S-acylation, commonly known as S-palmitoylation, is a reversible posttranslational lipid modification in which fatty acid, usually palmitic acid, covalently attaches to specific cysteine residues of proteins via thioester bonds. Palmitoylation enhances the hydrophobicity of proteins and contributes to their membrane association. It plays roles in protein trafficking, signalling, protein-protein interaction, protein stability and other important cellular functions. A family of Protein S-acyl Transferases (PATs) is responsible for this reaction. PATs are multi-pass transmembrane proteins that possess a catalytic Asp- His-His-Cys cysteine rich domain (DHHC-CRD) of ~50 amino acids. In Arabidopsis there are at least 24 such DHHC-CRD containing PAT proteins and they are named as AtPAT01 to AtPAT24. The function of only 2 AtPATs, AtPAT10 and AtPAT24 were studied in some detail, and a recent survey showed the ubiquitous expression pattern and different membrane localization habit of all 24 AtPATs. However, the biological function of the remaining 22 AtPATs in Arabidopsis was not reported when I started my project. Therefore, we carried out an initial screen of all the available T-DNA insertion lines of the 22 Arabidopsis PATs and identified transcriptional null mutants of 18 of the AtPATs. Among them, the k/o mutant plants of only 3 genes showed significantly altered phenotypes compared to wild-type Arabidopsis, and the mutants are named as atpat14, atpat21 and plp1(PAT-like Protein 1). This project aims to characterize these three putative PATs in details in terms of their PAT activities, catalytic domains, expression patterns, subcellular localizations and biological functions. AtPAT14 was proved as a PAT by yeast complementary and in vitro auto-acylation assays. Mutagenesis studies clearly demonstrated that the cysteine residue in the DHHCmotif is essential for the enzyme activity of AtPAT14. Transgenic Arabidopsis plants expressing AtPAT14-GFP were observed and it was shown that AtPAT14 is predominantly localized at the Trans-Golgi. The phenotype was observed in both atpat14-1 and atpat14-2 mutant lines and this showed that the leaves of both lines were aging much faster than the WT. Analysis of the levels of different phytohormones revealed that the mutant leaves contained much higher salicylic acid (SA) than the WT. This coincided with the increased transcript levels of genes involved in SA biosynthesis and signalling. Therefore, AtPAT14 mediated protein S-acylation plays important roles in leaf senescence via the regulation of SA biosynthesis and signalling pathways. AtPAT21 was also confirmed as a PAT and the DHHC its functional domain by similar approaches as for AtPAT14. The plasma membrane (PM) localized AtPAT21 plays essential roles in both male and female gametogenesis. As such, loss-of-function by TDNA insertion in AtPAT21 leads to the plant being completely sterile. Therefore, AtPAT21-mediated S-acylation of proteins(s) plays important roles in the reproduction of Arabidopsis. AtPLP1 (PAT-like Protein 1) contains the signature DHHC-CRD. However, it does not rescue the growth defects of akr1, pfa3 and swf1, the 3 yeast PAT mutants used in enzyme activity assays of other known PATs from plant and animals. Further, the cysteine residue in the DHHC motif was not essential for the function of AtPLP1 as mutated variant containing serine in place of cysteine of the DHHC motif can still rescue the growth defects of atplp1-1. Seedling establishment of atplp1-1 was impaired without external carbon source. This is because the efficiency in converting the seed storage lipid to sugar in the mutant is much lower than WT due to the defective β-oxidation process involved in the degradation of free fatty acids released from lipid during post-germinative growth. In addition, atplp1-1 seedlings are also de-etiolated in the dark, and this was coincided with more cytokinin (CK) and less active gibberellin (GA) related pathway in the mutant. Other defects were also found in atplp1-1, such as hypersensitive to abscisic acid (ABA) and sugar during seed germination and abnormal shoot apical meristem (SAM) in older plants. Therefore, protein S-acyltransferases play distinct and diverse roles throughout the life cycle, from seed germination, seedling growth to seed production in Arabidopsis. This is most likely through the palmitoylation of an array of proteins they modify. Hence, our results provide vital clues for future studies on the molecular mechanism as to how AtPATs operate in plant.

Acyl Coenzyme A: Isopenicillin N Acyl Transferase (AT) is a key enzyme in the biosynthesis of β-lactam antibiotics in penicillin producing organisms such as P. chrysogenum and A. nidulans. Its natural activity is to exchange the side chain of the low activity antibiotic IPN [18] for the phenylacetyl side chain resulting in the more active antibiotic Penicillin G [5]. The biosynthesis of β-lactams has been exploited towards producing these compounds for therapeutic use. However, increasing bacterial resistance means new analogues in this compound class are constantly sought.As well as improving current production methods of β-lactam antibiotics, AT's broad substrate specificity means it could potentially play a role in the development and production of alternative β-lactam antibiotics that are able to overcome resistance.This thesis describes the identification of an AT mutant with improved acylation activity (AAT activity) via screening of an AT library using a previously developed screening method. Approaches towards the development of a method for the identification of AT mutants with improved hydrolysis activity were also explored. The main problem to overcome in developing such a screen is the inhibitory effect of 6-APA [1], the product of hydrolysis, on AT's IAT activity. The first approach investigated the potential of increasing the sensitivity of an assay by targeting AT to the periplasm. A second approach using β-lactamases to hydrolyse 6-APA [1] thus freeing up the active site of AT was also investigated.

Exocytosis, the fusion of a vesicle to a cellular membrane, involves a protein named SNAP-25. This protein, containing two alpha helices connected with a linker region, is localized to the cell membrane via palmitic acids attached to the cysteine residues of its linker region in a process called palmitoylation. Are cysteine residues of the SNAP-25 linker region palmitoylated in an ordered manner and to a particular extent? The answer to this question may give insight into the regulated nature of exocytosis. While it is generally accepted that SNAP-25 must be palmitoylated in order to perform its exocytotic functions, the details surrounding this process are still being discovered, defined, and understood. In these studies we replicate the oxidation, reduction, and palmitoylation of SNAP-25 in vitro. Palmitoylating SNAP-25 in vitro, a process which occurs regularly in vivo, allows us to determine the extent of palmitoylation. In vitro palmitoylation of SNAP-25 was studied both with and without a native palmitoyl acyl transferase (PAT), DHHC-17, the enzyme to attach palmitic acids to cysteines in the linker region of SNAP-25. These studies were done under a variety of conditions designed to identify (1) components necessary for optimal palmitoylation and (2) extent of palmitoylation with components that mimic native conditions. Palmitoylation is a common modification for a variety of proteins, both soluble and membrane-bound. Like phosphorylation, palmitoylation is reversible and may play an important role in regulation of cellular processes. Specifically, the palmitoylation of SNAP-25 may play a critical role in the regulation of exocytosis and therefore learning further details about this important process may help us to better understand a variety of neurodegenerative diseases and states of decreased or compromised exocytosis.

Ce travail a consisté à étudier la N et/ou O acylation enzymatique d’alcool aminés et de dipeptides.Une étude préliminaire consacrée à l’acylation enzymatique d’une molécule modèle, le 6-amino-1-hexanol a démontré la capacité de la lipase B de Candida antarctica immobilisée à catalyser l’acylation de ce substrat dans différents milieux réactionnels. La mise en œuvre de cette réaction en solvants organiques (hexane, 2-méthyl-2-butanol) a conduit à la formation du produit diacylé avec un rendement de 85 % montrant l’absence de chimio-sélectivité de la réaction. L’utilisation de système sans solvant à base d’acide gras libre et de CO2 supercritique a permis d’orienter la chimio-sélectivité de la réaction en faveur de la O-acylation. Les liquides ioniques à cation de type imidazolium et à anions faiblement nucléophiles ont conduit à un taux de conversion de l’alcool aminé de l’ordre de 99 % tout en conservant l’absence de chimio-sélectivité observée en solvant organique. L’étude s’est ensuite focalisée sur l’acylation de dipeptides modèles tels que la Lys-Ser,HCl et la Ser-Leu. L’étude de l’acylation catalysée par la lipase B de Candida antarctica immobilisée de la Lys-Ser,HCl a montré une sélectivité exclusive en faveur de l’acylation de la fonction amine en position e, indépendamment du milieu réactionnel. La O-acylation de la Ser-Leu a permis de mettre en évidence l’influence du groupe carboxylique Cterminal électro-attracteur de Lys-Ser sur la réactivité de la fonction hydroxyle de la sérine. Enfin, la N-acylation enzymatique d’un dipeptide naturel bioactif, la carnosine a été réalisée d’une part en solvant organique, catalysée par la lipase B de Candida antarctica immobilisée et d’autre part, en milieu aqueux biphasique catalysée par l’acyl-transférase de Candida parapsilosis. L’acylation de la carnosine, conduisant à la synthèse de N-oléyl carnosine, n’affecte pas son activité inhibitrice de la xanthine oxydase et semble améliorer son activité anti-radicalaire vis-à-vis de l’anion superoxyde / The present work consisted in studying the N and/or O-enzymatic acylation of amino alcohols and dipeptides. A preliminary study was firstly undertaken about the enzymatic acylation of a bifunctionnal model molecule, 6-amino-1-hexanol and demonstrated the ability of the lipase B of Candida antarctica to catalyze the acylation of this substrate in different reaction media. The reaction performed in organic solvents (hexane, 2-methyl-2-butanol) allowed to the synthesis of the diacylated product with a substrate conversion yield of 85 %, showing the absence of chimio-selectivity of the reaction. The use of a solvent-free system constituted of free fatty acid and the use of supercritical carbon dioxide permitted to orientate the selectivity of the reaction in favour of the O-acylation. Ionic liquids with imidazolium cation and few nucleophilic anions led to a substrate conversion of 99 % and to maintain the absence of chemo-selectivity observed in organic solvents. Then, the study focused on the acylation of model dipeptides like Lys-Ser, HCl and Ser-Leu. Results relative to the acylation of Lys-Ser, HCl catalyzed by the lipase B of Candida antarctica immobilized showed a selectivity in favour of the acylation of the e-amino function independently of the reaction medium. The Ser-Leu O-acylation permitted to demonstrate the influence of the molecular environment (electro-attractor C terminal carboxylic group) on the reactivity of the serine hydroxyl function. Finally, the enzymatic acylation of a bioactive dipeptide was catalyzed by the lipase B of Candida antarctica immobilized in organic solvent and by the acyl-transferase of Candida parapsilosis in lipid-aqueous biphasic medium. The acylation of carnosine allowed the N-oleyl carnosine synthesis. The acylation of carnosine did not affect its xanthine oxydase inhibition activity and seemed to improve its superoxyde anion scavenging property