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

A Homozygous Variant in NAA60 Is Associated with Primary Familial Brain Calcification

BACKGROUND

Primary familial brain calcification (PFBC) is a monogenic disorder characterized by bilateral calcifications in the brain. The genetic basis remains unknown in over half of the PFBC patients, indicating the existence of additional novel causative genes. NAA60 was a recently reported novel causative gene for PFBC.

OBJECTIVE

The aim was to identify the probable novel causative gene in an autosomal recessive inherited PFBC family.

METHODS

We performed a comprehensive genetic study on a consanguineous Chinese family with 3 siblings diagnosed with PFBC. We evaluated the effect of the variant in a probable novel causative gene on the protein level using Western blot, immunofluorescence, and coimmunoprecipitation. Possible downstream pathogenic mechanisms were further explored in gene knockout (KO) cell lines and animal models.

RESULTS

We identified a PFBC co-segregated homozygous variant of c.460_461del (p.D154Lfs*113) in NAA60. Functional assays showed that this variant disrupts NAA60 protein localization to Golgi and accelerated protein degradation. The mutant NAA60 protein alters its interaction with the PFBC-related proteins PiT2 and XPR1, affecting intracellular phosphate homeostasis. Further mass spectrometry analysis in NAA60 KO cell lines revealed decreased expression of multiple brain calcification-associated proteins, including reduced folate carrier (RFC), a folate metabolism-related protein.

CONCLUSIONS

Our study replicated the identification of NAA60 as a novel causative gene for autosomal recessive PFBC, demonstrating our causative variant leads to NAA60 loss of function. The NAA60 loss of function disrupts not only PFBC-related proteins (eg, PiT2 and XPR1) but also a wide range of other brain calcification-associated membrane protein substrates (eg, RFC), and provided a novel probable pathogenic mechanism for PFBC. © 2024 International Parkinson and Movement Disorder Society.

Biallelic NAA60 variants with impaired n-terminal acetylation capacity cause autosomal recessive primary familial brain calcifications

Primary familial brain calcification (PFBC) is characterized by calcium deposition in the brain, causing progressive movement disorders, psychiatric symptoms, and cognitive decline. PFBC is a heterogeneous disorder currently linked to variants in six different genes, but most patients remain genetically undiagnosed. Here, we identify biallelic NAA60 variants in ten individuals from seven families with autosomal recessive PFBC. The NAA60 variants lead to loss-of-function with lack of protein N-terminal (Nt)-acetylation activity. We show that the phosphate importer SLC20A2 is a substrate of NAA60 in vitro. In cells, loss of NAA60 caused reduced surface levels of SLC20A2 and a reduction in extracellular phosphate uptake. This study establishes NAA60 as a causal gene for PFBC, provides a possible biochemical explanation of its disease-causing mechanisms and underscores NAA60-mediated Nt-acetylation of transmembrane proteins as a fundamental process for healthy neurobiological functioning.

Impact of Protein Nα-Modifications on Cellular Functions and Human Health

Most human proteins are modified by enzymes that act on the α-amino group of a newly synthesized polypeptide. Methionine aminopeptidases can remove the initiator methionine and expose the second amino acid for further modification by enzymes responsible for myristoylation, acetylation, methylation, or other chemical reactions. Specific acetyltransferases can also modify the initiator methionine and sometimes the acetylated methionine can be removed, followed by further modifications. These modifications at the protein N-termini play critical roles in cellular protein localization, protein-protein interaction, protein-DNA interaction, and protein stability. Consequently, the dysregulation of these modifications could significantly change the development and progression status of certain human diseases. The focus of this review is to highlight recent progress in our understanding of the roles of these modifications in regulating protein functions and how these enzymes have been used as potential novel therapeutic targets for various human diseases.

NATs at a glance

Most proteins receive an acetyl group at the N terminus while in their nascency as the result of modification by co-translationally acting N-terminal acetyltransferases (NATs). The N-terminal acetyl group can influence several aspects of protein functionality. From studies of NAT-lacking cells, it is evident that several cellular processes are affected by this modification. More recently, an increasing number of genetic cases have demonstrated that N-terminal acetylation has crucial roles in human physiology and pathology. In this Cell Science at a Glance and the accompanying poster, we provide an overview of the human NAT enzymes and their properties, substrate coverage, cellular roles and connections to human disease.

Optimized bisubstrate inhibitors for the actin N-terminal acetyltransferase NAA80

Acetylation of protein N-termini is one of the most common protein modifications in the eukaryotic cell and is catalyzed by the N-terminal acetyltransferase family of enzymes. The N-terminal acetyltransferase NAA80 is expressed in the animal kingdom and was recently found to specifically N-terminally acetylate actin, which is the main component of the microfilament system. This unique animal cell actin processing is essential for the maintenance of cell integrity and motility. Actin is the only known substrate of NAA80, thus potent inhibitors of NAA80 could prove as important tool compounds to study the crucial roles of actin and how NAA80 regulates this by N-terminal acetylation. Herein we describe a systematic study toward optimizing the peptide part of a bisubstrate-based NAA80 inhibitor comprising of coenzyme A conjugated onto the N-terminus of a tetrapeptide amide via an acetyl linker. By testing various combinations of Asp and Glu which are found at the N-termini of β- and γ-actin, respectively, CoA-Ac-EDDI-NH₂ was identified as the best inhibitor with an IC₅₀ value of 120 nM.

NAA60 (HAT4): the newly discovered bi-functional Golgi member of the acetyltransferase family

Chromatin structural organization, gene expression and proteostasis are intricately regulated in a wide range of biological processes, both physiological and pathological. Protein acetylation, a major post-translational modification, is tightly involved in interconnected biological networks, modulating the activation of gene transcription and protein action in cells. A very large number of studies describe the pivotal role of the so-called acetylome (accounting for more than 80% of the human proteome) in orchestrating different pathways in response to stimuli and triggering severe diseases, including cancer. NAA60/NatF (N-terminal acetyltransferase F), also named HAT4 (histone acetyltransferase type B protein 4), is a newly discovered acetyltransferase in humans modifying N-termini of transmembrane proteins starting with M-K/M-A/M-V/M-M residues and is also thought to modify lysine residues of histone H4. Because of its enzymatic features and unusual cell localization on the Golgi membrane, NAA60 is an intriguing acetyltransferase that warrants biochemical and clinical investigation. Although it is still poorly studied, this review summarizes current findings concerning the structural hallmarks and biological role of this novel targetable epigenetic enzyme.

Pan-cancer analysis reveals NAA50 as a cancer prognosis and immune infiltration-related biomarker

Background: N-Alpha-Acetyltransferase 50 (NAA50) has acetyltransferase activity and is important for chromosome segregation. However, the function and mechanism of NAA50 expression in cancer development was still unclear. Here, we systematically researched the function and mechanism of NAA50 in pan-cancer, and further verified the results of NAA50 in lung adenocarcinoma (LUAD). Methods: In this study, using the online databases TIMER2.0, SangerBox3.0, HPA, UCSC, GEPIA, cBioPortal, UALCAN, TISIDB, CancerSEA and LinkedOmics, we focused on the relevance between NAA50 and oncogenesis, progression, methylation, immune infiltration, function and prognosis. In addition, the proliferation of cells was detected by CCK-8 and Edu assay. Finally, we analyzed the relationship between the expression of NAA50 and cell cycle related proteins. Results: Pan-cancer analysis indicated that NAA50 was overexpressed in most cancers. And there was a significant correlation between NAA50 expression and the prognosis of cancer patients. In the meantime, NAA50 gene changes occur in a variety of tumors. Compared with normal tissues, the methylation level of NAA50 promoter increased in most cancer tissues. In addition, the results exhibited that in most cancers, NAA50 was significantly positively correlated with bone myeloid-derived suppressor cell (MDSC) infiltration and negatively correlated with T cell NK infiltration. Moreover, functional enrichment indicated that NAA50 regulates cell cycle and proliferation in LUAD. In vitro experiments testified that knockout of NAA50 could significantly inhibit the proliferation of LUAD. Conclusion: NAA50 may be a potential biomarker and oncogene of pan-cancer, especially LUAD, which may promote the occurrence and development of tumors through different mechanisms. Furthermore, NAA50 was bound up with to immune cell infiltration in pan-cancer, meaning NAA50 may be an important therapeutic target for human cancers.

From Nucleus to Membrane: A Subcellular Map of the N-Acetylation Machinery in Plants

N-terminal acetylation (NTA) is an ancient protein modification conserved throughout all domains of life. N-terminally acetylated proteins are present in the cytosol, the nucleus, the plastids, mitochondria and the plasma membrane of plants. The frequency of NTA differs greatly between these subcellular compartments. While up to 80% of cytosolic and 20-30% of plastidic proteins are subject to NTA, NTA of mitochondrial proteins is rare. NTA alters key characteristics of proteins such as their three-dimensional structure, binding properties and lifetime. Since the majority of proteins is acetylated by five ribosome-bound N-terminal acetyltransferases (Nats) in yeast and humans, NTA was long perceived as an exclusively co-translational process in eukaryotes. The recent characterization of post-translationally acting plant Nats, which localize to the plasma membrane and the plastids, has challenged this view. Moreover, findings in humans, yeast, green algae and higher plants uncover differences in the cytosolic Nat machinery of photosynthetic and non-photosynthetic eukaryotes. These distinctive features of the plant Nat machinery might constitute adaptations to the sessile lifestyle of plants. This review sheds light on the unique role of plant N-acetyltransferases in development and stress responses as well as their evolution-driven adaptation to function in different cellular compartments.

Extended N-Terminal Acetyltransferase Naa50 in Filamentous Fungi Adds to Naa50 Diversity

Most eukaryotic proteins are N-terminally acetylated by a set of Nα acetyltransferases (NATs). This ancient and ubiquitous modification plays a fundamental role in protein homeostasis, while mutations are linked to human diseases and phenotypic defects. In particular, Naa50 features species-specific differences, as it is inactive in yeast but active in higher eukaryotes. Together with NatA, it engages in NatE complex formation for cotranslational acetylation. Here, we report Naa50 homologs from the filamentous fungi Chaetomium thermophilum and Neurospora crassa with significant N- and C-terminal extensions to the conserved GNAT domain. Structural and biochemical analyses show that CtNaa50 shares the GNAT structure and substrate specificity with other homologs. However, in contrast to previously analyzed Naa50 proteins, it does not form NatE. The elongated N-terminus increases Naa50 thermostability and binds to dynein light chain protein 1, while our data suggest that conserved positive patches in the C-terminus allow for ribosome binding independent of NatA. Our study provides new insights into the many facets of Naa50 and highlights the diversification of NATs during evolution.

A Continuous Assay Set to Screen and Characterize Novel Protein N-Acetyltransferases Unveils Rice General Control Non-repressible 5-Related N-Acetyltransferase2 Activity

Protein N-acetyltransferases (NATs) belong to the general control non-repressible 5 (Gcn5)-related N-acetyltransferases (GNATs) superfamily. GNATs catalyze the transfer of acetyl from acetyl-CoA to the reactive amine moiety of a wide range of acceptors. NAT sequences are difficult to distinguish from other members of the GNAT superfamily and there are many uncharacterized GNATs. To facilitate the discovery and characterization of new GNATs, we have developed a new continuous, non-radioactive assay. This assay is virtually independent of the substrate and can be used to get substrate specificity hints. We validated first the assay with the well-characterized Schizosaccharomyces pombe NatA (SpNatA). The SpNatA kinetic parameters were determined with various peptides confirming the robustness of the new assay. We reveal that the longer the peptide substrate the more efficient the enzyme. As a proof of concept of the relevance of the new assay, we characterized a NAA90 member from rice (Oryza sativa), OsGNAT2. We took advantage of an in vivo medium-scale characterization of OsGNAT2 specificity to identify and then validate in vitro several specific peptide substrates. With this assay, we reveal long-range synergic effects of basic residues on OsGNAT2 activity. Overall, this new, high-throughput assay allows better understanding of the substrate specificity and activity of any GNAT.

Novel Bisubstrate Inhibitors for Protein N-Terminal Acetyltransferase D

Protein N-terminal acetyltransferase D (NatD, NAA40) that specifically acetylates the alpha-N-terminus of histone H4 and H2A has been implicated in various diseases, but no inhibitor has been reported for this important enzyme. Based on the acetyl transfer mechanism of NatD, we designed and prepared a series of highly potent NatD bisubstrate inhibitors by covalently linking coenzyme A to different peptide substrates via an acetyl or propionyl spacer. The most potent bisubstrate inhibitor displayed an apparent Ki value of 1.0 nM. Biochemical studies indicated that bisubstrate inhibitors are competitive to the peptide substrate and noncompetitive to the cofactor, suggesting that NatD undergoes an ordered Bi-Bi mechanism. We also demonstrated that these inhibitors are highly specific toward NatD, displaying about 1000-fold selectivity over other closely related acetyltransferases. High-resolution crystal structures of NatD bound to two of these inhibitors revealed the molecular basis for their selectivity and inhibition mechanism, providing a rational path for future inhibitor development.

Molecular mechanism of N-terminal acetylation by the ternary NatC complex

Protein N-terminal acetylation is predominantly a ribosome-associated modification, with NatA-E serving as the major enzymes. NatC is the most unusual of these enzymes, containing one Naa30 catalytic subunit and two auxiliary subunits, Naa35 and Naa38; and substrate selectivity profile that overlaps with NatE. Here, we report the cryoelectron microscopy structure of S. pombe NatC with a NatE/C-type bisubstrate analog and inositol hexaphosphate (IP₆), and associated biochemistry studies. We find that the presence of three subunits is a prerequisite for normal NatC acetylation activity in yeast and that IP₆ binds tightly to NatC to stabilize the complex. We also describe the molecular basis for IP₆-mediated NatC complex stabilization and the overlapping yet distinct substrate profiles of NatC and NatE.

Evolution-Driven Versatility of N Terminal Acetylation in Photoautotrophs

N terminal protein α-acetylation (NTA) is a pervasive protein modification that has recently attracted renewed interest. Early studies on NTA were mostly conducted in yeast and metazoans, providing a detailed portrait of the modification, which was indirectly applied to all eukaryotes. However, new findings originating from photosynthetic organisms have expanded our knowledge of this modification, revealing strong similarities as well as idiosyncratic features. Here, we review the most recent advances on NTA and its dedicated machinery in photosynthetic organisms. We discuss the cytosolic and unique plastid NTA machineries and their critical biological roles in development, stress responses, protein translocation, and stability. These new findings suggest that the multitasking plastid and cytosolic machineries evolved to support the specific needs of photoautotrophs.

Structural basis of Naa20 activity towards a canonical NatB substrate

N-terminal acetylation is one of the most common protein modifications in eukaryotes and is carried out by N-terminal acetyltransferases (NATs). It plays important roles in protein homeostasis, localization, and interactions and is linked to various human diseases. NatB, one of the major co-translationally active NATs, is composed of the catalytic subunit Naa20 and the auxiliary subunit Naa25, and acetylates about 20% of the proteome. Here we show that NatB substrate specificity and catalytic mechanism are conserved among eukaryotes, and that Naa20 alone is able to acetylate NatB substrates in vitro. We show that Naa25 increases the Naa20 substrate affinity, and identify residues important for peptide binding and acetylation activity. We present the first Naa20 crystal structure in complex with the competitive inhibitor CoA-Ac-MDEL. Our findings demonstrate how Naa20 binds its substrates in the absence of Naa25 and support prospective endeavors to derive specific NAT inhibitors for drug development.

Structural and functional characterization of the N-terminal acetyltransferase Naa50

The majority of eukaryotic proteins is modified by N-terminal acetylation, which plays a fundamental role in protein homeostasis, localization, and complex formation. N-terminal acetyltransferases (NATs) mainly act co-translationally on newly synthesized proteins at the ribosomal tunnel exit. NatA is the major NAT consisting of Naa10 catalytic and Naa15 auxiliary subunits, and with Naa50 forms the NatE complex. Naa50 has recently been identified in Arabidopsis thaliana and is important for plant development and stress response regulation. Here, we determined high-resolution X-ray crystal structures of AtNaa50 in complex with AcCoA and a bisubstrate analog. We characterized its substrate specificity, determined its enzymatic parameters, and identified functionally important residues. Even though Naa50 is conserved among species, we highlight differences between Arabidopsis and yeast, where Naa50 is catalytically inactive but binds CoA conjugates. Our study provides insights into Naa50 conservation, species-specific adaptations, and serves as a basis for further studies of NATs in plants.

Classification and phylogeny for the annotation of novel eukaryotic GNAT acetyltransferases

The enzymes of the GCN5-related N-acetyltransferase (GNAT) superfamily count more than 870 000 members through all kingdoms of life and share the same structural fold. GNAT enzymes transfer an acyl moiety from acyl coenzyme A to a wide range of substrates including aminoglycosides, serotonin, glucosamine-6-phosphate, protein N-termini and lysine residues of histones and other proteins. The GNAT subtype of protein N-terminal acetyltransferases (NATs) alone targets a majority of all eukaryotic proteins stressing the omnipresence of the GNAT enzymes. Despite the highly conserved GNAT fold, sequence similarity is quite low between members of this superfamily even when substrates are similar. Furthermore, this superfamily is phylogenetically not well characterized. Thus functional annotation based on sequence similarity is unreliable and strongly hampered for thousands of GNAT members that remain biochemically uncharacterized. Here we used sequence similarity networks to map the sequence space and propose a new classification for eukaryotic GNAT acetyltransferases. Using the new classification, we built a phylogenetic tree, representing the entire GNAT acetyltransferase superfamily. Our results show that protein NATs have evolved more than once on the GNAT acetylation scaffold. We use our classification to predict the function of uncharacterized sequences and verify by in vitro protein assays that two fungal genes encode NAT enzymes targeting specific protein N-terminal sequences, showing that even slight changes on the GNAT fold can lead to change in substrate specificity. In addition to providing a new map of the relationship between eukaryotic acetyltransferases the classification proposed constitutes a tool to improve functional annotation of GNAT acetyltransferases.

Enzymes of the GCN5-related N-acetyltransferase (GNAT) superfamily transfer an acetyl group from one molecule to another. This reaction is called acetylation and is one of the most common reactions inside the cell. The GNAT superfamily counts more than 870 000 members through all kingdoms of life. Despite sharing the same fold the GNAT superfamily is very diverse in terms of amino acid sequence and substrates. The eight N-terminal acetyltransferases (NatA, NatB, etc.. to NatH) are a GNAT subtype which acetylates the free amine group of polypeptide chains. This modification is called N-terminal acetylation and is one of the most abundant protein modifications in eukaryotic cells. This subtype is also characterized by a high sequence diversity even though they share the same substrate. In addition, the phylogeny of the superfamily is not characterized. This hampers functional annotation based on sequence similarity, and discovery of novel NATs. In this work we set out to solve the problem of the classification of eukaryotic GCN5-related acetyltransferases and report the first classification framework of the superfamily. This framework can be used as a tool for annotation of all GCN5-related acetyltransferases. As an example of what can be achieved we report in this paper the computational prediction and in vitro verification of the function of two previously uncharacterized N-terminal acetyltransferases. We also report the first acetyltransferase phylogenetic tree of the GCN5 superfamily. It indicates that N-terminal acetyltransferases do not constitute one homogeneous protein family, but that the ability to bind and acetylate protein N-termini had evolved more than once on the same acetylation scaffold. We also show that even small changes in key positions can lead to altered enzyme specificity.

Divergent architecture of the heterotrimeric NatC complex explains N-terminal acetylation of cognate substrates

The heterotrimeric NatC complex, comprising the catalytic Naa30 and the two auxiliary subunits Naa35 and Naa38, co-translationally acetylates the N-termini of numerous eukaryotic target proteins. Despite its unique subunit composition, its essential role for many aspects of cellular function and its suggested involvement in disease, structure and mechanism of NatC have remained unknown. Here, we present the crystal structure of the Saccharomyces cerevisiae NatC complex, which exhibits a strikingly different architecture compared to previously described N-terminal acetyltransferase (NAT) complexes. Cofactor and ligand-bound structures reveal how the first four amino acids of cognate substrates are recognized at the Naa30–Naa35 interface. A sequence-specific, ligand-induced conformational change in Naa30 enables efficient acetylation. Based on detailed structure–function studies, we suggest a catalytic mechanism and identify a ribosome-binding patch in an elongated tip region of NatC. Our study reveals how NAT machineries have divergently evolved to N-terminally acetylate specific subsets of target proteins.

The conserved eukaryotic heterotrimeric NatC complex co-translationally acetylates the N-termini of numerous target proteins. Here, the authors provide insights into the catalytic mechanism of NatC by determining the crystal structures of Saccharomyces cerevisiae NatC in the absence and presence of cofactors and peptide substrates and reveal the molecular basis of substrate binding by further biochemical analyses.

Mechanism of aminoacyl-tRNA acetylation by an aminoacyl-tRNA acetyltransferase AtaT from enterohemorrhagic E. coli

Toxin-antitoxin systems in bacteria contribute to stress adaptation, dormancy, and persistence. AtaT, a type-II toxin in enterohemorrhagic E. coli, reportedly acetylates the α-amino group of the aminoacyl-moiety of initiator Met-tRNAf^Met, thus inhibiting translation initiation. Here, we show that AtaT has a broader specificity for aminoacyl-tRNAs than initially claimed. AtaT efficiently acetylates Gly-tRNA^Gly, Trp-tRNA^Trp, Tyr-tRNA^Tyr and Phe-tRNA^Phe isoacceptors, in addition to Met-tRNAf^Met, and inhibits global translation. AtaT interacts with the acceptor stem of tRNAf^Met, and the consecutive G-C pairs in the bottom-half of the acceptor stem are required for acetylation. Consistently, tRNA^Gly, tRNA^Trp, tRNA^Tyr and tRNA^Phe also possess consecutive G-C base-pairs in the bottom halves of their acceptor stems. Furthermore, misaminoacylated valyl-tRNAf^Met and isoleucyl-tRNAf^Met are not acetylated by AtaT. Therefore, the substrate selection by AtaT is governed by the specific acceptor stem sequence and the properties of the aminoacyl-moiety of aminoacyl-tRNAs.

AtaT is a type-II toxin from enterohemorrhagic E. coli, reported to acetylate the aminoacyl-moiety of initiator Met-tRNAf^Met, thus inhibiting translation initiation. Biochemical analysis suggests that AtaT has a broader specificity for aminoacyl-tRNAs and inhibits global translation. Structure of AtaT in complex with acetylated Met-tRNAf^Met offers insight into the substrate selection by the enzyme.

The Arabidopsis Nα -acetyltransferase NAA60 locates to the plasma membrane and is vital for the high salt stress response

In humans and plants, N-terminal acetylation plays a central role in protein homeostasis, affects 80% of proteins in the cytoplasm and is catalyzed by five ribosome-associated N-acetyltransferases (NatA-E). Humans also possess a Golgi-associated NatF (HsNAA60) that is essential for Golgi integrity. Remarkably, NAA60 is absent in fungi and has not been identified in plants. Here we identify and characterize the first plasma membrane-anchored post-translationally acting N-acetyltransferase AtNAA60 in the reference plant Arabidopsis thaliana by the combined application of reverse genetics, global proteomics, live-cell imaging, microscale thermophoresis, circular dichroism spectroscopy, nano-differential scanning fluorometry, intrinsic tryptophan fluorescence and X-ray crystallography. We demonstrate that AtNAA60, like HsNAA60, is membrane-localized in vivo by an α-helical membrane anchor at its C-terminus, but in contrast to HsNAA60, AtNAA60 localizes to the plasma membrane. The AtNAA60 crystal structure provides insights into substrate-binding, the broad substrate specificity and the catalytic mechanism probed by structure-based mutagenesis. Characterization of the NAA60 loss-of-function mutants (naa60-1 and naa60-2) uncovers a plasma membrane-localized substrate of AtNAA60 and the importance of NAA60 during high salt stress. Our findings provide evidence for the plant-specific evolution of a plasma membrane-anchored N-acetyltransferase that is vital for adaptation to stress.

Substrate specificities of Escherichia coli ItaT that acetylates aminoacyl-tRNAs

Escherichia coli ItaT toxin reportedly acetylates the α-amino group of the aminoacyl-moiety of Ile-tRNA^Ile specifically, using acetyl-CoA as an acetyl donor, thereby inhibiting protein synthesis. The mechanism of the substrate specificity of ItaT had remained elusive. Here, we present functional and structural analyses of E. coli ItaT, which revealed the mechanism of ItaT recognition of specific aminoacyl-tRNAs for acetylation. In addition to Ile-tRNA^Ile, aminoacyl-tRNAs charged with hydrophobic residues, such as Val-tRNA^Val and Met-tRNA^Met, were acetylated by ItaT in vivo. Ile-tRNA^Ile, Val-tRNA^Val and Met-tRNA^Met were acetylated by ItaT in vitro, while aminoacyl-tRNAs charged with other hydrophobic residues, such as Ala-tRNA^Ala, Leu-tRNA^Leu and Phe-tRNA^Phe, were less efficiently acetylated. A comparison of the structures of E. coli ItaT and the protein N-terminal acetyltransferase identified the hydrophobic residues in ItaT that possibly interact with the aminoacyl moiety of aminoacyl-tRNAs. Mutations of the hydrophobic residues of ItaT reduced the acetylation activity of ItaT toward Ile-tRNA^Ilein vitro, as well as the ItaT toxicity in vivo. Altogether, the size and shape of the hydrophobic pocket of ItaT are suitable for the accommodation of the specific aminoacyl-moieties of aminoacyl-tRNAs, and ItaT has broader specificity toward aminoacyl-tRNAs charged with certain hydrophobic amino acids.

Protein N-Terminal Acetylation: Structural Basis, Mechanism, Versatility, and Regulation

N-terminal acetylation (NTA) is one of the most widespread protein modifications, which occurs on most eukaryotic proteins, but is significantly less common on bacterial and archaea proteins. This modification is carried out by a family of enzymes called N-terminal acetyltransferases (NATs). To date, 12 NATs have been identified, harboring different composition, substrate specificity, and in some cases, modes of regulation. Recent structural and biochemical analysis of NAT proteins allows for a comparison of their molecular mechanisms and modes of regulation, which are described here. Although sharing an evolutionarily conserved fold and related catalytic mechanism, each catalytic subunit uses unique elements to mediate substrate-specific activity, and use NAT-type specific auxiliary and regulatory subunits, for their cellular functions.

Molecular basis for N-terminal alpha-synuclein acetylation by human NatB

NatB is one of three major N-terminal acetyltransferase (NAT) complexes (NatA-NatC), which co-translationally acetylate the N-termini of eukaryotic proteins. Its substrates account for about 21% of the human proteome, including well known proteins such as actin, tropomyosin, CDK2, and α-synuclein (αSyn). Human NatB (hNatB) mediated N-terminal acetylation of αSyn has been demonstrated to play key roles in the pathogenesis of Parkinson's disease and as a potential therapeutic target for hepatocellular carcinoma. Here we report the cryo-EM structure of hNatB bound to a CoA-αSyn conjugate, together with structure-guided analysis of mutational effects on catalysis. This analysis reveals functionally important differences with human NatA and Candida albicans NatB, resolves key hNatB protein determinants for αSyn N-terminal acetylation, and identifies important residues for substrate-specific recognition and acetylation by NatB enzymes. These studies have implications for developing small molecule NatB probes and for understanding the mode of substrate selection by NAT enzymes.

Dual lysine and N-terminal acetyltransferases reveal the complexity underpinning protein acetylation

Protein acetylation is a highly frequent protein modification. However, comparatively little is known about its enzymatic machinery. N-α-acetylation (NTA) and ε-lysine acetylation (KA) are known to be catalyzed by distinct families of enzymes (NATs and KATs, respectively), although the possibility that the same GCN5-related N-acetyltransferase (GNAT) can perform both functions has been debated. Here, we discovered a new family of plastid-localized GNATs, which possess a dual specificity. All characterized GNAT family members display a number of unique features. Quantitative mass spectrometry analyses revealed that these enzymes exhibit both distinct KA and relaxed NTA specificities. Furthermore, inactivation of GNAT2 leads to significant NTA or KA decreases of several plastid proteins, while proteins of other compartments were unaffected. The data indicate that these enzymes have specific protein targets and likely display partly redundant selectivity, increasing the robustness of the acetylation process in vivo. In summary, this study revealed a new layer of complexity in the machinery controlling this prevalent modification and suggests that other eukaryotic GNATs may also possess these previously underappreciated broader enzymatic activities.

Dynamics-function relationship in the catalytic domains of N-terminal acetyltransferases

N-terminal acetyltransferases (NATs) belong to the superfamily of acetyltransferases. They are enzymes catalysing the transfer of an acetyl group from acetyl coenzyme A to the N-terminus of polypeptide chains. N-terminal acetylation is one of the most common protein modifications. To date, not much is known on the molecular basis for the exclusive substrate specificity of NATs. All NATs share a common fold called GNAT. A characteristic of NATs is the β6β7 hairpin loop covering the active site and forming with the α1α2 loop a narrow tunnel surrounding the catalytic site in which cofactor and polypeptide meet and exchange an acetyl group. We investigated the dynamics-function relationships of all available structures of NATs covering the three domains of Life. Using an elastic network model and normal mode analysis, we found a common dynamics pattern conserved through the GNAT fold; a rigid V-shaped groove formed by the β4 and β5 strands and splitting the fold in two dynamical subdomains. Loops α1α2, β3β4 and β6β7 all show clear displacements in the low frequency normal modes. We characterized the mobility of the loops and show that even limited conformational changes of the loops along the low-frequency modes are able to significantly change the size and shape of the ligand binding sites. Based on the fact that these movements are present in most low-frequency modes, and common to all NATs, we suggest that the α1α2 and β6β7 loops may regulate ligand uptake and the release of the acetylated polypeptide.

Molecular basis for N-terminal acetylation by human NatE and its modulation by HYPK

The human N-terminal acetyltransferase E (NatE) contains NAA10 and NAA50 catalytic, and NAA15 auxiliary subunits and associates with HYPK, a protein with intrinsic NAA10 inhibitory activity. NatE co-translationally acetylates the N-terminus of half the proteome to mediate diverse biological processes, including protein half-life, localization, and interaction. The molecular basis for how NatE and HYPK cooperate is unknown. Here, we report the cryo-EM structures of human NatE and NatE/HYPK complexes and associated biochemistry. We reveal that NAA50 and HYPK exhibit negative cooperative binding to NAA15 in vitro and in human cells by inducing NAA15 shifts in opposing directions. NAA50 and HYPK each contribute to NAA10 activity inhibition through structural alteration of the NAA10 substrate-binding site. NAA50 activity is increased through NAA15 tethering, but is inhibited by HYPK through structural alteration of the NatE substrate-binding site. These studies reveal the molecular basis for coordinated N-terminal acetylation by NatE and HYPK.

Biochemical and structural analysis of N-terminal acetyltransferases

N-terminal acetylation is a co- and post-translational modification catalyzed by the conserved N-terminal acetyltransferase (NAT) family of enzymes. A majority of the human proteome is modified by the human NATs (NatA-F and H), which are minimally composed of a catalytic subunit and as many as two auxiliary subunits. Together, NATs specifically regulate many cellular functions by influencing protein activities such as their degradation, membrane targeting, and protein-protein interactions. This chapter will describe methods developed for their preparation, and their biochemical and structural characterization. This will include methodologies for expression and purification of recombinant NAT protein, kinetic assays, biochemical and biophysical assays, and strategies for structural studies.

Structure and Mechanism of Acetylation by the N-Terminal Dual Enzyme NatA/Naa50 Complex

NatA co-translationally acetylates the N termini of over 40% of eukaryotic proteins and can associate with another catalytic subunit, Naa50, to form a ternary NatA/Naa50 dual enzyme complex (also called NatE). The molecular basis of association between Naa50 and NatA and the mechanism for how their association affects their catalytic activities in yeast and human are poorly understood. Here, we determined the X-ray crystal structure of yeast NatA/Naa50 as a scaffold to understand coregulation of NatA/Naa50 activity in both yeast and human. We find that Naa50 makes evolutionarily conserved contacts to both the Naa10 and Naa15 subunits of NatA. These interactions promote catalytic crosstalk within the human complex, but do so to a lesser extent in the yeast complex, where Naa50 activity is compromised. These studies have implications for understanding the role of the NatA/Naa50 complex in modulating the majority of the N-terminal acetylome in diverse species.

A novel NAA10 p.(R83H) variant with impaired acetyltransferase activity identified in two boys with ID and microcephaly

Background

N-terminal acetylation is a common protein modification in human cells and is catalysed by N-terminal acetyltransferases (NATs), mostly cotranslationally. The NAA10-NAA15 (NatA) protein complex is the major NAT, responsible for acetylating ~ 40% of human proteins. Recently, NAA10 germline variants were found in patients with the X-linked lethal Ogden syndrome, and in other familial or de novo cases with variable degrees of developmental delay, intellectual disability (ID) and cardiac anomalies.

Methods

Here we report a novel NAA10 (NM_003491.3) c.248G > A, p.(R83H) missense variant in NAA10 which was detected by whole exome sequencing in two unrelated boys with intellectual disability, developmental delay, ADHD like behaviour, very limited speech and cardiac abnormalities. We employ in vitro acetylation assays to functionally test the impact of this variant on NAA10 enzyme activity.

Results

Functional characterization of NAA10-R83H by in vitro acetylation assays revealed a reduced enzymatic activity of monomeric NAA10-R83H. This variant is modelled to have an altered charge density in the acetyl-coenzyme A (Ac-CoA) binding region of NAA10.

Conclusions

We show that NAA10-R83H has a reduced monomeric catalytic activity, likely due to impaired enzyme-Ac-CoA binding. Our data support a model where reduced NAA10 and/or NatA activity cause the phenotypes observed in the two patients.

Electronic supplementary material

The online version of this article (10.1186/s12881-019-0803-1) contains supplementary material, which is available to authorized users.

Spotlight on protein N-terminal acetylation

N-terminal acetylation (Nt-acetylation) is a widespread protein modification among eukaryotes and prokaryotes alike. By appending an acetyl group to the N-terminal amino group, the charge, hydrophobicity, and size of the N-terminus is altered in an irreversible manner. This alteration has implications for the lifespan, folding characteristics and binding properties of the acetylated protein. The enzymatic machinery responsible for Nt-acetylation has been largely described, but significant knowledge gaps remain. In this review, we provide an overview of eukaryotic N-terminal acetyltransferases (NATs) and the impact of Nt-acetylation. We also discuss other functions of known NATs and outline methods for studying Nt-acetylation.

A novel NAA10 variant with impaired acetyltransferase activity causes developmental delay, intellectual disability, and hypertrophic cardiomyopathy

The NAA10-NAA15 complex (NatA) is an N-terminal acetyltransferase that catalyzes N-terminal acetylation of ~40% of all human proteins. N-terminal acetylation has several different roles in the cell, including altering protein stability and degradation, protein localization and protein-protein interactions. In recent years several X-linked NAA10 variants have been associated with genetic disorders. We have identified a previously undescribed NAA10 c.215T>C p.(Ile72Thr) variant in three boys from two unrelated families with a milder phenotypic spectrum in comparison to most of the previously described patients with NAA10 variants. These boys have development delay, intellectual disability, and cardiac abnormalities as overlapping phenotypes. Functional studies reveal that NAA10 Ile72Thr is destabilized, while binding to NAA15 most likely is intact. Surprisingly, the NatA activity of NAA10 Ile72Thr appears normal while its monomeric activity is decreased. This study further broadens the phenotypic spectrum associated with NAA10 deficiency, and adds to the evidence that genotype-phenotype correlations for NAA10 variants are much more complex than initially anticipated.

Structure of Human NatA and Its Regulation by the Huntingtin Interacting Protein HYPK

Co-translational N-terminal protein acetylation regulates many protein functions including degradation, folding, interprotein interactions, and targeting. Human NatA (hNatA), one of six conserved metazoan N-terminal acetyltransferases, contains Naa10 catalytic and Naa15 auxiliary subunits, and associates with the intrinsically disordered Huntingtin yeast two-hybrid protein K (HYPK). We report on the crystal structures of hNatA and hNatA/HYPK, and associated biochemical and enzymatic analyses. We demonstrate that hNatA contains unique features: a stabilizing inositol hexaphosphate (IP₆) molecule and a metazoan-specific Naa15 domain that mediates high-affinity HYPK binding. We find that HYPK harbors intrinsic hNatA-specific inhibitory activity through a bipartite structure: a ubiquitin-associated domain that binds a hNaa15 metazoan-specific region and an N-terminal loop-helix region that distorts the hNaa10 active site. We show that HYPK binding blocks hNaa50 targeting to hNatA, likely limiting Naa50 ribosome localization in vivo. These studies provide a model for metazoan NAT activity and HYPK regulation of N-terminal acetylation.

Structural determinants and cellular environment define processed actin as the sole substrate of the N-terminal acetyltransferase NAA80

N-terminal acetylation performed by N-terminal acetyltransferases (NATs) is a common protein modification in human cells. A unique NAT, NAA80, was recently found to control actin N-terminal acetylation and cytoskeletal dynamics. In this study, we developed potent and specific bisubstrate inhibitors against NAA80 and determined the crystal structure of NAA80 in complex with an inhibitor mimicking the β-actin N terminus, thus revealing molecular determinants for the substrate specificity and selective inhibition of NAA80. A yeast model uncovered how a cellular determinant, the NatB enzyme, acts to restrict the number of in vivo NAA80 substrates relative to the broader intrinsic capacity of NAA80. Our data provide a starting point for further development of inhibitors for the regulation of actin and cytoskeletal functions.

N-terminal (Nt) acetylation is a major protein modification catalyzed by N-terminal acetyltransferases (NATs). Methionine acidic N termini, including actin, are cotranslationally Nt acetylated by NatB in all eukaryotes, but animal actins containing acidic N termini, are additionally posttranslationally Nt acetylated by NAA80. Actin Nt acetylation was found to regulate cytoskeletal dynamics and motility, thus making NAA80 a potential target for cell migration regulation. In this work, we developed potent and selective bisubstrate inhibitors for NAA80 and determined the crystal structure of NAA80 in complex with such an inhibitor, revealing that NAA80 adopts a fold similar to other NAT enzymes but with a more open substrate binding region. Furthermore, in contrast to most other NATs, the substrate specificity of NAA80 is mainly derived through interactions between the enzyme and the acidic amino acids at positions 2 and 3 of the actin substrate and not residues 1 and 2. A yeast model revealed that ectopic expression of NAA80 in a strain lacking NatB activity partially restored Nt acetylation of NatB substrates, including yeast actin. Thus, NAA80 holds intrinsic capacity to posttranslationally Nt acetylate NatB-type substrates in vivo. In sum, the presence of a dominant cotranslational NatB in all eukaryotes, the specific posttranslational actin methionine removal in animals, and finally, the unique structural features of NAA80 leave only the processed actins as in vivo substrates of NAA80. Together, this study reveals the molecular and cellular basis of NAA80 Nt acetylation and provides a scaffold for development of inhibitors for the regulation of cytoskeletal properties.

NAA10 dysfunction with normal NatA-complex activity in a girl with non-syndromic ID and a de novo NAA10 p.(V111G) variant - a case report

Background

The NAA10-NAA15 (NatA) protein complex is an N-terminal acetyltransferase responsible for acetylating ~ 40% of eukaryotic proteins. In recent years, NAA10 variants have been found in patients with an X-linked developmental disorder called Ogden syndrome in its most severe form and, in other familial or de novo cases, with variable degrees of syndromic intellectual disability (ID) affecting both sexes.

Case presentation

Here we report and functionally characterize a novel and de novo NAA10 (NM_003491.3) c.332 T > G p.(V111G) missense variant, that was detected by trio-based whole exome sequencing in an 11 year old girl with mild/moderate non-syndromic intellectual disability. She had delayed motor and language development, but normal behavior without autistic traits. Her blood leukocyte X-inactivation pattern was within normal range (80/20). Functional characterization of NAA10-V111G by cycloheximide chase experiments suggests that NAA10-V111G has a reduced stability compared to NAA10-WT, and in vitro acetylation assays revealed a reduced enzymatic activity of monomeric NAA10-V111G but not for NAA10-V111G in complex with NAA15 (NatA enzymatic activity).

Conclusions

We show that NAA10-V111G has a reduced stability and monomeric catalytic activity, while NatA function remains unaltered. This is the first example of isolated NAA10 dysfunction in a case of ID, suggesting that the syndromic cases may also require a degree of compromised NatA function.

Identification of an alternatively spliced nuclear isoform of human N-terminal acetyltransferase Naa30

N-terminal acetylation is a highly abundant and important protein modification in eukaryotes catalyzed by N-terminal acetyltransferases (NATs). In humans, six different NATs have been identified (NatA-NatF), each composed of individual subunits and acetylating a distinct set of substrates. Along with most NATs, NatC acts co-translationally at the ribosome. The NatC complex consists of the catalytic subunit Naa30 and the auxiliary subunits Naa35 and Naa38, and can potentially Nt-acetylate cytoplasmic proteins when the initiator methionine is followed by a bulky hydrophobic/amphipathic residue at position 2. Here, we have identified a splice variant of human NAA30, which encodes a truncated protein named Naa30₂₈₈. The splice variant was abundantly present in thyroid cancer tissues and in several different human cancer cell lines. Surprisingly, Naa30₂₈₈ localized predominantly to the nucleus, as opposed to annotated Naa30 which has a cytoplasmic localization. Full-length Naa30 acetylated a classical NatC substrate peptide in vitro, whereas no significant NAT activity was detected for Naa30_288. Due to the nuclear localization, we also examined acetyltransferase activity towards lysine residues. Neither full-length Naa30 nor Naa30₂₈₈ displayed any lysine acetyltransferase activity. Overexpression of full-length Naa30 increased cell viability via inhibition of apoptosis. In contrast, Naa30₂₈₈ did not exert an anti-apoptotic effect. In sum, we identified a novel and widely expressed Naa30 isoform with a potential non-catalytic role in the nucleus.

Molecular Basis of Substrate Specific Acetylation by N-Terminal Acetyltransferase NatB

The NatB N-terminal acetyltransferase specifically acetylates the N-terminal group of substrate protein peptides starting with Met-Asp/Glu/Asn/Gln. How NatB recognizes and acetylates these substrates remains unknown. Here, we report crystal structures of a NatB holoenzyme from Candida albicans in the presence of its co-factor CoA and substrate peptides. The auxiliary subunit Naa25 of NatB forms a horseshoe-like deck to hold specifically its catalytic subunit Naa20. The first two amino acids Met and Asp of a substrate peptide mediate the major interactions with the active site in the Naa20 subunit. The hydrogen bonds between the substrate Asp and pocket residues of Naa20 are essential to determine the NatB substrate specificity. Moreover, a hydrogen bond between the amino group of the substrate Met and a carbonyl group in the Naa20 active site directly anchors the substrate toward acetyl-CoA. Together, these structures define a unique molecular mechanism of specific N-terminal acetylation acted by NatB.

Probing the interaction between NatA and the ribosome for co-translational protein acetylation

N-terminal acetylation is among the most abundant protein modifications in eukaryotic cells. Over the last decade, significant progress has been made in elucidating the function of N-terminal acetylation for a number of diverse systems, involved in a wide variety of biological processes. The enzymes responsible for the modification are the N-terminal acetyltransferases (NATs). The NATs are a highly conserved group of enzymes in eukaryotes, which are responsible for acetylating over 80% of the soluble proteome in human cells. Importantly, many of these NATs act co-translationally; they interact with the ribosome near the exit tunnel and acetylate the nascent protein chain as it is being translated. While the structures of many of the NATs have been determined, the molecular basis for the interaction with ribosome is not known. Here, using purified ribosomes and NatA, a very well-studied NAT, we show that NatA forms a stable complex with the ribosome in the absence of other stabilizing factors and through two conserved regions; primarily through an N-terminal domain and an internal basic helix. These regions may orient the active site of the NatA to face the peptide emerging from the exit tunnel. This work provides a framework for understanding how NatA and potentially other NATs interact with the ribosome for co-translational protein acetylation and sets the foundation for future studies to decouple N-terminal acetyltransferase activity from ribosome association.

Blocking an N-terminal acetylation-dependent protein interaction inhibits an E3 ligase

N-terminal acetylation is an abundant modification influencing protein functions. Since ≈80% of mammalian cytosolic proteins are N-terminally acetylated, this potentially represents an untapped target for chemical control of their functions. Structural studies have revealed that, like lysine acetylation, N-terminal acetylation converts a positively charged amine into a hydrophobic handle that mediates protein interactions, suggesting it may be a druggable target. We report the development of chemical probes targeting the N-terminal acetylation-dependent interaction between an E2 conjugating enzyme (UBE2M, aka UBC12) and DCN1 (aka DCUN1D1), a subunit of a multiprotein E3 ligase for the ubiquitin-like protein NEDD8. The inhibitors are highly selective with respect to other protein acetyl amide binding sites, inhibit NEDD8 ligation in vitro and in cells, and suppress the anchorage-independent growth of a cell line harboring DCN1 amplification. Overall, the data demonstrate that N-terminal acetyl-dependent protein interactions are druggable targets, and provide insights into targeting multiprotein E2–E3 ligases.

Molecular determinants of the N-terminal acetyltransferase Naa60 anchoring to the Golgi membrane

Nα-Acetyltransferase 60 (Naa60 or NatF) was recently identified as an unconventional N-terminal acetyltransferase (NAT) because it localizes to organelles, in particular the Golgi apparatus, and has a preference for acetylating N termini of the transmembrane proteins. This knowledge challenged the prevailing view of N-terminal acetylation as a co-translational ribosome-associated process and suggested a new mechanistic functioning for the enzymes responsible for this increasingly recognized protein modification. Crystallography studies on Naa60 were unable to resolve the C-terminal tail of Naa60, which is responsible for the organellar localization. Here, we combined modeling, in vitro assays, and cellular localization studies to investigate the secondary structure and membrane interacting capacity of Naa60. The results show that Naa60 is a peripheral membrane protein. Two amphipathic helices within the Naa60 C terminus bind the membrane directly in a parallel position relative to the lipid bilayer via hydrophobic and electrostatic interactions. A peptide corresponding to the C terminus was unstructured in solution and only folded into an α-helical conformation in the presence of liposomes. Computational modeling and cellular mutational analysis revealed the hydrophobic face of two α-helices to be critical for membranous localization. Furthermore, we found a strong and specific binding preference of Naa60 toward membranes containing the phosphatidylinositol PI(4)P, thus possibly explaining the primary residency of Naa60 at the PI(4)P-rich Golgi. In conclusion, we have defined the mode of cytosolic Naa60 anchoring to the Golgi apparatus, most likely occurring post-translationally and specifically facilitating post-translational N-terminal acetylation of many transmembrane proteins.

Microscopy-based Saccharomyces cerevisiae complementation model reveals functional conservation and redundancy of N-terminal acetyltransferases

N-terminal acetylation is a highly abundant protein modification catalyzed by N-terminal acetyltransferases (NATs) NatA-NatG. The Saccharomyces cerevisiae protein Arl3 depends on interaction with Sys1 for its localization to the Golgi and this targeting strictly requires NatC-mediated N-terminal acetylation of Arl3. We utilized the Arl3 acetylation-dependent localization phenotype as a model system for assessing the functional conservation and in vivo redundancy of several human NATs. The catalytic subunit of human NatC, hNaa30 (Mak3), restored Arl3 localization in the absence of yNaa30, but only in the presence of either yeast or human Naa35 subunit (Mak10). In contrast, hNaa35 was not able to replace its yeast orthologue without the co-expression of hNaa30, suggesting co-evolution of the two NatC subunits. The most recently discovered and organellar human NAT, NatF/Naa60, restored the Golgi localization of Arl3 in the absence of yNaa30. Interestingly, this was also true for hNaa60 lacking its membrane-binding domain whereas hNaa50 did not complement NatC function. This in vivo redundancy reflects NatC and NatF´s overlapping in vitro substrate specificities. The yeast model presented here provides a robust and rapid readout of NatC and NatF activity in vivo, and revealed evolutionary conservation of the NatC complex and redundancy between NatC and NatF.

First Things First: Vital Protein Marks by N-Terminal Acetyltransferases

N-terminal (Nt) acetylation is known to be a highly abundant co-translational protein modification, but the recent discovery of Golgi- and chloroplast-resident N-terminal acetyltransferases (NATs) revealed that it can also be added post-translationally. Nt-acetylation may act as a degradation signal in a novel branch of the N-end rule pathway, whose functions include the regulation of human blood pressure. Nt-acetylation also modulates protein interactions, targeting, and folding. In plants, Nt-acetylation plays a role in the control of resistance to drought and in regulation of immune responses. Mutations of specific human NATs that decrease their activity can cause either the lethal Ogden syndrome or severe intellectual disability and cardiovascular defects. In sum, recent advances highlight Nt-acetylation as a key factor in many biological pathways.