Protein acetylation
Protein acetylation are acetylation reactions that occur within living cells as drug metabolism, by enzymes in the liver and other organs. Pharmaceuticals frequently employ acetylation to enable such esters to cross the blood–brain barrier, where they are deacetylated by enzymes in a manner similar to acetylcholine. Examples of acetylated pharmaceuticals are diacetylmorphine, acetylsalicylic acid, THC-O-acetate, and diacerein. Conversely, drugs such as isoniazid are acetylated within the liver during drug metabolism. A drug that depends on such metabolic transformations in order to act is termed a prodrug.
Acetylation is an important modification of proteins in cell biology; and proteomics studies have identified thousands of acetylated mammalian proteins. Acetylation occurs as a co-translational and post-translational modification of proteins, for example, histones, p53, and tubulins. Among these proteins, chromatin proteins and metabolic enzymes are highly represented, indicating that acetylation has a considerable impact on gene expression and metabolism. In bacteria, 90% of proteins involved in central metabolism of Salmonella enterica are acetylated.
N-terminal acetylation
acetylation is one of the most common co-translational covalent modifications of proteins in eukaryotes, and it is crucial for the regulation and function of different proteins. N-terminal acetylation plays an important role in the synthesis, stability and localization of proteins. About 85% of all human proteins and 68% in yeast are acetylated at their Nα-terminus. Several proteins from prokaryotes and archaea are also modified by N-terminal acetylation.N-terminal Acetylation is catalyzed by a set of enzyme complexes, the N-terminal acetyltransferases. NATs transfer an acetyl group from acetyl-coenzyme A to the α-amino group of the first amino acid residue of the protein. Different NATs are responsible for the acetylation of nascent protein N-terminal, and the acetylation was found to be irreversible so far.
N-terminal acetyltransferases
To date, seven different NATs have been found in humans - NatA, NatB, NatC, NatD, NatE, NatF and NatH. Each of these different enzyme complexes is specific for different amino acids or amino acid sequences which is shown in the following table.Table 1. The Composition and Substrate specificity of NATs.
| NAT | Subunits | Substrates |
| NatA | Naa10 Naa15 | Ser-, Ala-, Gly-, Thr-, Val-, Cys- N-termini |
| NatB | Naa20 Naa25 | Met-Glu-, Met-Asp-, Met-Asn-, Met-Gln- N-termini |
| NatC | Naa30 Naa35 Naa38 | Met-Leu-, Met-Ile-, Met-Trp-, Met-Phe- N-termini |
| NatD | Naa40 | Ser-Gly-Gly-, Ser-Gly-Arg- N-termini |
| NatE | Naa50 Naa10 Naa15 | Met-Leu-, Met-Ala-, Met-Lys-, Met-Met- N-termini |
| NatF | Naa60 | Met-Lys-, Met-Leu-, Met-Ile-, Met-Trp-, Met-Phe- N-termini |
| NatH | Naa80 | Actin- N-termini |
NatA
NatA is composed of two subunits, the catalytic subunit Naa10 and the auxiliary subunit Naa15. NatA subunits are more complex in higher eukaryotes than in lower eukaryotes. In addition to the genes NAA10 and NAA15, the mammal-specific genes NAA11 and NAA16, make functional gene products, which form different active NatA complexes. Four possible hNatA catalytic-auxiliary dimers are formed by these four proteins. However, Naa10/Naa15 is the most abundant NatA.NatA acetylates Ser, Ala-, Gly-, Thr-, Val- and Cys N-termini after the initiator methionine is removed by methionine amino-peptidases. These amino acids are more frequently expressed in the N-terminal of proteins in eukaryotes, so NatA is the major NAT corresponding to the whole number of its potential substrates.
Several different interaction partners are involved in the N-terminal acetylation by NatA. Huntingtin interacting protein K interacts with hNatA on the ribosome to affect the N-terminal acetylation of a subset of NatA substrates. Subunits hNaa10 and hNaa15 will increase the tendency for aggregation of Huntingtin if HYPK is depleted. Hypoxia-inducible factor -1α has also been found to interact with hNaa10 to inhibit hNaa10-mediated activation of β-catenin transcriptional activity.
NatB
NatB complexes are composed of the catalytic subunit Naa20p and the auxiliary subunit Naa25p, which are both found in yeast and humans. In yeast, all the NatB subunits are ribosome-associated; but in humans, NatB subunits are both found to be ribosome-associated and non-ribosomal form. NatB acetylates the N-terminal methionine of substrates starting with Met-Glu-, Met-Asp-, Met-Asn- or Met-Gln- N termini.NatC
NatC complex consists of one catalytic subunit Naa30p and two auxiliary subunits Naa35p and Naa38p. All three subunits are found on the ribosome in yeast, but they are also found in non-ribosomal NAT forms like Nat2. NatC complex acetylates the N-terminal methionine of substrates Met-Leu-, Met-Ile-, Met-Trp- or Met-Phe N-termini.NatD
NatD is only composed with the catalytic unit Naa40p and Naa40p and it is conceptually different form the other NATs. At first, only two substrates, H2A and H4 have been identified in yeast and humans. Secondly, the substrate specificity of Naa40p lies within the first 30-50 residues which are quite larger than the substrate specificity of other NATs. The acetylation of histones by NatD is partially associate with ribosomes and the amino acids substrates are the very N-terminal residues, which makes it different from lysine N-acetyltransferases.NatE
NatE complex consists with subunit Naa50p and two NatA subunits, Naa10p and Naa15p. The N terminus of Naa50p substrates is different from those acetylated by the NatA activity of Naa10p. NAA50 in plants is essential to control plant growth, development, and stress responses and NAA50 function is highly conserved between humans and plants.NatF
NatF is a NAT that is composed of the Naa60 enzyme. Initially, it was thought that NatF was only found in higher eukaryotes, since it was absent from yeast. However, it was later found that Naa60 is found throughout the eukaryotic domain, but was secondarily lost in the fungi lineage. Compared to yeast, NatF contributes to the higher abundance of N-terminal acetylation in humans. NatF complex acetylates the N-terminal methionine of substrates Met-Lys-, Met-Leu-, Met-Ile-, Met-Trp- and Met-Phe N termini which are partly overlapping with NatC and NatE. NatF has been shown to have an organellar localization and acetylates cytosolic N-termini of transmembrane proteins. The organellar localization of Naa60 is mediated by its unique C-terminus, which consists of two alpha helices that peripherally associate with the membrane and mediate interactions with PI(4)P.NAA80/NatH
NAA80/NatH is an N-terminal acetyltransferase that specifically acetylates the N-terminus of actin.N-terminal acetylation function
Protein stability
N-terminal acetylation of proteins can affect protein stability, but the results and mechanism were not very clear until now. It was believed that N-terminal acetylation protects proteins from being degraded as Nα-acetylation N-termini were supposed to block N-terminal ubiquitination and subsequent protein degradation. However, several studies have shown that the N-terminal acetylated protein have a similar degradation rate as proteins with a non-blocked N-terminus.Protein localization
N-terminal acetylation has been shown that it can steer the localization of proteins. Arl3p is one of the 'Arf-like' GTPases, which is crucial for the organization of membrane traffic. It requires its Nα-acetyl group for its targeting to the Golgi membrane by the interaction with Golgi membrane-residing protein Sys1p. If the Phe or Tyr is replaced by an Ala at the N-terminal of Arl3p, it can no longer localized to the Golgi membrane, indicating that Arl3p needs its natural N-terminal residues which could be acetylated for proper localization.Metabolism and apoptosis
Protein N-terminal acetylation has also been proved to relate with cell cycle regulation and apoptosis with protein knockdown experiments. Knockdown of the NatA or the NatC complex leads to the induction of p53-dependent apoptosis, which may indicate that the anti-apoptotic proteins were less or no longer functional because of reduced protein N-terminal acetylation. But in contrast, the caspase-2, which is acetylated by NatA, can interact with the adaptor protein RIP associated Ich-1/Ced-3 homologous protein with a death domain. This could activate caspase-2 and induce cell apoptosis.Protein synthesis
proteins play an important role in the protein synthesis, which could also be N-terminal acetylated. The N-terminal acetylation of the ribosome proteins may have an effect on protein synthesis. A decrease of 27% and 23% in the protein synthesis rate was observed with NatA and NatB deletion strains. A reduction of translation fidelity was observed in the NatA deletion strain and a defect in ribosome was noticed in the NatB deletion strain.Cancer
NATs have been suggested to act as both onco-proteins and tumor suppressors in human cancers, and NAT expression may be increased and decreased in cancer cells. Ectopic expression of hNaa10p increased cell proliferation and up regulation of gene involved in cell survival proliferation and metabolism. Overexpression of hNaa10p was in the urinary bladder cancer, breast cancer and cervical carcinoma. But a high level expression of hNaa10p could also suppress tumor growth and a reduced level of expressed hNaa10p is associated with a poor prognosis, large tumors and more lymph node metastases.Table 2. Overview of the expression of NatA subunits in various cancer tissues
| Nat subunits | Cancer tissue | Expression pattern |
| hNaa10 | lung cancer, breast cancer, colorectal cancer, hepatocellular carcinoma | high in tumors |
| hNaa10 | lung cancer, breast cancer, pancreatic cancer, ovarian cancer | loss of heterozygosity in tumors |
| hNaa10 | breast cancer, gastric cancer, lung cancer | high in primary tumors, but low with lymph node metastases |
| hNaa10 | Non-small cell lung cancer | low in tumors |
| hNaa15 | papillary thyroid carcinoma, gastric cancer | high in tumors |
| hNaa15 | neuroblastoma | high in advanced stage tumors |
| hNaa11 | hepatocellular carcinoma | loss of heterozygosity in tumors |
Lysine acetylation and deacetylation
Proteins are typically acetylated on lysine residues and this reaction relies on acetyl-coenzyme A as the acetyl group donor.In histone acetylation and deacetylation, histone proteins are acetylated and deacetylated on lysine residues in the N-terminal tail as part of gene regulation. Typically, these reactions are catalyzed by enzymes with histone acetyltransferase or histone deacetylase activity, although HATs and HDACs can modify the acetylation status of non-histone proteins as well.
The regulation of transcription factors, effector proteins, molecular chaperones, and cytoskeletal proteins by acetylation and deacetylation is a significant post-translational regulatory mechanism These regulatory mechanisms are analogous to phosphorylation and dephosphorylation by the action of kinases and phosphatases. Not only can the acetylation state of a protein modify its activity but there has been recent suggestion that this post-translational modification may also crosstalk with phosphorylation, methylation, ubiquitination, sumoylation, and others for dynamic control of cellular signaling. The regulation of tubulin protein is an example of this in mouse neurons and astroglia. A tubulin acetyltransferase is located in the axoneme, and acetylates the α-tubulin subunit in an assembled microtubule. Once disassembled, this acetylation is removed by another specific deacetylase in the cell cytosol. Thus axonemal microtubules, which have a long half-life, carry a "signature acetylation," which is absent from cytosolic microtubules that have a shorter half-life.
In the field of epigenetics, histone acetylation have been shown to be important mechanisms in the regulation of gene transcription. Histones, however, are not the only proteins regulated by posttranslational acetylation. The following are examples of various other proteins with roles in regulating signal transduction, whose activities are also affected by acetylation and deacetylation.