Nα-acetyltransferase 60 (NAA60) also known as NatF is a member of the N-terminal

acetyltransferase An acetyltransferase (also referred to as a transacetylase) is any of a class of transferase enzymes that transfers an acetyl group in a reaction called acetylation. In biological organisms, post-translational modification of a protein via acetyl ...

(NAT) family of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s. NATs bind to acetyl-coenzyme A (Ac-CoA) as well as to the protein N-terminus, and

enzymatically An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as produc ...

transfer the

acetyl In organic chemistry, an acetyl group is a functional group denoted by the chemical formula and the structure . It is sometimes represented by the symbol Ac (not to be confused with the element actinium). In IUPAC nomenclature, an acetyl grou ...

group from Ac-CoA to the free backbone

amino group In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

(NH₃⁺) on the first residue of the protein. NATs are mono- or multisubunit enzymes consisting of one catalytic subunit and up to two auxiliary subunits, however, only a catalytic subunit of NAA60 has been identified so far.

Structure

The crystal structure of NAA60 was solved in 2016, though it was missing the C-terminal region due to difficulties purifying the whole protein. The structure revealed an overall fold that is similar to the catalytic subunit of ribosome-associated NATs, and the two residues Y164 and Y165 of the β6-β7 loop important for

peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

anchoring is conserved in NAA60 as in multiple NATs. Most similarities in the active site of NAA60 were found to NAA50. The two NATs share several residues that make up a hydrophobic core recognizing the Nt-Met of their substrates. However, this cavity is larger and more solvent exposed in NAA60 to accommodate larger and more polar residues in position 2 and 3 of its substrates. Through biochemical analysis, the C-terminal end has been found to be important for binding to intracellular membranes but not for its biochemical function. Two amphipathic α-helices near the C-terminus (residues 190-202 and 211-224) have been identified as mediators of the membrane interaction

Subcellular localization

NAA60 localizes to the

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic Cell (biology), cells. Part of the endomembrane system in the cytoplasm, it protein targeting, packages proteins ...

, making it the only organellar bound NAT described to date. NAA10 through NAA50 all exhibit a cytoplasmic or nuclear localization. Both endogenous and overexpressed NAA60 colocalizes with the cis-Golgi marker GM130, as well as identifying it in vesicles colocalizing with markers for peroxisomes,

endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of the endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membra ...

lysosome A lysosome () is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation cent ...

s, and secretory vesicles NAA60 has shown binding preference to membranes containing the phosphatidylinositol 4-phosphate lipid, possibly explaining its residence in Golgi membranes which are rich in these

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

s. All parts of the protein, including the GNAT domain, face the cytosol which limits NAA60`s catalytic activity to the cytosolic side of membranes.

Substrate specificity

NAA60, together with NatC and NatE, is able to acetylate methionine starting N-termini followed by hydrophobic or amphiphatic-type amino acids (ML-, MI-, MF-, MY-, and MK-). NAA60 show further substrate specificity towards membrane proteins with their N-termini facing the cytosol. From an N-terminomic analysis of NAA60 knock-down (KD) cells, 23 substrates of NAA60 were found, of which 21 were membrane proteins located either in the ER, plasma membrane, Golgi, mitochondria, or vesicular membranes.

Disease

NAA60 was found to be important in brain phosphate homeostasis. It was identified as causative of the genetic disease primary familial brian calcification (PFBC). PFBC is a

neurodegenerative disease A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, mul ...

characterized by bilateral calcification distributed in the basal ganglia, thalamus, and cerebellum. Symptoms could be generally grouped as

movement disorder Movement disorders are clinical syndromes with either an excess of movement or a paucity of voluntary and involuntary movements, unrelated to weakness or spasticity. Movement disorders present with extrapyramidal symptoms and are caused by basa ...

cognitive decline Cognitive impairment is an inclusive term to describe any characteristic that acts as a barrier to the cognition process or different areas of cognition. Cognition, also known as cognitive function, refers to the mental processes of how a person ...

, and psychiatric disturbances. In the study by Chelban and Aksnes ''et al''. 10 individuals with PFBC from 7 different families were presented with biallelic variants of NAA60. In NAA60 KD cells, one have previously observed Golgi fragmentation leading to the hypothesis that NAA60 is essential for the N-terminal acetylation of membrane proteins critical for normal Golgi function. Findings from the aforementioned study indicate a decrease in surface levels of SLC20A2 (another kown PFBC-related protein) when NAA60 is lacking, but no clear explanation of the molecular mechanism behind PFBC exists at this time.

References

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