Functional evaluation of 16 SCHAD missense variants: Only amino acid substitutions causing congenital hyperinsulinism of infancy lead to loss-of-function phenotypes in vitro.

Short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), encoded by the HADH gene, is a ubiquitously expressed mitochondrial enzyme involved in fatty acid oxidation. This protein also plays a role in insulin secretion as recessive HADH mutations cause congenital hyperinsulinism of infancy (CHI) via loss of an inhibitory interaction with glutamate dehydrogenase (GDH). Here, we present a functional evaluation of 16 SCHAD missense variants identified either in CHI patients or by high-throughput sequencing projects in various populations. To avoid interactions with endogenously produced SCHAD protein, we assessed protein stability, subcellular localization, and GDH interaction in a SCHAD knockout HEK293 cell line constructed by CRISPR-Cas9 methodology. We also established methods for efficient SCHAD expression and purification in E. coli, and tested enzymatic activity of the variants. Our analyses showed that rare variants of unknown significance identified in populations generally had similar properties as normal SCHAD. However, the CHI-associated variants p.Gly34Arg, p.Ile184Phe, p.Pro258Leu, and p.Gly303Ser were unstable with low protein levels detectable when expressed in HEK293 cells. Moreover, CHI variants p.Lys136Glu, p.His170Arg, and p.Met188Val presented normal protein levels but displayed clearly impaired enzymatic activity in vitro, and their interaction with GDH appeared reduced. Our results suggest that pathogenic missense variants of SCHAD either make the protein target of a post-translational quality control system or can impair the function of SCHAD without influencing its steady-state protein level. We did not find any evidence that rare SCHAD missense variants observed only in the general population and not in CHI patients are functionally affected.

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

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.

A Role for Human N-alpha Acetyltransferase 30 (Naa30) in Maintaining Mitochondrial Integrity.

N-terminal acetylation (Nt-acetylation) by N-terminal acetyltransferases (NATs) is one of the most common protein modifications in eukaryotes. The NatC complex represents one of three major NATs of which the substrate profile remains largely unexplored. Here, we defined the in vivo human NatC Nt-acetylome on a proteome-wide scale by combining knockdown of its catalytic subunit Naa30 with positional proteomics. We identified 46 human NatC substrates, expanding our current knowledge on the substrate repertoire of NatC which now includes proteins harboring Met-Leu, Met-Ile, Met-Phe, Met-Trp, Met-Val, Met-Met, Met-His and Met-Lys N termini. Upon Naa30 depletion the expression levels of several organellar proteins were found reduced, in particular mitochondrial proteins, some of which were found to be NatC substrates. Interestingly, knockdown of Naa30 induced the loss of mitochondrial membrane potential and fragmentation of mitochondria. In conclusion, NatC Nt-acetylates a large variety of proteins and is essential for mitochondrial integrity and function.

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.

Biochemical and cellular analysis of Ogden syndrome reveals downstream Nt-acetylation defects.

The X-linked lethal Ogden syndrome was the first reported human genetic disorder associated with a mutation in an N-terminal acetyltransferase (NAT) gene. The affected males harbor an Ser37Pro (S37P) mutation in the gene encoding Naa10, the catalytic subunit of NatA, the major human NAT involved in the co-translational acetylation of proteins. Structural models and molecular dynamics simulations of the human NatA and its S37P mutant highlight differences in regions involved in catalysis and at the interface between Naa10 and the auxiliary subunit hNaa15. Biochemical data further demonstrate a reduced catalytic capacity and an impaired interaction between hNaa10 S37P and Naa15 as well as Naa50 (NatE), another interactor of the NatA complex. N-Terminal acetylome analyses revealed a decreased acetylation of a subset of NatA and NatE substrates in Ogden syndrome cells, supporting the genetic findings and our hypothesis regarding reduced Nt-acetylation of a subset of NatA/NatE-type substrates as one etiology for Ogden syndrome. Furthermore, Ogden syndrome fibroblasts display abnormal cell migration and proliferation capacity, possibly linked to a perturbed retinoblastoma pathway. N-Terminal acetylation clearly plays a role in Ogden syndrome, thus revealing the in vivo importance of N-terminal acetylation in human physiology and disease.

RACK1 (receptor for activated C-kinase 1) interactions with spectrin repeat elements.

Receptor for activated C-kinase 1 (RACK1) is an intracellular scaffolding protein involved in a multitude of signalling pathways. The cytoskeleton is fundamental for intracellular cell signalling as it forms an interconnected network of regulatory proteins. Here, spectrin is a central component as it forms the actin-spectrin network that serves as docking surfaces for cellular components. The interaction between RACK1 and components of spectrin, the single spectrin repeats R16, R17 and the double spectrin repeat R1617 from the α-spectrin chain were investigated by biosensor technology and docking analysis. RACK1 associated only weakly to R16 (KD = 1.0 ± 0.5 × 10(-6) M), about 20 times stronger to R1617 (KD = 5.3 ± 0.7 × 10(-8) M) and 100 times stronger to R17 (KD = 0.9 ± 0.3 × 10(-8) M). Docking analysis showed that while R16 alone preferentially docked with its B-helix, R17 docked through its A-helix and BC loop. The double repeat and RACK1 mainly formed two different complex conformations. R1617 docked tangentially to the N/C-terminal of RACK1 or radially along a groove on the outer surface of RACK1. These configurations could account for the slight increase in entropic and the decrease in enthalpic interactions for the R1617-RACK1 interaction, compared with the interactions of RACK1 to the two single repeats. Our results suggest a mode of interaction that allows spectrin to attach to the N/C part of RACK through the inter-helical AB and BC loops and adopt a multitude of configurations in between the two limiting configurations.

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 ß- and γ-actin, respectively, CoA-Ac-EDDI-NH2 was identified as the best inhibitor with an IC50 value of 120 nM.

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 Naa30288. 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.

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.

LEDGF/p75 has increased expression in blasts from chemotherapy-resistant human acute myelogenic leukemia patients and protects leukemia cells from apoptosis in vitro.

BACKGROUND: Relapse due to chemoresistant residual disease is a major cause of death in acute myelogenous leukemia (AML). The present study was undertaken to elucidate the molecular mechanisms of chemoresistance by comparing differential gene expression in blasts from patients with resistant relapsing AML and chemosensitive AML. RESULTS: About 20 genes were identified as preferentially expressed in blasts pooled from patients with resistant disease, as compared to chemosensitive AML blasts, based on differential gene expression screening. Half of these genes encoded proteins related to protein translation, of these a novel protein related to the ribosomal stalk protein P0. Other upregulated mRNAs coded for cytochrome C oxidase III, the transcription factors ERF-2/TIS11d, and the p75 and p52 splice variants of Lens Epithelial Derived Growth Factor (LEDGF). Analysis of blasts from single patients disclosed that LEDGF/p75 was the most consistently upregulated mRNA in resistant AML. Transfection experiments demonstrated that LEDGF/p75 and p52b antagonized daunorubicin-induced and cAMP-induced apoptosis in an AML cell line. Also HEK-293 cells were protected against daunorubicin by LEDGF/p75 and p52b, whereas LEDGF/p52 splice variants lacking exon 6 had proapoptotic effects. Interestingly, full length LEDGF/p75 protected against truncated pro-apoptotic LEDGF/p75. CONCLUSION: Our results provide evidence for an association between the overexpression of genes encoding survival proteins like LEDGF/p75 and chemo-resistance in acute myelogenous leukemia. LEDGF/p75 has previously not been shown to protect against chemotherapy, and is a potential drug target in AML.

The N-terminal acetyltransferase Naa10 is essential for zebrafish development.

N-terminal acetylation, catalysed by N-terminal acetyltransferases (NATs), is among the most common protein modifications in eukaryotes and involves the transfer of an acetyl group from acetyl-CoA to the α-amino group of the first amino acid. Functions of N-terminal acetylation include protein degradation and sub-cellular targeting. Recent findings in humans indicate that a dysfunctional Nα-acetyltransferase (Naa) 10, the catalytic subunit of NatA, the major NAT, is associated with lethality during infancy. In the present study, we identified the Danio rerio orthologue zebrafish Naa 10 (zNaa10). In vitro N-terminal acetylation assays revealed that zNaa10 has NAT activity with substrate specificity highly similar to that of human Naa10. Spatiotemporal expression pattern was determined by in situ hybridization, showing ubiquitous expression with especially strong staining in brain and eye. By morpholino-mediated knockdown, we demonstrated that naa10 morphants displayed increased lethality, growth retardation and developmental abnormalities like bent axis, abnormal eyes and bent tails. In conclusion, we identified the zebrafish Naa10 orthologue and revealed that it is essential for normal development and viability of zebrafish.

De novo missense mutations in the NAA10 gene cause severe non-syndromic developmental delay in males and females.

Recent studies revealed the power of whole-exome sequencing to identify mutations in sporadic cases with non-syndromic intellectual disability. We now identified de novo missense variants in NAA10 in two unrelated individuals, a boy and a girl, with severe global developmental delay but without any major dysmorphism by trio whole-exome sequencing. Both de novo variants were predicted to be deleterious, and we excluded other variants in this gene. This X-linked gene encodes N-alpha-acetyltransferase 10, the catalytic subunit of the NatA complex involved in multiple cellular processes. A single hypomorphic missense variant p.(Ser37Pro) was previously associated with Ogden syndrome in eight affected males from two different families. This rare disorder is characterized by a highly recognizable phenotype, global developmental delay and results in death during infancy. In an attempt to explain the discrepant phenotype, we used in vitro N-terminal acetylation assays which suggested that the severity of the phenotype correlates with the remaining catalytic activity. The variant in the Ogden syndrome patients exhibited a lower activity than the one seen in the boy with intellectual disability, while the variant in the girl was the most severe exhibiting only residual activity in the acetylation assays used. We propose that N-terminal acetyltransferase deficiency is clinically heterogeneous with the overall catalytic activity determining the phenotypic severity.

Downregulation of N-terminal acetylation triggers ABA-mediated drought responses in Arabidopsis.

N-terminal acetylation (NTA) catalysed by N-terminal acetyltransferases (Nats) is among the most common protein modifications in eukaryotes, but its significance is still enigmatic. Here we characterize the plant NatA complex and reveal evolutionary conservation of NatA biochemical properties in higher eukaryotes and uncover specific and essential functions of NatA for development, biosynthetic pathways and stress responses in plants. We show that NTA decreases significantly after drought stress, and NatA abundance is rapidly downregulated by the phytohormone abscisic acid. Accordingly, transgenic downregulation of NatA induces the drought stress response and results in strikingly drought resistant plants. Thus, we propose that NTA by the NatA complex acts as a cellular surveillance mechanism during stress and that imprinting of the proteome by NatA is an important switch for the control of metabolism, development and cellular stress responses downstream of abscisic acid.

Receptor for activated protein C kinase 1 (RACK1) is overexpressed in papillary thyroid carcinoma.

BACKGROUND: The receptor for activated C kinase 1 (RACK1) has been shown to be overexpressed in several types of cancers such as breast, colon, melanomas, and lung. RACK1 is linked to Ras-Raf-mediated signal transduction and transformed foci formation of 3T3 cells in vitro, and since this pathway is central in papillary thyroid carcinoma (PTC) oncogenesis, we hypothesized that RACK1 could play a role in the development or maintenance of PTC. No report on RACK1 expression in thyroid tissue is available; the present study was therefore aimed at identifying possible correlation of RACK1 expression at the mRNA or protein level in normal thyroid tissue compared to PTC. METHODS: We used TaqMan quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry to study the RACK1 gene and protein expression in matched tumor and nontumor samples from 59 PTC patients. The tumor samples were divided into two main categories, low-risk (group 1-3) and high-risk (group 4-6), in accordance with both histological classification and clinical appearance. RESULTS: RACK1 mRNA and protein levels were found highly overexpressed in tumor samples, whereas Ki-Ras mRNA was found to be relatively unchanged. B-Raf mRNA expression was low and detected only in tumor samples. Sequencing analysis detected no mutations in RACK1 or Ki-Ras, but 62.7% of the patients harbored the B-Raf single-nucleotide substitution T1799A (codon V600E). Phosphorylated extracellular signal-regulated kinase (pERK) immunohistochemistry analysis demonstrated activation of the mitogen-activated protein kinase (MAPK) pathway in tumor cells. Poorly differentiated and undifferentiated PTCs expressed significantly higher RACK1 mRNA levels than well-differentiated PTCs (p<0.017). CONCLUSIONS: Taken together, our findings point to an important role of RACK1 protein in PTC development and progression. Our data also emphasize the importance of assessing protein expression and not only mRNA levels.

The MDM2 promoter SNP285C/309G haplotype diminishes Sp1 transcription factor binding and reduces risk for breast and ovarian cancer in Caucasians.

MDM2 plays a key role in modulating p53 function. The MDM2 SNP309T > G promoter polymorphism enhances Sp1 binding and has been linked to cancer risk and young age at diagnosis although with conflicting evidence. We report a second MDM2 promoter polymorphism, SNP285G > C, residing on the SNP309G allele. SNP285C occurs in Caucasians only, where 7.7% (95% CI 7.6%-7.8%) of healthy individuals carry the SNP285C/309G haplotype. In vitro analyses reveals that SNP309G enhances but SNP285C strongly reduces Sp1 promoter binding. Comparing MDM2 promoter status among different cohorts of ovarian (n = 1993) and breast (n = 1973) cancer patients versus healthy controls (n = 3646), SNP285C reduced the risk of both ovarian (OR 0.74; CI 0.58-0.94) and breast cancer (OR 0.79; CI 0.62-1.00) among SNP309G carriers.

RACK1 regulates Ki-Ras-mediated signaling and morphological transformation of NIH 3T3 cells.

Activating Ras mutations are involved in a significant fraction of human tumors. A suppressor screen using a retroviral mouse fibroblast cDNA library was performed to identify novel factors in Ras-mediated transformation. We identified a novel potent inhibitor of Ras-mediated morphological transformation encoded by a truncated version of the receptor for activated C-kinase (RACK1). The truncated protein, designated RACK1DeltaWD1, lacked the N-terminal 49 amino acids encoding the first of the 7 WD40 repeats in RACK1. RACK1DeltaWD1 expression restored contact inhibition, stress fiber formation and reduced ERK phosphorylation in Ki-Ras transformed NIH 3T3 cells. We demonstrate that truncated RACK1 is involved in complexes consisting of wild-type RACK1 and protein kinase C isoforms alpha, betaI and delta, compromising the transduction of an activated Ras signal to the Raf-MEK-ERK pathway. The cellular localization of RACK1DeltaWD1 differed from wtRACK1, indicating that signaling complexes containing the truncated version of RACK1 are incorrectly localized. Notably, 12-O-tetradecanoyl-13-phorbol acetate (TPA) mediated intracellular translocation of RACK1-interacting PKC alpha and delta was abrogated in RACK1DeltaWD1-expressing cells. Our data support a model where RACK1 acts as a key factor in Ki-Ras-mediated morphological transformation.