Actin finally matures: uncovering machinery and impact.

Actin, one of the most abundant proteins in nature and a key component of the cytoskeleton, undergoes a unique multistep N-terminal (Nt) maturation. In a recent report, Haahr et al. identified actin maturation protease (ACTMAP) as the dedicated actin aminopeptidase and showed that its absence is associated with abnormal muscle physiology.

Protein Termini 2022: central roles of protein ends.

Although locating at the protein ends, N- and C-termini are at the center of numerous cellular functions. This topic engages an increasing number of scientists, recently forming the International Society of Protein Termini (ISPT). Protein Termini 2022 gathered this interdisciplinary community to discuss how protein ends may steer protein functionality.

Co-translational, Post-translational, and Non-catalytic Roles of N-Terminal Acetyltransferases.

Recent studies of N-terminal acetylation have identified new N-terminal acetyltransferases (NATs) and expanded the known functions of these enzymes beyond their roles as ribosome-associated co-translational modifiers. For instance, the identification of Golgi- and chloroplast-associated NATs shows that acetylation of N termini also happens post-translationally. In addition, we now appreciate that some NATs are highly specific; for example, a dedicated NAT responsible for post-translational N-terminal acetylation of actin was recently revealed. Other studies have extended NAT function beyond Nt acetylation, including functions as lysine acetyltransferases (KATs) and non-catalytic roles. Finally, emerging studies emphasize the physiological relevance of N-terminal acetylation, including roles in calorie-restriction-induced longevity and pathological α-synuclein aggregation in Parkinson's disease. Combined, the NATs rise as multifunctional proteins, and N-terminal acetylation is gaining recognition as a major cellular regulator.

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.

Expanded in vivo substrate profile of the yeast N-terminal acetyltransferase NatC.

N-terminal acetylation is a conserved protein modification among eukaryotes. The yeast Saccharomyces cerevisiae is a valuable model system for studying this modification. The bulk of protein N-terminal acetylation in S. cerevisiae is catalyzed by the N-terminal acetyltransferases NatA, NatB, and NatC. Thus far, proteome-wide identification of the in vivo protein substrates of yeast NatA and NatB has been performed by N-terminomics. Here, we used S. cerevisiae deleted for the NatC catalytic subunit Naa30 and identified 57 yeast NatC substrates by N-terminal combined fractional diagonal chromatography analysis. Interestingly, in addition to the canonical N-termini starting with ML, MI, MF, and MW, yeast NatC substrates also included MY, MK, MM, MA, MV, and MS. However, for some of these substrate types, such as MY, MK, MV, and MS, we also uncovered (residual) non-NatC NAT activity, most likely due to the previously established redundancy between yeast NatC and NatE/Naa50. Thus, we have revealed a complex interplay between different NATs in targeting methionine-starting N-termini in yeast. Furthermore, our results showed that ectopic expression of human NAA30 rescued known NatC phenotypes in naa30Δ yeast, as well as partially restored the yeast NatC Nt-acetylome. Thus, we demonstrate an evolutionary conservation of NatC from yeast to human thereby underpinning future disease models to study pathogenic NAA30 variants. Overall, this work offers increased biochemical and functional insights into NatC-mediated N-terminal acetylation and provides a basis for future work to pinpoint the specific molecular mechanisms that link the lack of NatC-mediated N-terminal acetylation to phenotypes of NatC deletion yeast.

Missense NAA20 variants impairing the NatB protein N-terminal acetyltransferase cause autosomal recessive developmental delay, intellectual disability, and microcephaly.

PURPOSE: N-terminal acetyltransferases modify proteins by adding an acetyl moiety to the first amino acid and are vital for protein and cell function. The NatB complex acetylates 20% of the human proteome and is composed of the catalytic subunit NAA20 and the auxiliary subunit NAA25. In five individuals with overlapping phenotypes, we identified recessive homozygous missense variants in NAA20. METHODS: Two different NAA20 variants were identified in affected individuals in two consanguineous families by exome and genome sequencing. Biochemical studies were employed to assess the impact of the NAA20 variants on NatB complex formation and catalytic activity. RESULTS: Two homozygous variants, NAA20 p.Met54Val and p.Ala80Val (GenBank: NM_016100.4, c.160A>G and c.239C>T), segregated with affected individuals in two unrelated families presenting with developmental delay, intellectual disability, and microcephaly. Both NAA20-M54V and NAA20-A80V were impaired in their capacity to form a NatB complex with NAA25, and in vitro acetylation assays revealed reduced catalytic activities toward different NatB substrates. Thus, both NAA20 variants are impaired in their ability to perform cellular NatB-mediated N-terminal acetylation. CONCLUSION: We present here a report of pathogenic NAA20 variants causing human disease and data supporting an essential role for NatB-mediated N-terminal acetylation in human development and physiology.

N-terminal acetylation of actin by NAA80 is essential for structural integrity of the Golgi apparatus.

N-alpha-acetyltransferase 80 (NAA80) was recently demonstrated to acetylate the N-terminus of actin, with NAA80 knockout cells showing actin cytoskeleton-related phenotypes, such as increased formation of membrane protrusions and accelerated migration. Here we report that NAA80 knockout cells additionally display fragmentation of the Golgi apparatus. We further employed rescue assays to demonstrate that this phenotype is connected to the ability of NAA80 to modify actin. Thus, re-expression of NAA80, which leads to re-establishment of actin's N-terminal acetyl group, rescued the Golgi fragmentation, whereas a catalytic dead NAA80 mutant could neither restore actin Nt-acetylation nor Golgi structure. The Golgi phenotype of NAA80 KO cells was shared by both migrating and non-migrating cells and live-cell imaging indicated increased Golgi dynamics in migrating NAA80 KO cells. Finally, we detected a drastic increase in the amount of F-actin in cells lacking NAA80, suggesting a causal relationship between this effect and the observed re-organization of Golgi structure. The findings further underscore the importance of actin Nt-acetylation and provide novel insight into its cellular roles, suggesting a mechanistic link between actin modification state and Golgi organization.

NAA80 is actin's N-terminal acetyltransferase and regulates cytoskeleton assembly and cell motility.

Actin, one of the most abundant proteins in nature, participates in countless cellular functions ranging from organelle trafficking and pathogen motility to cell migration and regulation of gene transcription. Actin's cellular activities depend on the dynamic transition between its monomeric and filamentous forms, a process exquisitely regulated in cells by a large number of actin-binding and signaling proteins. Additionally, several posttranslational modifications control the cellular functions of actin, including most notably N-terminal (Nt)-acetylation, a prevalent modification throughout the animal kingdom. However, the biological role and mechanism of actin Nt-acetylation are poorly understood, and the identity of actin's N-terminal acetyltransferase (NAT) has remained a mystery. Here, we reveal that NAA80, a suggested NAT enzyme whose substrate specificity had not been characterized, is Nt-acetylating actin. We further show that actin Nt-acetylation plays crucial roles in cytoskeletal assembly in vitro and in cells. The absence of Nt-acetylation leads to significant differences in the rates of actin filament depolymerization and elongation, including elongation driven by formins, whereas filament nucleation by the Arp2/3 complex is mostly unaffected. NAA80-knockout cells display severely altered cytoskeletal organization, including an increase in the ratio of filamentous to globular actin, increased filopodia and lamellipodia formation, and accelerated cell motility. Together, the results demonstrate NAA80's role as actin's NAT and reveal a crucial role for actin Nt-acetylation in the control of cytoskeleton structure and dynamics.

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.

N-terminal Acetylation Levels Are Maintained During Acetyl-CoA Deficiency in Saccharomyces cerevisiae.

N-terminal acetylation (Nt-acetylation) is a highly abundant protein modification in eukaryotes and impacts a wide range of cellular processes, including protein quality control and stress tolerance. Despite its prevalence, the mechanisms regulating Nt-acetylation are still nebulous. Here, we present the first global study of Nt-acetylation in yeast cells as they progress to stationary phase in response to nutrient starvation. Surprisingly, we found that yeast cells maintain their global Nt-acetylation levels upon nutrient depletion, despite a marked decrease in acetyl-CoA levels. We further observed two distinct sets of protein N termini that display differential and opposing Nt-acetylation behavior upon nutrient starvation, indicating a dynamic process. The first protein cluster was enriched for annotated N termini showing increased Nt-acetylation in stationary phase compared with exponential growth phase. The second protein cluster was conversely enriched for alternative nonannotated N termini (i.e. N termini indicative of shorter N-terminal proteoforms) and, like histones, showed reduced acetylation levels in stationary phase when acetyl-CoA levels were low. Notably, the degree of Nt-acetylation of Pcl8, a negative regulator of glycogen biosynthesis and two components of the pre-ribosome complex (Rsa3 and Rpl7a) increased during starvation. Moreover, the steady-state levels of these proteins were regulated both by starvation and NatA activity. In summary, this study represents the first comprehensive analysis of metabolic regulation of Nt-acetylation and reveals that specific, rather than global, Nt-acetylation events are subject to metabolic regulation.

(Hyper)tension release by N-terminal acetylation.

A recent study links N-terminal acetylation and N-end rule degradation to blood pressure regulation. N-terminal mutants of Rgs2, a key G-protein regulator, are differentially processed by N-terminal acetyltransferases and the two branches of the N-end rule pathway. This leads to an imbalance in the signaling governing blood pressure.

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.

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.

Loss of N-terminal acetyltransferase A activity induces thermally unstable ribosomal proteins and increases their turnover in Saccharomyces cerevisiae.

Protein N-terminal (Nt) acetylation is one of the most abundant modifications in eukaryotes, covering ~50-80 % of the proteome, depending on species. Cells with defective Nt-acetylation display a wide array of phenotypes such as impaired growth, mating defects and increased stress sensitivity. However, the pleiotropic nature of these effects has hampered our understanding of the functional impact of protein Nt-acetylation. The main enzyme responsible for Nt-acetylation throughout the eukaryotic kingdom is the N-terminal acetyltransferase NatA. Here we employ a multi-dimensional proteomics approach to analyze Saccharomyces cerevisiae lacking NatA activity, which causes global proteome remodeling. Pulsed-SILAC experiments reveals that NatA-deficient strains consistently increase degradation of ribosomal proteins compared to wild type. Explaining this phenomenon, thermal proteome profiling uncovers decreased thermostability of ribosomes in NatA-knockouts. Our data are in agreement with a role for Nt-acetylation in promoting stability for parts of the proteome by enhancing the avidity of protein-protein interactions and folding.

Exploiting the potential of commercial digital holographic microscopy by combining it with 3D matrix cell culture assays.

3D cell culture assays are becoming increasingly popular due to their higher resemblance to tissue environment. These provide an increased complexity compared to the growth on 2D surface and therefore allow studies of advanced cellular properties such as invasion. We report here on the use of 3D Matrigel cell preparations combined with a particular gentle and informative type of live-cell microscopy: quantitative digital holographic microscopy (DHM), here performed by a commercial software-integrated system, currently mostly used for 2D cell culture preparations. By demonstrating this compatibility, we highlight the possible time-efficient quantitative analysis obtained by using a commercial software-integrated DHM system, also for cells in a more advanced 3D culture environment. Further, we demonstrate two very different examples making use of this advantage by performing quantitative DHM analysis of: (1) wound closure cell monolayer Matrigel invasion assay and (2) Matrigel-trapped single and clumps of suspension cells. For both these, we benefited from the autofocus functionality of digital phase holographic imaging to obtain 3D information for cells migrating in a 3D environment. For the latter, we demonstrate that it is possible to quantitatively measure tumourigenic properties like growth of cell clump (or spheroid) over time, as well as single-cell invasion out of cell clump and into the surrounding extracellular matrix. Overall, our findings highlight several possibilities for 3D digital holographic microscopy applications combined with 3D cell preparations, therein studies of drug response or genetic alterations on invasion capacity as well as on tumour growth and metastasis.

Efficient and crucial quality control of HAP1 cell ploidy status.

The near-haploid human cell line HAP1 recently became a popular subject for CRISPR/Cas9 editing, since only one allele requires modification. Through the gene-editing service at Horizon Discovery, there are at present more than 7500 edited cell lines available and the number continuously increases. The haploid nature of HAP1 is unstable as cultures become diploid with time. Here, we demonstrated some fundamental differences between haploid and diploid HAP1 cells, hence underlining the need for taking control over ploidy status in HAP1 cultures prior to phenotyping. Consequently, we optimized a procedure to determine the ploidy of HAP1 by flow cytometry in order to obtain diploid cultures and avoid ploidy status as an interfering variable in experiments. Furthermore, in order to facilitate this quality control, we validated a size-based cell sorting procedure to obtain the diploid culture more rapidly. Hence, we provide here two streamlined protocols for quality controlling the ploidy of HAP1 cells and document their validity and necessity.This article has an associated First Person interview with the co-first authors of the paper.

Actin polymerization and cell motility are affected by NAA80-mediated posttranslational N-terminal acetylation of actin.

Actin is the most abundant protein in our cells, and also one of the most studied. Nevertheless, an important modifier of actin, the N-terminal acetyltransferase (NAT) for actin, remained unknown until now. The recent identification of the enzyme that catalyzes actin acetylation, has opened up for functional studies of unacetylated actin using knockout cells. This enzyme, called NAA80 (Nα-acetyltransferase 80) or NatH, belongs to the NAT family of enzymes, which together provides N-terminal acetylation for around 80 % of the human proteome. In many cases, N-terminal acetylation is essential. In the case of actin, the acetyl group that NAA80 attaches to actin plays an important role in actin's polymerization properties as well as in actin's function in cell migration.

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.

Molecular, cellular, and physiological significance of N-terminal acetylation.

Protein N-terminal acetylation is catalyzed by N-terminal acetyltransferases and represents one of the most common protein modifications in eukaryotes. An increasing number of studies report on the importance of N-terminal acetylation for protein degradation, complex formation, subcellular targeting, and protein folding. N-terminal acetyltransferases are recognized to play important roles in a diversity of cellular processes like apoptosis, cell proliferation, sister chromatid cohesion, and chromatin silencing and are even linked to the development of rare genetic disorders and cancer. This article summarizes our current knowledge on the implications of N-terminal acetylation at the protein, cellular, and physiological levels.