Differential requirements of a mitotic acetyltransferase in somatic and germ line cells

Metabolic regulation of proteome stability via N-terminal acetylation controls male germline stem cell differentiation and reproduction

The molecular mechanisms connecting cellular metabolism with differentiation remain poorly understood. Here, we find that metabolic signals contribute to stem cell differentiation and germline homeostasis during Drosophila melanogaster spermatogenesis. We discovered that external citrate, originating outside the gonad, fuels the production of Acetyl-coenzyme A by germline ATP-citrate lyase (dACLY). We show that this pathway is essential during the final spermatogenic stages, where a high Acetyl-coenzyme A level promotes NatB-dependent N-terminal protein acetylation. Using genetic and biochemical experiments, we establish that N-terminal acetylation shields key target proteins, essential for spermatid differentiation, from proteasomal degradation by the ubiquitin ligase dUBR1. Our work uncovers crosstalk between metabolism and proteome stability that is mediated via protein post-translational modification. We propose that this system coordinates the metabolic state of the organism with gamete production. More broadly, modulation of proteome turnover by circulating metabolites may be a conserved regulatory mechanism to control cell functions.

Characterisation and preliminary functional analysis of N-acetyltransferase 13 from Schistosoma japonicum

Background

N-acetyltransferase 13 (NAT13) is a probable catalytic component of the ARD1A-NARG1 complex possessing alpha (N-terminal) acetyltransferase activity.

Results

In this study, a full-length complementary DNA (cDNA) encoding Schistosoma japonicum NAT13 (SjNAT13) was isolated from schistosome cDNAs. The 621 bp open reading frame of SjNAT13 encodes a polypeptide of 206 amino acids. Real-time PCR analysis revealed SjNAT13 expression in all tested developmental stages. Transcript levels were highest in cercariae and 21-day-old worms, and higher in male adult worms than female adult worms. The rSjNAT13 protein induced high levels of anti-rSjNAT13 IgG antibodies. In two independent immunoprotection trials, rSjNAT13 induced 24.23% and 24.47% reductions in the numbers of eggs in liver. RNA interference (RNAi) results showed that small interfering RNA (siRNA) Sj-514 significantly reduced SjNAT13 transcript levels in worms and decreased egg production in vitro.

Conclusions

Thus, rSjNAT13 might play an important role in the development and reproduction of schistosomes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-03045-y.

Characterization of Specific N-α-Acetyltransferase 50 (Naa50) Inhibitors Identified Using a DNA Encoded Library

Two novel compounds were identified as Naa50 binders/inhibitors using DNA-encoded technology screening. Biophysical and biochemical data as well as cocrystal structures were obtained for both compounds (3a and 4a) to understand their mechanism of action. These data were also used to rationalize the binding affinity differences observed between the two compounds and a MLGP peptide-containing substrate. Cellular target engagement experiments further confirm the Naa50 binding of 4a and demonstrate its selectivity toward related enzymes (Naa10 and Naa60). Additional analogs of inhibitor 4a were also evaluated to study the binding mode observed in the cocrystal structures.

Naa50/San-dependent N-terminal acetylation of Scc1 is potentially important for sister chromatid cohesion

The gene separation anxiety (san) encodes Naa50/San, a N-terminal acetyltransferase required for chromosome segregation during mitosis. Although highly conserved among higher eukaryotes, the mitotic function of this enzyme is still poorly understood. Naa50/San was originally proposed to be required for centromeric sister chromatid cohesion in Drosophila and human cells, yet, more recently, it was also suggested to be a negative regulator of microtubule polymerization through internal acetylation of beta Tubulin. We used genetic and biochemical approaches to clarify the function of Naa50/San during development. Our work suggests that Naa50/San is required during tissue proliferation for the correct interaction between the cohesin subunits Scc1 and Smc3. Our results also suggest a working model where Naa50/San N-terminally acetylates the nascent Scc1 polypeptide, and that this co-translational modification is subsequently required for the establishment and/or maintenance of sister chromatid cohesion.

Early programming of the oocyte epigenome temporally controls late prophase I transcription and chromatin remodelling

Oocytes are arrested for long periods of time in the prophase of the first meiotic division (prophase I). As chromosome condensation poses significant constraints to gene expression, the mechanisms regulating transcriptional activity in the prophase I-arrested oocyte are still not entirely understood. We hypothesized that gene expression during the prophase I arrest is primarily epigenetically regulated. Here we comprehensively define the Drosophila female germ line epigenome throughout oogenesis and show that the oocyte has a unique, dynamic and remarkably diversified epigenome characterized by the presence of both euchromatic and heterochromatic marks. We observed that the perturbation of the oocyte's epigenome in early oogenesis, through depletion of the dKDM5 histone demethylase, results in the temporal deregulation of meiotic transcription and affects female fertility. Taken together, our results indicate that the early programming of the oocyte epigenome primes meiotic chromatin for subsequent functions in late prophase I.

Crystal Structure of the Golgi-Associated Human Nα-Acetyltransferase 60 Reveals the Molecular Determinants for Substrate-Specific Acetylation

N-Terminal acetylation is a common and important protein modification catalyzed by N-terminal acetyltransferases (NATs). Six human NATs (NatA-NatF) contain one catalytic subunit each, Naa10 to Naa60, respectively. In contrast to the ribosome-associated NatA to NatE, NatF/Naa60 specifically associates with Golgi membranes and acetylates transmembrane proteins. To gain insight into the molecular basis for the function of Naa60, we developed an Naa60 bisubstrate CoA-peptide conjugate inhibitor, determined its X-ray structure when bound to CoA and inhibitor, and carried out biochemical experiments. We show that Naa60 adapts an overall fold similar to that of the catalytic subunits of ribosome-associated NATs, but with the addition of two novel elongated loops that play important roles in substrate-specific binding. One of these loops mediates a dimer to monomer transition upon substrate-specific binding. Naa60 employs a catalytic mechanism most similar to Naa50. Collectively, these data reveal the molecular basis for Naa60-specific acetyltransferase activity with implications for its Golgi-specific functions.

The world of protein acetylation

Acetylation is one of the major post-translational protein modifications in the cell, with manifold effects on the protein level as well as on the metabolome level. The acetyl group, donated by the metabolite acetyl-coenzyme A, can be co- or post-translationally attached to either the α-amino group of the N-terminus of proteins or to the ε-amino group of lysine residues. These reactions are catalyzed by various N-terminal and lysine acetyltransferases. In case of lysine acetylation, the reaction is enzymatically reversible via tightly regulated and metabolism-dependent mechanisms. The interplay between acetylation and deacetylation is crucial for many important cellular processes. In recent years, our understanding of protein acetylation has increased significantly by global proteomics analyses and in depth functional studies. This review gives a general overview of protein acetylation and the respective acetyltransferases, and focuses on the regulation of metabolic processes and physiological consequences that come along with protein acetylation.

Absence of N-terminal acetyltransferase diversification during evolution of eukaryotic organisms

Protein N-terminal acetylation is an ancient and ubiquitous co-translational modification catalyzed by a highly conserved family of N-terminal acetyltransferases (NATs). Prokaryotes have at least 3 NATs, whereas humans have six distinct but highly conserved NATs, suggesting an increase in regulatory complexity of this modification during eukaryotic evolution. Despite this, and against our initial expectations, we determined that NAT diversification did not occur in the eukaryotes, as all six major human NATs were most likely present in the Last Eukaryotic Common Ancestor (LECA). Furthermore, we also observed that some NATs were actually secondarily lost during evolution of major eukaryotic lineages; therefore, the increased complexity of the higher eukaryotic proteome occurred without a concomitant diversification of NAT complexes.

Developmental roles of protein N-terminal acetylation

Discovered more than 50 years ago, N-terminal acetylation (N-Ac) is one of the most common protein modifications. Catalyzed by different N-terminal acetyltransferases (NATs), N-Ac was originally believed to mostly promote protein stability. However, several functional consequences at substrate level were recently described that yielded important new insights about the distinct molecular functions for this modification. The ubiquitous and apparent irreversible nature of this protein modification leads to the assumption that N-Ac mostly executes constitutive functions. In spite of the large number of substrates for each NAT, recent studies in multicellular organisms have nevertheless indicated very specific phenotypes after NAT loss. This raises the hypothesis that in vivo N-Ac is only functionally rate limiting for a small subset of substrates. In this review, we will discuss the function of N-Ac in the context of a developing organism. We will propose that some rate limiting NAT substrates may be tissue-specific leading to differential functions of N-Ac during development of multicellular organisms. Moreover, we will also propose the existence of tissue and developmental-specific mechanisms that differentially regulate N-Ac.

N-terminal acetylome analysis reveals the specificity of Naa50 (Nat5) and suggests a kinetic competition between N-terminal acetyltransferases and methionine aminopeptidases

Cotranslational N-terminal (Nt-) acetylation of nascent polypeptides is mediated by N-terminal acetyltransferases (NATs). The very N-terminal amino acid sequence largely determines whether or not a given protein is Nt-acetylated. Currently, there are six distinct NATs characterized, NatA-NatF, in humans of which the in vivo substrate specificity of Naa50 (Nat5)/NatE, an alternative catalytic subunit of the human NatA, so far remained elusive. In this study, we quantitatively compared the Nt-acetylomes of wild-type yeast S. cerevisiae expressing the endogenous yeast Naa50 (yNaa50), the congenic strain lacking yNaa50, and an otherwise identical strain expressing human Naa50 (hNaa50). Six canonical yeast NatA substrates were Nt-acetylated less in yeast lacking yNaa50 than in wild-type yeast. In contrast, the ectopically expressed hNaa50 resulted, predominantly, in the Nt-acetylation of N-terminal Met (iMet) starting N-termini, including iMet-Lys, iMet-Val, iMet-Ala, iMet-Tyr, iMet-Phe, iMet-Leu, iMet-Ser, and iMet-Thr N-termini. This identified hNaa50 as being similar, in its substrate specificity, to the previously characterized hNaa60/NatF. In addition, the identification, in yNaa50-lacking yeast expressing hNaa50, of Nt-acetylated iMet followed by a small residue such as Ser, Thr, Ala, or Val, revealed a kinetic competition between Naa50 and Met-aminopeptidases (MetAPs), and implied that Nt-acetylated iMet followed by a small residue cannot be removed by MetAPs, a deduction supported by our in vitro data. As such, Naa50-mediated Nt-acetylation may act to retain the iMet of proteins of otherwise MetAP susceptible N-termini and the fraction of retained and Nt-acetylated iMet (followed by a small residue) in such a setting would be expected to depend on the relative levels of ribosome-associated Naa50/NatA and MetAPs.

The biological functions of Naa10 - From amino-terminal acetylation to human disease

N-terminal acetylation (NTA) is one of the most abundant protein modifications known, and the N-terminal acetyltransferase (NAT) machinery is conserved throughout all Eukarya. Over the past 50 years, the function of NTA has begun to be slowly elucidated, and this includes the modulation of protein-protein interaction, protein-stability, protein function, and protein targeting to specific cellular compartments. Many of these functions have been studied in the context of Naa10/NatA; however, we are only starting to really understand the full complexity of this picture. Roughly, about 40% of all human proteins are substrates of Naa10 and the impact of this modification has only been studied for a few of them. Besides acting as a NAT in the NatA complex, recently other functions have been linked to Naa10, including post-translational NTA, lysine acetylation, and NAT/KAT-independent functions. Also, recent publications have linked mutations in Naa10 to various diseases, emphasizing the importance of Naa10 research in humans. The recent design and synthesis of the first bisubstrate inhibitors that potently and selectively inhibit the NatA/Naa10 complex, monomeric Naa10, and hNaa50 further increases the toolset to analyze Naa10 function.

An organellar nα-acetyltransferase, naa60, acetylates cytosolic N termini of transmembrane proteins and maintains Golgi integrity

N-terminal acetylation is a major and vital protein modification catalyzed by N-terminal acetyltransferases (NATs). NatF, or Nα-acetyltransferase 60 (Naa60), was recently identified as a NAT in multicellular eukaryotes. Here, we find that Naa60 differs from all other known NATs by its Golgi localization. A new membrane topology assay named PROMPT and a selective membrane permeabilization assay established that Naa60 faces the cytosolic side of intracellular membranes. An Nt-acetylome analysis of NAA60-knockdown cells revealed that Naa60, as opposed to other NATs, specifically acetylates transmembrane proteins and has a preference for N termini facing the cytosol. Moreover, NAA60 knockdown causes Golgi fragmentation, indicating an important role in the maintenance of the Golgi's structural integrity. This work identifies a NAT associated with membranous compartments and establishes N-terminal acetylation as a common modification among transmembrane proteins, a thus-far poorly characterized part of the N-terminal acetylome.

Meiosis in male Drosophila

Meiosis entails sorting and separating both homologous and sister chromatids. The mechanisms for connecting sister chromatids and homologs during meiosis are highly conserved and include specialized forms of the cohesin complex and a tightly regulated homolog synapsis/recombination pathway designed to yield regular crossovers between homologous chromatids. Drosophila male meiosis is of special interest because it dispenses with large segments of the standard meiotic script, particularly recombination, synapsis and the associated structures. Instead, Drosophila relies on a unique protein complex composed of at least two novel proteins, SNM and MNM, to provide stable connections between homologs during meiosis I. Sister chromatid cohesion in Drosophila is mediated by cohesins, ring-shaped complexes that entrap sister chromatids. However, unlike other eukaryotes Drosophila does not rely on the highly conserved Rec8 cohesin in meiosis, but instead utilizes two novel cohesion proteins, ORD and SOLO, which interact with the SMC1/3 cohesin components in providing meiotic cohesion.

Drosophila protein kinase N (Pkn) is a negative regulator of actin-myosin activity during oogenesis

Nurse cell dumping is an actin-myosin based process, where 15 nurse cells of a given egg chamber contract and transfer their cytoplasmic content through the ring canals into the growing oocyte. We isolated two mutant alleles of protein kinase N (pkn) and showed that Pkn negatively-regulates activation of the actin-myosin cytoskeleton during the onset of dumping. Using live-cell imaging analysis we observed that nurse cell dumping rates sharply increase during the onset of fast dumping. Such rate increase was severely impaired in pkn mutant nurse cells due to excessive nurse cell actin-myosin activity and/or loss of tissue integrity. Our work demonstrates that the transition between slow and fast dumping is a discrete event, with at least a five to six-fold dumping rate increase. We show that Pkn negatively regulates nurse cell actin-myosin activity. This is likely to be important for directional cytoplasmic flow. We propose Pkn provides a negative feedback loop to help avoid excessive contractility after local activation of Rho GTPase.

Chemoproteomic profiling of lysine acetyltransferases highlights an expanded landscape of catalytic acetylation

Lysine acetyltransferases (KATs) play a critical role in the regulation of gene expression, metabolism, and other key cellular functions. One shortcoming of traditional KAT assays is their inability to study KAT activity in complex settings, a limitation that hinders efforts at KAT discovery, characterization, and inhibitor development. To address this challenge, here we describe a suite of cofactor-based affinity probes capable of profiling KAT activity in biological contexts. Conversion of KAT bisubstrate inhibitors to clickable photoaffinity probes enables the selective covalent labeling of three phylogenetically distinct families of KAT enzymes. Cofactor-based affinity probes report on KAT activity in cell lysates, where KATs exist as multiprotein complexes. Chemical affinity purification and unbiased LC–MS/MS profiling highlights an expanded landscape of orphan lysine acetyltransferases present in the human genome and provides insight into the global selectivity and sensitivity of CoA-based proteomic probes that will guide future applications. Chemoproteomic profiling provides a powerful method to study the molecular interactions of KATs in native contexts and will aid investigations into the role of KATs in cell state and disease.

Requirement for highly efficient pre-mRNA splicing during Drosophila early embryonic development

Drosophila syncytial nuclear divisions limit transcription unit size of early zygotic genes. As mitosis inhibits not only transcription, but also pre-mRNA splicing, we reasoned that constraints on splicing were likely to exist in the early embryo, being splicing avoidance a possible explanation why most early zygotic genes are intronless. We isolated two mutant alleles for a subunit of the NTC/Prp19 complexes, which specifically impaired pre-mRNA splicing of early zygotic but not maternally encoded transcripts. We hypothesized that the requirements for pre-mRNA splicing efficiency were likely to vary during development. Ectopic maternal expression of an early zygotic pre-mRNA was sufficient to suppress its splicing defects in the mutant background. Furthermore, a small early zygotic transcript with multiple introns was poorly spliced in wild-type embryos. Our findings demonstrate for the first time the existence of a developmental pre-requisite for highly efficient splicing during Drosophila early embryonic development and suggest in highly proliferative tissues a need for coordination between cell cycle and gene architecture to ensure correct gene expression and avoid abnormally processed transcripts. DOI: http://dx.doi.org/10.7554/eLife.02181.001.

Arabidopsis CHROMOSOME TRANSMISSION FIDELITY 7 (AtCTF7/ECO1) is required for DNA repair, mitosis and meiosis

The proper transmission of DNA in dividing cells is crucial for the survival of eukaryotic organisms. During cell division, faithful segregation of replicated chromosomes requires their tight attachment, known as sister chromatid cohesion, until anaphase. Sister chromatid cohesion is established during S-phase in a process requiring an acetyltransferase that in yeast is known as Establishment of cohesion 1 (Eco1). Inactivation of Eco1 typically disrupts chromosome segregation and homologous recombination-dependent DNA repair in dividing cells, ultimately resulting in lethality. We report here the isolation and detailed characterization of two homozygous T-DNA insertion mutants for the Arabidopsis thaliana Eco1 homolog, CHROMOSOME TRANSMISSION FIDELITY 7/ESTABLISHMENT OF COHESION 1 (CTF7/ECO1), called ctf7-1 and ctf7-2. Mutants exhibited dwarfism, poor anther development and sterility. Analysis of somatic tissues by flow cytometry, scanning electron microscopy and quantitative real-time PCR identified defects in DNA repair and cell division, including an increase in the area of leaf epidermal cells, an increase in DNA content and the upregulation of genes involved in DNA repair including BRCA1 and PARP2. No significant change was observed in the expression of genes that influence entry into the endocycle. Analysis of meiocytes identified changes in chromosome morphology and defective segregation; the abundance of chromosomal-bound cohesion subunits was also reduced. Transcript levels for several meiotic genes, including the recombinase genes DMC1 and RAD51C and the S-phase licensing factor CDC45 were elevated in mutant anthers. Taken together our results demonstrate that Arabidopsis CTF7/ECO1 plays important roles in the preservation of genome integrity and meiosis.

Design, synthesis, and kinetic characterization of protein N-terminal acetyltransferase inhibitors

The N-termini of 80-90% of human proteins are acetylated by the N-terminal acetyltransferases (NATs), NatA-NatF. The major NAT complex, NatA, and particularly the catalytic subunit hNaa10 (ARD1) has been implicated in cancer development. For example, knockdown of hNaa10 results in cancer cell death and the arrest of cell proliferation. It also sensitized cancer cells to drug-induced cytotoxicity. Human NatE has a distinct substrate specificity and is essential for normal chromosome segregation. Thus, NAT inhibitors may potentially be valuable anticancer therapeutics, either directly or as adjuvants. Herein, we report the design and synthesis of the first inhibitors targeting these enzymes. Using the substrate specificity of the enzymes as a guide, we synthesized three bisubstrate analogues that potently and selectively inhibit the NatA complex (CoA-Ac-SES4; IC50 = 15.1 μM), hNaa10, the catalytic subunit of NatA (CoA-Ac-EEE4; Ki = 1.6 μM), and NatE/hNaa50 (CoA-Ac-MLG7; Ki* = 8 nM); CoA-Ac-EEE4 is a reversible competitive inhibitor of hNaa10, and CoA-Ac-MLG7 is a slow tight binding inhibitor of hNaa50. Our demonstration that it is possible to develop NAT selective inhibitors should assist future efforts to develop NAT inhibitors with more drug-like properties that can be used to chemically interrogate in vivo NAT function.

Mutations at the flavin binding site of ETF:QO yield a MADD-like severe phenotype in Drosophila

Following a screening on EMS-induced Drosophila mutants defective for formation and morphogenesis of epithelial cells, we have identified three lethal mutants defective for the production of embryonic cuticle. The mutants are allelic to the CG12140 gene, the fly homologue of electron transfer flavoprotein:ubiquinone oxidoreductase (ETF:QO). In humans, inherited defects in this inner membrane protein account for multiple acyl-CoA dehydrogenase deficiency (MADD), a metabolic disease of β-oxidation, with a broad range of clinical phenotypes, varying from embryonic lethal to mild forms. The three mutant alleles carried distinct missense mutations in ETF:QO (G65E, A68V and S104F) and maternal mutant embryos for ETF:QO showed lethal morphogenetic defects and a significant induction of apoptosis following germ-band elongation. This phenotype is accompanied by an embryonic accumulation of short- and medium-chain acylcarnitines (C4, C8 and C12) as well as long-chain acylcarnitines (C14 and C16:1), whose elevation is also found in severe MADD forms in humans under intense metabolic decompensation. In agreement the ETF:QO activity in the mutant embryos is markedly decreased in relation to wild type activity. Amino acid sequence analysis and structural mapping into a molecular model of ETF:QO show that all mutations map at FAD interacting residues, two of which at the nucleotide-binding Rossmann fold. This structural domain is composed by a β-strand connected by a short loop to an α-helix, and its perturbation results in impaired cofactor association via structural destabilisation and consequently enzymatic inactivation. This work thus pinpoints the molecular origins of a severe MADD-like phenotype in the fruit fly and establishes the proof of concept concerning the suitability of this organism as a potential model organism for MADD.

Human protein N-terminal acetyltransferase hNaa50p (hNAT5/hSAN) follows ordered sequential catalytic mechanism: combined kinetic and NMR study

N(α)-acetylation is a common protein modification catalyzed by different N-terminal acetyltransferases (NATs). Their essential role in the biogenesis and degradation of proteins is becoming increasingly evident. The NAT hNaa50p preferentially modifies peptides starting with methionine followed by a hydrophobic amino acid. hNaa50p also possesses N(ε)-autoacetylation activity. So far, no eukaryotic NAT has been mechanistically investigated. In this study, we used NMR spectroscopy, bisubstrate kinetic assays, and product inhibition experiments to demonstrate that hNaa50p utilizes an ordered Bi Bi reaction of the Theorell-Chance type. The NMR results, both the substrate binding study and the dynamic data, further indicate that the binding of acetyl-CoA induces a conformational change that is required for the peptide to bind to the active site. In support of an ordered Bi Bi reaction mechanism, addition of peptide in the absence of acetyl-CoA did not alter the structure of the protein. This model is further strengthened by the NMR results using a catalytically inactive hNaa50p mutant.

Protein N-terminal acetyltransferases: when the start matters

The majority of eukaryotic proteins are subjected to N-terminal acetylation (Nt-acetylation), catalysed by N-terminal acetyltransferases (NATs). Recently, the structure of an NAT-peptide complex was determined, and detailed proteome-wide Nt-acetylation patterns were revealed. Furthermore, Nt-acetylation just emerged as a multifunctional regulator, acting as a protein degradation signal, an inhibitor of endoplasmic reticulum (ER) translocation, and a mediator of protein complex formation. Nt-acetylation is regulated by acetyl-coenzyme A (Ac-CoA) levels, and thereby links metabolic cell states to cell death. The essentiality of NATs in humans is stressed by the recent discovery of a human hereditary lethal disease caused by a mutation in an NAT gene. Here, we discuss how these recent findings shed light on NATs as major protein regulators and key cellular players.

Protein N-terminal acetyltransferases in cancer

The human N-terminal acetyltransferases (NATs) catalyze the transfer of acetyl moieties to the N-termini of 80-90% of all human proteins. Six NAT types are present in humans, NatA-NatF, each is composed of specific subunits and each acetylates a set of substrates defined by the N-terminal amino-acid sequence. NATs have been suggested to act as oncoproteins as well as tumor suppressors in human cancers, and NAT expression may be both elevated and decreased in cancer versus non-cancer tissues. Manipulation of NATs in cancer cells induced cell-cycle arrest, apoptosis or autophagy, implying that these enzymes target a variety of pathways. Of particular interest is hNaa10p (human ARD1), the catalytic subunit of the NatA complex, which was coupled to a number of signaling molecules including hypoxia inducible factor-1α, β-catenin/cyclin D1, TSC2/mammalian target of rapamycin, myosin light chain kinase , DNA methyltransferase1/E-cadherin and p21-activated kinase-interacting exchange factors (PIX)/Cdc42/Rac1. The variety of mechanistic links where hNaa10p acts as a NAT, a lysine acetyltransferase or displaying a non-catalytic role, provide insights to how hNaa10p may act as both a tumor suppressor and oncoprotein.

Drosophila aPKC is required for mitotic spindle orientation during symmetric division of epithelial cells

Epithelial cells mostly orient the spindle along the plane of the epithelium (planar orientation) for mitosis to produce two identical daughter cells. The correct orientation of the spindle relies on the interaction between cortical polarity components and astral microtubules. Recent studies in mammalian tissue culture cells suggest that the apically localised atypical protein kinase C (aPKC) is important for the planar orientation of the mitotic spindle in dividing epithelial cells. Yet, in chicken neuroepithelial cells, aPKC is not required in vivo for spindle orientation, and it has been proposed that the polarization cues vary between different epithelial cell types and/or developmental processes. In order to investigate whether Drosophila aPKC is required for spindle orientation during symmetric division of epithelial cells, we took advantage of a previously isolated temperature-sensitive allele of aPKC. We showed that Drosophila aPKC is required in vivo for spindle planar orientation and apical exclusion of Pins (Raps). This suggests that the cortical cues necessary for spindle orientation are not only conserved between Drosophila and mammalian cells, but are also similar to those required for spindle apicobasal orientation during asymmetric cell division.

Structure of a ternary Naa50p (NAT5/SAN) N-terminal acetyltransferase complex reveals the molecular basis for substrate-specific acetylation

The co-translational modification of N-terminal acetylation is ubiquitous among eukaryotes and has been reported to have a wide range of biological effects. The human N-terminal acetyltransferase (NAT) Naa50p (NAT5/SAN) acetylates the α-amino group of proteins containing an N-terminal methionine residue and is essential for proper sister chromatid cohesion and chromosome condensation. The elevated activity of NATs has also been correlated with cancer, making these enzymes attractive therapeutic targets. We report the x-ray crystal structure of Naa50p bound to a native substrate peptide fragment and CoA. We found that the peptide backbone of the substrate is anchored to the protein through a series of backbone hydrogen bonds with the first methionine residue specified through multiple van der Waals contacts, together creating an α-amino methionine-specific pocket. We also employed structure-based mutagenesis; the results support the importance of the α-amino methionine-specific pocket of Naa50p and are consistent with the proposal that conserved histidine and tyrosine residues play important catalytic roles. Superposition of the ternary Naa50p complex with the peptide-bound Gcn5 histone acetyltransferase revealed that the two enzymes share a Gcn5-related N-acetyltransferase fold but differ in their respective substrate-binding grooves such that Naa50p can accommodate only an α-amino substrate and not a side chain lysine substrate that is acetylated by lysine acetyltransferase enzymes such as Gcn5. The structure of the ternary Naa50p complex also provides the first molecular scaffold for the design of NAT-specific small molecule inhibitors with possible therapeutic applications.

The human N-alpha-acetyltransferase 40 (hNaa40p/hNatD) is conserved from yeast and N-terminally acetylates histones H2A and H4

Protein N^α-terminal acetylation (Nt-acetylation) is considered one of the most common protein modification in eukaryotes, and 80-90% of all soluble human proteins are modified in this way, with functional implications ranging from altered protein function and stability to translocation potency amongst others. Nt-acetylation is catalyzed by N-terminal acetyltransferases (NATs), and in yeast five NAT types are identified and denoted NatA-NatE. Higher eukaryotes additionally express NatF. Except for NatD, human orthologues for all yeast NATs are identified. yNatD is defined as the catalytic unit Naa40p (Nat4) which co-translationally Nt-acetylates histones H2A and H4.

In this study we identified and characterized hNaa40p/hNatD, the human orthologue of the yeast Naa40p. An in vitro proteome-derived peptide library Nt-acetylation assay indicated that recombinant hNaa40p acetylates N-termini starting with the consensus sequence Ser-Gly-Gly-Gly-Lys-, strongly resembling the N-termini of the human histones H2A and H4. This was confirmed as recombinant hNaa40p Nt-acetylated the oligopeptides derived from the N-termini of both histones. In contrast, a synthetically Nt-acetylated H4 N-terminal peptide with all lysines being non-acetylated, was not significantly acetylated by hNaa40p, indicating that hNaa40p catalyzed H4 N^α-acetylation and not H4 lysine N^ε-acetylation. Also, immunoprecipitated hNaa40p specifically Nt-acetylated H4 in vitro. Heterologous expression of hNaa40p in a yeast naa40-Δ strain restored Nt-acetylation of yeast histone H4, but not H2A in vivo, probably reflecting the fact that the N-terminal sequences of human H2A and H4 are highly similar to each other and to yeast H4 while the N-terminal sequence of yeast H2A differs. Thus, Naa40p seems to have co-evolved with the human H2A sequence. Finally, a partial co-sedimentation with ribosomes indicates that hNaa40p co-translationally acetylates H2A and H4. Combined, our results strongly suggest that human Naa40p/NatD is conserved from yeast. Thus, the NATs of all classes of N-terminally acetylated proteins in humans now appear to be accounted for.

NatF contributes to an evolutionary shift in protein N-terminal acetylation and is important for normal chromosome segregation

N-terminal acetylation (N-Ac) is a highly abundant eukaryotic protein modification. Proteomics revealed a significant increase in the occurrence of N-Ac from lower to higher eukaryotes, but evidence explaining the underlying molecular mechanism(s) is currently lacking. We first analysed protein N-termini and their acetylation degrees, suggesting that evolution of substrates is not a major cause for the evolutionary shift in N-Ac. Further, we investigated the presence of putative N-terminal acetyltransferases (NATs) in higher eukaryotes. The purified recombinant human and Drosophila homologues of a novel NAT candidate was subjected to in vitro peptide library acetylation assays. This provided evidence for its NAT activity targeting Met-Lys- and other Met-starting protein N-termini, and the enzyme was termed Naa60p and its activity NatF. Its in vivo activity was investigated by ectopically expressing human Naa60p in yeast followed by N-terminal COFRADIC analyses. hNaa60p acetylated distinct Met-starting yeast protein N-termini and increased general acetylation levels, thereby altering yeast in vivo acetylation patterns towards those of higher eukaryotes. Further, its activity in human cells was verified by overexpression and knockdown of hNAA60 followed by N-terminal COFRADIC. NatF's cellular impact was demonstrated in Drosophila cells where NAA60 knockdown induced chromosomal segregation defects. In summary, our study revealed a novel major protein modifier contributing to the evolution of N-Ac, redundancy among NATs, and an essential regulator of normal chromosome segregation. With the characterization of NatF, the co-translational N-Ac machinery appears complete since all the major substrate groups in eukaryotes are accounted for.

Small chemical groups are commonly attached to proteins in order to control their activity, localization, and stability. An abundant protein modification is N-terminal acetylation, in which an N-terminal acetyltransferase (NAT) catalyzes the transfer of an acetyl group to the very N-terminal amino acid of the protein. When going from lower to higher eukaryotes there is a significant increase in the occurrence of N-terminal acetylation. We demonstrate here that this is partly because higher eukaryotes uniquely express NatF, an enzyme capable of acetylating a large group of protein N-termini including those previously found to display an increased N-acetylation potential in higher eukaryotes. Thus, the current study has possibly identified the last major component of the eukaryotic machinery responsible for co-translational N-acetylation of proteins. All eukaryotic proteins start with methionine, which is co-translationally cleaved when the second amino acid is small. Thereafter, NatA may acetylate these newly exposed N-termini. Interestingly, NatF also has the potential to act on these types of N-termini where the methionine was not cleaved. At the cellular level, we further found that NatF is essential for normal chromosome segregation during cell division.

Proteome-derived peptide libraries allow detailed analysis of the substrate specificities of N(alpha)-acetyltransferases and point to hNaa10p as the post-translational actin N(alpha)-acetyltransferase

The impact of N(α)-terminal acetylation on protein stability and protein function in general recently acquired renewed and increasing attention. Although the substrate specificity profile of the conserved enzymes responsible for N(α)-terminal acetylation in yeast has been well documented, the lack of higher eukaryotic models has hampered the specificity profile determination of N(α)-acetyltransferases (NATs) of higher eukaryotes. The fact that several types of protein N termini are acetylated by so far unknown NATs stresses the importance of developing tools for analyzing NAT specificities. Here, we report on a method that implies the use of natural, proteome-derived modified peptide libraries, which, when used in combination with two strong cation exchange separation steps, allows for the delineation of the in vitro specificity profiles of NATs. The human NatA complex, composed of the auxiliary hNaa15p (NATH/hNat1) subunit and the catalytic hNaa10p (hArd1) and hNaa50p (hNat5) subunits, cotranslationally acetylates protein N termini initiating with Ser, Ala, Thr, Val, and Gly following the removal of the initial Met. In our studies, purified hNaa50p preferred Met-Xaa starting N termini (Xaa mainly being a hydrophobic amino acid) in agreement with previous data. Surprisingly, purified hNaa10p preferred acidic N termini, representing a group of in vivo acetylated proteins for which there are currently no NAT(s) identified. The most prominent representatives of the group of acidic N termini are γ- and β-actin. Indeed, by using an independent quantitative assay, hNaa10p strongly acetylated peptides representing the N termini of both γ- and β-actin, and only to a lesser extent, its previously characterized substrate motifs. The immunoprecipitated NatA complex also acetylated the actin N termini efficiently, though displaying a strong shift in specificity toward its known Ser-starting type of substrates. Thus, complex formation of NatA might alter the substrate specificity profile as compared with its isolated catalytic subunits, and, furthermore, NatA or hNaa10p may function as a post-translational actin N(α)-acetyltransferase.

Drosophila variable nurse cells encodes arrest defective 1 (ARD1), the catalytic subunit of the major N-terminal acetyltransferase complex

Mutations in the Drosophila variable nurse cells (vnc) gene result in female sterility and oogenesis defects, including egg chambers with too many or too few nurse cells. We show that vnc corresponds to Arrest Defective1 (Ard1) and encodes the catalytic subunit of NatA, the major N-terminal acetyl-transferase complex. While N-terminal acetylation is one of the most prevalent covalent protein modifications in eukaryotes, analysis of its role in development has been challenging since mutants that compromise NatA activity have not been described in any multicellular animal. Our data show that reduced ARD1 levels result in pleiotropic oogenesis defects including abnormal cyst encapsulation, desynchronized cystocyte division, disrupted nurse cell chromosome dispersion, and abnormal chorion patterning, consistent with the wide range of predicted NatA substrates. Furthermore, we find that loss of Ard1 affects cell survival/proliferation and is lethal for the animal, providing the first demonstration that this modification is essential in higher eukaryotes.

Human Naa50p (Nat5/San) displays both protein N alpha- and N epsilon-acetyltransferase activity

Protein acetylation is a widespread modification that is mediated by site-selective acetyltransferases. KATs (lysine N(epsilon)-acetyltransferases), modify the side chain of specific lysines on histones and other proteins, a central process in regulating gene expression. N(alpha)-terminal acetylation occurs on the ribosome where the alpha amino group of nascent polypeptides is acetylated by NATs (N-terminal acetyltransferase). In yeast, three different NAT complexes were identified NatA, NatB, and NatC. NatA is composed of two main subunits, the catalytic subunit Naa10p (Ard1p) and Naa15p (Nat1p). Naa50p (Nat5) is physically associated with NatA. In man, hNaa50p was shown to have acetyltransferase activity and to be important for chromosome segregation. In this study, we used purified recombinant hNaa50p and multiple oligopeptide substrates to identify and characterize an N(alpha)-acetyltransferase activity of hNaa50p. As the preferred substrate this activity acetylates oligopeptides with N termini Met-Leu-Xxx-Pro. Furthermore, hNaa50p autoacetylates lysines 34, 37, and 140 in vitro, modulating hNaa50p substrate specificity. In addition, histone 4 was detected as a hNaa50p KAT substrate in vitro. Our findings thus provide the first experimental evidence of an enzyme having both KAT and NAT activities.

Composition and biological significance of the human Nalpha-terminal acetyltransferases

Protein N^α-terminal acetylation is one of the most common protein modifications in eukaryotic cells, occurring on approximately 80% of soluble human proteins. An increasing number of studies links N^α-terminal acetylation to cell differentiation, cell cycle, cell survival, and cancer. Thus, N^α-terminal acetylation is an essential modification for normal cell function in humans. Still, little is known about the functional role of N^α-terminal acetylation. Recently, the three major human N-acetyltransferase complexes, hNatA, hNatB and hNatC, were identified and characterized. We here summarize the identified N-terminal acetyltransferase complexes in humans, and we review the biological studies on N^α-terminal acetylation in humans and other higher eukaryotes.