP-1 after tumor necrosis factor alpha (TNFalpha) or lipopolysaccharide (LPS) treatment. PARP-1 and p300 synergistically coactivated NF-kappaB-dependent gene expression in response to TNFalpha and LPS. Furthermore, PARP-1 interacted directly with p300 and enhanced the interaction of NF-kappaB1/p50 to p300. The C terminus, harboring the catalytic domain of PARP-1 but not its enzymatic activity, was required for complete transcriptional coactivation of NF-kappaB by p300 in response to TNFalpha and LPS. Together, these results indicate that PARP-1 acts synergistically with p300 and plays an essential regulatory role in NF-kappaB-dependent gene expression.

Arginine methyltransferase CARM1 is a promoter-specific regulator of NF-kappaB-dependent gene expression

Nuclear factor kappaB (NF-kappaB) plays an important role in the transcriptional regulation of genes involved in inflammation and cell survival. Here, we show that coactivator-associated arginine methyltransferase CARM1/PRMT4 is a novel transcriptional coactivator of NF-kappaB and functions as a promoter-specific regulator of NF-kappaB recruitment to chromatin. Carm1 knockout cells showed impaired expression of a subset of NF-kappaB-dependent genes upon TNFalpha or LPS stimulation. CARM1 forms a complex with p300 and NF-kappaB in vivo and interacts directly with the NF-kappaB subunit p65 in vitro. CARM1 seems to act in a gene-specific manner mainly by enhancing NF-kappaB recruitment to cognate sites. Moreover, CARM1 synergistically coactivates NF-kappaB-mediated transactivation, in concert with the transcriptional coactivators p300/CREB-binding protein and the p160 family of steroid receptor coactivators. For at least a subset of CARM1-dependent NF-kappaB target genes, the enzymatic activities of both CARM1 and p300 are necessary for the observed synergy between CARM1 and p300. Our results suggest that the cooperative action between protein arginine methyltransferases and protein lysine acetyltransferases regulates NF-kappaB-dependent gene activation in vivo.

Transcription coactivator p300 binds PCNA and may have a role in DNA repair synthesis

The transcriptional coactivator p300 interacts with many transcription factors that participate in a broad spectrum of biological activities, such as cellular differentiation, homeostasis and growth control. Mouse embryos lacking both p300 alleles die around mid-gestation, with pleiotropic defects in morphogenesis, in cell differentiation and, unexpectedly, in cell proliferation because of reduced DNA synthesis. Here we show that p300 may have a role in DNA repair synthesis through its interaction with proliferating cell nuclear antigen (PCNA). We show that in vitro and in vivo p300 forms a complex with PCNA that does not depend on the S phase of cell cycle. A large fraction of both p300 and PCNA colocalize to speckled structures in the nucleus. Furthermore, the endogenous p300-PCNA complex stimulates DNA synthesis in vitro. Chromatin immunoprecipitation experiments indicate that p300 is associated with freshly synthesized DNA after ultraviolet irradiation. Our results suggest that p300 may participate in chromatin remodelling at DNA lesion sites to facilitate PCNA function in DNA repair synthesis.

Regulation of human flap endonuclease-1 activity by acetylation through the transcriptional coactivator p300

We describe a role for the transcriptional coactivator p300 in DNA metabolism. p300 formed a complex with flap endonuclease-1 (Fen1) and acetylated Fen1 in vitro. Furthermore, Fen1 acetylation was observed in vivo and was enhanced upon UV treatment of human cells. Remarkably, acetylation of the Fen1 C terminus by p300 significantly reduced Fen1's DNA binding and nuclease activity. Proliferating cell nuclear antigen (PCNA) was able to stimulate both acetylated and unacetylated Fen1 activity to the same extent. Our results identify acetylation as a novel regulatory modification of Fen1 and implicate that p300 is not only a component of the chromatin remodeling machinery but might also play a critical role in regulating DNA metabolic events.

Acetylation regulates the DNA end-trimming activity of DNA polymerase beta

We describe a novel regulatory mechanism for DNA polymerase beta (Polbeta), a protein involved in DNA base excision repair (BER). Polbeta colocalized in vivo and formed a complex with the transcriptional coactivator p300. p300 interacted with Polbeta through distinct domains and acetylated Polbeta in vitro. Polbeta acetylation was furthermore observed in vivo. Lysine 72 of Polbeta was identified as the main target for acetylation by p300. Interestingly, acetylated Polbeta showed a severely reduced ability to participate in a reconstituted BER assay. This was due to an impairment of the dRP-lyase activity of Polbeta. Acetylation of Polbeta thus acts as an intranuclear regulatory mechanism and implies that p300 plays a critical regulatory role in BER.

The value of Doppler assessment of the uteroplacental circulation in predicting preeclampsia or intrauterine growth retardation

Flow velocity waveforms of the uteroplacental arteries were analyzed at 20 and 24 weeks of gestation, by means of duplex pulsed Doppler ultrasonography, in 93 women at risk for preeclampsia or intrauterine growth retardation. The ability of an elevated resistance index to predict these conditions was tested. At 20 and 24 weeks an abnormal resistance index was significantly associated with intrauterine growth retardation but not with preeclampsia, with or without proteinuria. A low fetal abdominal circumference at 20 or 24 weeks or an increasing maternal plasma uric acid concentration at 24 weeks was as predictive as an elevated resistance index. In a second group of 43 women, screened in the same way, the only association was of an elevated resistance index at 20 weeks with intrauterine growth retardation. Although elevated resistance indices occur more commonly in women who develop intrauterine growth retardation and/or preeclampsia, the correlation is not close enough to be clinically useful as a screening test.

Analysis of PARP inhibitor toxicity by multidimensional fluorescence microscopy reveals mechanisms of sensitivity and resistance

Exploiting the full potential of anti-cancer drugs necessitates a detailed understanding of their cytotoxic effects. While standard omics approaches are limited to cell population averages, emerging single cell techniques currently lack throughput and are not applicable for compound screens. Here, we employed a versatile and sensitive high-content microscopy-based approach to overcome these limitations and quantify multiple parameters of cytotoxicity at the single cell level and in a cell cycle resolved manner. Applied to PARP inhibitors (PARPi) this approach revealed an S-phase-specific DNA damage response after only 15 min, quantitatively differentiated responses to several clinically important PARPi, allowed for cell cycle resolved analyses of PARP trapping, and predicted conditions of PARPi hypersensitivity and resistance. The approach illuminates cellular mechanisms of drug synergism and, through a targeted multivariate screen, could identify a functional interaction between PARPi olaparib and NEDD8/SCF inhibition, which we show is dependent on PARP1 and linked to PARP1 trapping.

Methods to study anti-cancer drugs cytotoxicity are often low throughput and rely on population average. Here the authors present an automated image-based cytometry method to quantify multiple cytotoxicity parameters in single cells, and use it to study the effect of PARP inhibitors in cancer cells.

Efficient Pre-mRNA Cleavage Prevents Replication-Stress-Associated Genome Instability

Cellular mechanisms that safeguard genome integrity are often subverted in cancer. To identify cancer-related genome caretakers, we employed a convergent multi-screening strategy coupled to quantitative image-based cytometry and ranked candidate genes according to multivariate readouts reflecting viability, proliferative capacity, replisome integrity, and DNA damage signaling. This unveiled regulators of replication stress resilience, including components of the pre-mRNA cleavage and polyadenylation complex. We show that deregulation of pre-mRNA cleavage impairs replication fork speed and leads to excessive origin activity, rendering cells highly dependent on ATR function. While excessive formation of RNA:DNA hybrids under these conditions was tightly associated with replication-stress-induced DNA damage, inhibition of transcription rescued fork speed, origin activation, and alleviated replication catastrophe. Uncoupling of pre-mRNA cleavage from co-transcriptional processing and export also protected cells from replication-stress-associated DNA damage, suggesting that pre-mRNA cleavage provides a mechanism to efficiently release nascent transcripts and thereby prevent gene gating-associated genomic instability.

Comprehensive ADP-ribosylome analysis identifies tyrosine as an ADP-ribose acceptor site

Despite recent mass spectrometry (MS)-based breakthroughs, comprehensive ADP-ribose (ADPr)-acceptor amino acid identification and ADPr-site localization remain challenging. Here, we report the establishment of an unbiased, multistep ADP-ribosylome data analysis workflow that led to the identification of tyrosine as a novel ARTD1/PARP1-dependent in vivo ADPr-acceptor amino acid. MS analyses of in vitro ADP-ribosylated proteins confirmed tyrosine as an ADPr-acceptor amino acid in RPS3A (Y155) and HPF1 (Y238) and demonstrated that trans-modification of RPS3A is dependent on HPF1. We provide an ADPr-site Localization Spectra Database (ADPr-LSD), which contains 288 high-quality ADPr-modified peptide spectra, to serve as ADPr spectral references for correct ADPr-site localizations.

Identification of lysines 36 and 37 of PARP-2 as targets for acetylation and auto-ADP-ribosylation

Poly-ADP-ribose polymerase-2 (PARP-2) was described to regulate cellular functions comprising DNA surveillance, inflammation and cell differentiation by co-regulating different transcription factors. Using an in vitro and in vivo approach, we identified PARP-2 as a new substrate for the histone acetyltransferases PCAF and GCN5L. Site directed mutagenesis indicated that lysines 36 and 37, located in the nuclear localization signal of PARP-2, are the main targets for PCAF and GCN5L activity in vitro. Interestingly, acetylation of the same two PARP-2 residues reduces the DNA binding and enzymatic activity of PARP-2. Finally, PARP-2 with mutated lysines 36 and 37 showed reduced auto-mono-ADP-ribosylation when compared to wild type PARP-2. Together, our results provide evidence that acetylation of PARP-2 is a key post-translational modification that may regulate DNA binding and consequently also the enzymatic activity of PARP-2.

[3H]Ro 19-6327: a reversible ligand and affinity labelling probe for monoamine oxidase-B

This study demonstrated the existence of specific binding sites for [3H]Ro 19-6327 in human platelet membranes. This compound is a novel, time-dependent inhibitor of monoamine oxidase type B (MAO-B) and is structurally closely related to [3H]Ro 16-6491. The density of the sites labelled with high affinity by [3H]Ro 19-6327 was similar to that observed in previous studies with [3H]Ro 16-6491 as ligand. Binding was reversible at 20 degrees C and showed a relatively slow dissociation (t1/2 = 220 min). The dissociation rate was markedly decreased (t1/2 = greater than 24h) at 0 degrees C. MAO-B, but not MAO-A inhibitors, effectively prevented the binding of [3H]Ro 19-6327. Like [3H]Ro 16-6491, [3H]Ro 19-6327 is recognized as a substrate by MAO-B, being eventually deaminated by the enzyme. Since the deaminated aldehyde derivative of Ro 19-6327 did not inhibit MAO-B, a still unidentified reversible adduct, formed at the MAO-B active site, might explain the high potency and selectivity of [3H]Ro 19-6327. Incubation of the radioligand-enzyme complex from platelet and brain membranes with NaBH3CN and acetic acid (to pH 4.5) caused the irreversible incorporation of the radioactivity into a single polypeptide as shown by SDS-PAGE analysis. This polypeptide had a molecular weight identical to that of the MAO-B subunit, i.e. 58,000. The presence of unlabelled MAO-B inhibitors in the incubation mixture prevented the covalent incorporation of [3H]Ro 19-6327. The irreversible MAO-B inhibitor, [3H] pargyline, labelled a protein with a molecular weight identical to the protein labelled by [3H]Ro 19-6327.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of [3H]Ro 16-6491, a reversible inhibitor of monoamine oxidase type B, to human brain mitochondria and platelet membranes

The reversible inhibitor of monoamine oxidase type B (MAO-B) [3H]Ro 16-6491 binds specifically and with high affinity to a single population of binding sites in human frontal cortex crude mitochondria and platelet membranes. In both tissues binding equilibrium was reached after 1 h incubation at 20 degrees C. Dissociation of bound radioactivity was relatively fast at 20 degrees C (t1/2 = 90-120 min) whereas at 0 degrees C [3H]Ro 16-6491 showed the characteristics of a slowly dissociating ligand. Inhibitors and substrates of MAO-B inhibited binding of [3H]Ro 16-6491, whereas MAO-A blockers were much less potent. Ro 16-6491 was also a substrate for MAO-B and a stable unidentified intermediate of the oxidation of Ro 16-6491 possessing high affinity for the enzyme may account for the marked MAO-B inhibitory effect of the drug. According to this hypothesis Ro 16-6491 would behave as a mechanism-based reversible inhibitor. In conclusion, [3H]Ro 16-6491 binds selectively to MAO-B and represents an excellent new radioligand probe for studying the regional tissue distribution of this enzyme in normal and pathological conditions.

Listeria monocytogenes infection in ruminants: Is there a link to the environment, food and human health? A review

Listeria (L.) monocytogenes is widely distributed in the environment, but also has the ability to cause serious invasive disease in ruminants and humans. This review provides an overview of listeriosis in ruminants and discusses our insufficient understanding of reservoirs and possible cycling ofL. monocytogenes between animal and human hosts, food and the environment. It indicates gaps in our knowledge of the role of genetic subtypes in L. monocytogenes ecology and virulence as well as risk factors, in vivo diagnostics and pathogenesis of listeriosis in ruminants. Filling these gaps will contribute to improving the control of L. monocytogenes and enhancing disease prevention. As the prevalence of listeriosis in ruminants in Switzerland is likely to be underestimated, propositions concerning improvement options for surveillance of listeriosis in ruminants are provided.

Impact of ribonucleotide incorporation by DNA polymerases β and λ on oxidative base excision repair

Oxidative stress is a very frequent source of DNA damage. Many cellular DNA polymerases (Pols) can incorporate ribonucleotides (rNMPs) during DNA synthesis. However, whether oxidative stress-triggered DNA repair synthesis contributes to genomic rNMPs incorporation is so far not fully understood. Human specialized Pols β and λ are the important enzymes involved in the oxidative stress tolerance, acting both in base excision repair and in translesion synthesis past the very frequent oxidative lesion 7,8-dihydro-8-oxoguanine (8-oxo-G). We found that Pol β, to a greater extent than Pol λ can incorporate rNMPs opposite normal bases or 8-oxo-G, and with a different fidelity. Further, the incorporation of rNMPs opposite 8-oxo-G delays repair by DNA glycosylases. Studies in Pol β- and λ-deficient cell extracts suggest that Pol β levels can greatly affect rNMP incorporation opposite oxidative DNA lesions.

Oxidative stress is a common source of DNA damage and is repaired by the base excision repair machinery, including polymerase beta. Here the authors find that polymerase beta, and to a lesser extent lambda, can mistakenly incorporate ribonucleotides during synthesis.

Design, synthesis, and X-ray data of novel potential antipsychotic agents. Substituted 7-phenylquinolizidines: stereospecific, neuroleptic, and antinociceptive properties

The semirigid 7-phenylquinolizidine system was selected for the design of new potential antipsychotic agents because it fulfills earlier considerations on the minimal structural and steric requirements necessary to attain a high affinity to the dopamine receptors and, in addition, leaves open many opportunities for variations. Whereas the initial compound 5a, modeled after the structure of butaclamol, was virtually inactive, the introduction of substituents on the phenyl ring and of further optional structural elements improved the dopamine antagonistic properties by supplying additional binding forces. Initially, a compound in the potency range of chlorpromazine was obtained: (9aH)-2-tert-butyl-7-(2,4-dichlorophenyl)octahydro-2H-qui nolizin-2-ol (5e). By the optical resolution of 5c, it was demonstrated that the biological activity resides in the (-) enantiomer. The absolute configuration of (-)-5e was determined by single-crystal X-ray analysis to be 2S,7R,9aR. Further variations of the optional structural elements led to the unexpected finding of compounds with strong antinociceptive properties, e.g., (2R,7S,9aS)-2-butylocta-hydro-7-phenyl-2H-quinolizin-2 -yl acetate [(+)-26]. Interestingly, this compound belongs to the enantiomeric series opposite to that of the neuroleptic-like (-)-5e.

Investigation on the structure of the active site of monoamine oxidase-B by affinity labeling with the selective inhibitor lazabemide and by site-directed mutagenesis

The structural features of the active site of human monoamine oxidase B (MAO-B) were investigated by affinity labeling and site-directed mutagenesis. The pseudosubstrate inhibitor N-[2-aminoethyl]-5-chloro-2-pyridine carboxamide HCl (lazabemide) can be irreversibly linked to MAO-B by reduction of the enzyme-inhibitor complex with NaBH(3)CN. Analysis of the flavin spectrum of [(3)H]lazabemide-labeled human MAO-B indicated that insertion of the inhibitor did not occur into the isoalloxazine ring of FAD. After trypsin digestion and HPLC peptide mapping of the radiolabeled enzyme, two labeled peptides were observed. Sequence analysis showed that both peptides started at Val371 of human MAO-B. These results indicate that [(3)H]lazabemide is incorporated into the MAO-B peptide stretch containing the FAD-modified Cys397. The function of putative active-site residues contained in this region was investigated by site-directed mutagenesis and expression of the mutant proteins in HEK-293 cells. Substitution of His382 of MAO-B with an Arg greatly reduced the enzymic activity, suggesting that this residue may represent a nucleophile relevant for the MAO-B catalytic mechanism. Whereas it has been shown that mutation of Cys389 with a Ser residue does not markedly affect the activity of the enzyme [Wu, H.-F., Chen, K. and Shih, J.C. (1993) Mol. Pharmacol. 43, 888-893] the mutant carrying an Ala at this position was virtually inactive. Conversely, substitution of Lys386 (to Met) and Ser394 (to Ala) did not markedly modify the kinetic properties of the enzyme. We also report that mutation of MAO-B Thr158 (to Ala) resulted in a dramatic loss of enzymic activity.

[3H]Ro 16-6491, a selective probe for affinity labelling of monoamine oxidase type B in human brain and platelet membranes

[3H]Ro 16-6491 [N-(2-aminoethyl)-p-chlorobenzamide HCl], a reversible "mechanism-based" inhibitor of monoamine oxidase (MAO) type B, binds selectively and with high affinity to the active site of MAO-B in brain and platelet membranes. Under normal conditions, the binding of [3H]Ro 16-6491 is fully reversible. However, [3H]Ro 16-6491 could be irreversibly bound (covalently) to membranes by the addition of the reducing agent NaBH3CN to the sample and adjusting to pH 4.5 with acetic acid. No irreversible labelling occurred in the absence of NaBH3CN and at neutral pH. The presence of the irreversible MAO-B inhibitor l-deprenyl completely abolished the irreversible labelling of the membranes by [3H]Ro 16-6491. The selective inactivation of MAO-B, e.g., by l-deprenyl prevented the covalent incorporation of [3H]Ro 16-6491 whereas selective inhibition of the MAO-A by clorgyline was without effect. The covalent linkage to membranes of unlabelled Ro 16-6491 and Ro 19-6327 (a selective and reversible MAO-B inhibitor closely related to Ro 16-6491) after the addition of NaBH3CN at pH 4.5 irreversibly inactivated MAO-B activity whereas MAO-A activity was unaffected. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of labelled membranes showed that [3H]Ro 16-6491 was incorporated into a single polypeptide with a molecular mass identical to the one labelled by [3H]pargyline (58 kilodaltons). Our results indicate that the polypeptide that is covalently labelled by [3H]Ro 16-6491 corresponds to one of the two MAO-B subunits.(ABSTRACT TRUNCATED AT 250 WORDS)