Substrate specificity and inhibition studies of human serotonin N-acetyltransferase.
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Arylalkylamine N-acetyltransferase (AANAT) catalyzes the reaction of serotonin with acetyl-CoA to form N-acetylserotonin and plays a major role in the regulation of the melatonin circadian rhythm in vertebrates. In the present study, the human cloned enzyme has been expressed in bacteria, purified, cleaved, and characterized. The specificity of the human enzyme toward substrates (natural as well as synthetic arylethylamines) and cosubstrates (essentially acyl homologs of acetyl-CoA) has been investigated. Peptide combinatorial libraries of tri-, tetra-, and pentapeptides with various amino acid compositions were also screened as potential sources of inhibitors. We report the findings of several peptides with low micromolar inhibitory potency. For activity measurement as well as for specificity studies, an original and rapid method of analysis was developed. The assay was based on the separation and detection of N-[(3)H]acetylarylethylamine formed from various arylethylamines and tritiated acetyl-CoA, by means of high performance liquid chromatography with radiochemical detection. The assay proved to be robust and flexible, could accommodate the use of numerous synthetic substrates, and was successfully used throughout this study. We also screened a large number of pharmacological bioamines among which only one, tranylcypromine, behaved as a substrate. The synthesis and survey of simple arylethylamines also showed that AANAT has a large recognition pattern, including compounds as different as phenyl-, naphthyl-, benzothienyl-, or benzofuranyl-ethylamine derivatives. An extensive enzymatic study allowed us to pinpoint the amino acid residue of the pentapeptide inhibitor, S 34461, which interacts with the cosubstrate-binding site area, in agreement with an in silico study based on the available coordinates of the hAANAT crystal. (+info)
cAmp regulation of arylalkylamine N-acetyltransferase (AANAT, EC 2.3.1.87): a new cell line (1E7) provides evidence of intracellular AANAT activation.
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Arylalkylamine N-acetyltransferase (serotonin N-acetyltransferase, AANAT, EC ) is the penultimate enzyme in melatonin synthesis. As described here, a cell line (1E7) expressing human AANAT (hAANAT) has been developed to study the human enzyme. 1E7 hAANAT is detectable in immunoblots as a 23-kDa band and is immunocytochemically visualized in the cytoplasm. The specific concentration of hAANAT in homogenates is comparable to that of the night rat pineal gland. Kinetics of AANAT extracted from 1E7 cells are the same as those of bacterially expressed hAANAT; both preparations of hAANAT are equally sensitive to the inhibitor CoA-S-N-acetyltryptamine. Studies of cAMP regulation indicate that treatment with forskolin, dibutyryl cAMP, isobutylmethylxanthine, or isoproterenol activate cellular hAANAT within intact 1E7 cells approximately 8-fold without markedly increasing the abundance of AANAT protein or the activity of AANAT in broken cell preparations; and, that forskolin, isobutylmethylxanthine and isoproterenol elevate cyclic AMP production. These observations extend our understanding of cAMP regulation of AANAT activity, because it is currently thought that this only involves changes in the steady-state levels of AANAT protein. This previously unrecognized switching mechanism could function physiologically to control melatonin production without changing AANAT protein levels. (+info)
Role of a pineal cAMP-operated arylalkylamine N-acetyltransferase/14-3-3-binding switch in melatonin synthesis.
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The daily rhythm in melatonin levels is controlled by cAMP through actions on the penultimate enzyme in melatonin synthesis, arylalkylamine N-acetyltransferase (AANAT; serotonin N-acetyltransferase, EC ). Results presented here describe a regulatory/binding sequence in AANAT that encodes a cAMP-operated binding switch through which cAMP-regulated protein kinase-catalyzed phosphorylation [RRHTLPAN --> RRHpTLPAN] promotes formation of a complex with 14-3-3 proteins. Formation of this AANAT/14-3-3 complex enhances melatonin production by shielding AANAT from dephosphorylation and/or proteolysis and by decreasing the K(m) for 5-hydroxytryptamine (serotonin). Similar switches could play a role in cAMP signal transduction in other biological systems. (+info)
Retinal ganglion cells are autonomous circadian oscillators synthesizing N-acetylserotonin during the day.
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Retinal ganglion cells send visual and circadian information to the brain regarding the environmental light-dark cycles. We investigated the capability of retinal ganglion cells of synthesizing melatonin, a highly reliable circadian marker that regulates retinal physiology, as well as the capacity of these cells to function as autonomous circadian oscillators. Chick retinal ganglion cells presented higher levels of melatonin assessed by radioimmunoassay during both the subjective day in constant darkness and the light phase of a light-dark cycle. Similar changes were observed in mRNA levels and activity of arylalkylamine N-acetyltransferase, a key enzyme in melatonin biosynthesis, with the highest levels of both parameters during the subjective day. These daily variations were preceded by the elevation of cyclic-AMP content, the second messenger involved in the regulation of melatonin biosynthesis. Moreover, cultures of immunopurified retinal ganglion cells at embryonic day 8 synchronized by medium exchange synthesized a [3H]melatonin-like indole from [3H]tryptophan. This [3H]indole was rapidly released to the culture medium and exhibited a daily variation, with levels peaking 8 h after synchronization, which declined a few hours later. Cultures of embryonic retinal ganglion cells also showed self-sustained daily rhythms in arylalkylamine N-acetyltransferase mRNA expression during at least three cycles with a period near 24 h. These rhythms were also observed after the application of glutamate. The results demonstrate that chick retinal ganglion cells may function as autonomous circadian oscillators synthesizing a melatonin-like indole during the day. (+info)
Cellular stability of serotonin N-acetyltransferase conferred by phosphonodifluoromethylene alanine (Pfa) substitution for Ser-205.
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Large changes in the activity of serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) in the pineal gland control the rhythmic production of the time-keeping hormone melatonin. The activity of AANAT reflects changes in the amount and activation state of the AANAT protein, both of which increase at night. The molecular basis of this regulation is now becoming known, and recent data indicate that this involves phosphorylation-dependent binding to the 14-3-3 protein at two sites, one of which, Ser-205, is located several residues from the C terminus. In this study, we determined whether substitution of this residue with a non-hydrolyzable the phosphoserine/phosphothreonine mimetic would promote binding to the 14-3-3 protein and enhance cellular stability. To accomplish this, a C-terminal AANAT peptide containing the phosphonodifluoromethylene alanine at Ser-205 was synthesized and fused to bacterially expressed AANAT(30-199) using expressed protein ligation. The resulting semisynthetic protein has enhanced affinity for the expressed 14-3-3 protein and exhibits greater cellular stability in microinjection experiments, as compared with the unmodified AANAT. Enhanced 14-3-3 binding was also observed using humanized ovine AANAT, which has a different C-terminal sequence (Gly-Cys) than the ovine enzyme (Asp-Arg), indicating that that characteristic is not unique to the ovine enzyme. These studies provide the first evidence that substitution of Ser-205 with the stable phosphomimetic amino acid phosphonodifluoromethylene alanine enhances binding to 14-3-3 and the cellular stability of AANAT and are consistent with the view that Ser-205 phosphorylation plays a critical role in the regulation of AANAT activity and melatonin production. (+info)
Melatonin synthesis: 14-3-3-dependent activation and inhibition of arylalkylamine N-acetyltransferase mediated by phosphoserine-205.
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The nocturnal increase in circulating melatonin in vertebrates is regulated by the activity of arylalkylamine N-acetyltransferase (AANAT), the penultimate enzyme in the melatonin pathway (serotonin --> N-acetylserotonin --> melatonin). Large changes in activity are linked to cyclic AMP-dependent protein kinase-mediated phosphorylation of AANAT T31. Phosphorylation of T31 promotes binding of AANAT to the dimeric 14-3-3 protein, which activates AANAT by increasing arylalkylamine affinity. In the current study, a putative second AANAT cyclic AMP-dependent protein kinase phosphorylation site, S205, was found to be approximately 55% phosphorylated at night, when T31 is approximately 40% phosphorylated. These findings indicate that ovine AANAT is dual-phosphorylated. Moreover, light exposure at night decreases T31 and S205 phosphorylation, consistent with a regulatory role of both sites. AANAT peptides containing either T31 or S205 associate with 14-3-3zeta in a phosphorylation-dependent manner; binding through phosphorylated (p)T31 is stronger than that through pS205, consistent with the location of only pT31 in a mode I binding motif, one of two recognized high-affinity 14-3-3-binding motifs AANAT protein binds to 14-3-3zeta through pT31 or pS205. Two-site binding lowers the Km for arylalkylamine substrate to approximately 30 microM. In contrast, single-site pS205 binding increases the Km to approximately 1,200 microM. Accordingly, the switch from dual to single pS205 binding of AANAT to 14-3-3 changes the Km for substrates by approximately 40-fold. pS205 seems to be part of a previously unrecognized 14-3-3-binding motif-pS/pT (X1-2)-COOH, referred to here as mode III. (+info)
Homeobox-clock protein interaction in zebrafish. A shared mechanism for pineal-specific and circadian gene expression.
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In non-mammalian vertebrates, the pineal gland is photoreceptive and contains an intrinsic circadian oscillator that drives rhythmic production and secretion of melatonin. These features require an accurate spatiotemporal expression of an array of specific genes in the pineal gland. Among these is the arylalkylamine N-acetyltransferase, a key enzyme in the melatonin production pathway. In zebrafish, pineal specificity of zfaanat2 is determined by a region designated the pineal-restrictive downstream module (PRDM), which contains three photoreceptor conserved elements (PCEs) and an E-box, elements that are generally associated with photoreceptor-specific and rhythmic expression, respectively. Here, by using in vivo and in vitro approaches, it was found that the PCEs and E-box of the PRDM mediate a synergistic effect of the photoreceptor-specific homeobox OTX5 and rhythmically expressed clock protein heterodimer, BMAL/CLOCK, on zfaanat2 expression. Furthermore, the distance between the PCEs and the E-box was found to be critical for PRDM function, suggesting a possible physical feature of this synergistic interaction. OTX5-BMAL/CLOCK may act through this mechanism to simultaneously control pineal-specific and rhythmic expression of zfaanat2 and possibly also other pineal and retinal genes. (+info)
Rhythmic serotonin N-acetyltransferase mRNA degradation is essential for the maintenance of its circadian oscillation.
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Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase [AANAT]) is the key enzyme in melatonin synthesis regulated by circadian rhythm. To date, our understanding of the oscillatory mechanism of melatonin has been limited to autoregulatory transcriptional and posttranslational regulations of AANAT mRNA. In this study, we identify three proteins from pineal glands that associate with cis-acting elements within species-specific AANAT 3' untranslated regions to mediate mRNA degradation. These proteins include heterogeneous nuclear ribonucleoprotein R (hnRNP R), hnRNP Q, and hnRNP L. Their RNA-destabilizing function was determined by RNA interference and overexpression approaches. Expression patterns of these factors in pineal glands display robust circadian rhythm. The enhanced levels detected after midnight correlate with an abrupt decline in AANAT mRNA level. A mathematical model for the AANAT mRNA profile and its experimental evidence with rat pinealocytes indicates that rhythmic AANAT mRNA degradation mediated by hnRNP R, hnRNP Q, and hnRNP L is a key process in the regulation of its circadian oscillation. (+info)