Interaction of the N-Acetyl Transferase Complex NatA with the Ribosome

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Abstract

Proteins are synthesized on cytosolic ribosomes, but in order for proteins to become fully-functional, they often have to undergo a series of modifications. One of the earliest steps of modifications they undertake is the N-terminal protein modification. N-terminal protein modifications can occur co-translationally on the ribosome. One of these modifications is N-terminal acetylation, which is the addition of an acetyl moiety to the αNH2 group of the protein. This modification occurs on over 50% of yeast proteins and over 80% of human proteins, hence it is one of the most common protein modifications. Some of the few known functions of N-terminal acetylation are related to protein stability and degradation, protein-protein interaction, protein-membrane interaction and protein secretion. N-terminal acetyl transferases (NATs) catalyse the N-terminal acetylation reaction. In eukaryotes, there are six types of NATs (NatA-F), which differ in their substrate specificity and also subcellular localisation. NatA is the most abundant NAT and is highly conserved across eukaryotes. NatA modifies proteins typically with an Ala, Cys, Gly, Ser, Val or Thr at the second residue position (P2) following the removal of the N-terminal initiator methionine, by the action of methionine aminopeptidase (MetAP). NatA is composed of two subunits, an auxiliary subunit (Naa15) and a catalytic subunit (Naa10). Mutations of the NAA10 gene can cause human disease, such as Ogden syndrome and intellectual disability. The auxiliary subunit is typically required for ribosome interaction of the NAT complexes. NatE is distinct in that it lacks a dedicated auxiliary subunit of its own; instead its catalytic subunit (Naa50) associates with the NatA complex. In yeast, NatA/E binds quantitatively to the ribosome in a salt-sensitive manner. Therefore, the aims of this project are: identify the binding site of NatA on the ribosome, define components of Naa15 required for ribosome binding and NatE complex formation, and investigate the significance of ribosome interaction on NatA functions in S. cerevisiae. Data obtained from this project suggests that Naa15 binds quantitatively to the ribosome and facilitates ribosome-association of the catalytic Naa10 subunit. Ribosome binding involves an ionic interaction between basic residues in the N-terminus of Naa15 and the negatively charged surface of the ribosome. Naa15 binds adjacent to ribosomal proteins eL38 and eL31 as well as the nascent polypeptide-associated complex (NAC). Furthermore, ribosome association of Naa50 is dependent upon Naa10 as well as Naa15, as deletion of any of the NatA subunits can inhibit Naa50 interaction with the ribosome. In addition, Mutations that mimic human disease-associated mutations S37P (S39P) and R116W (R159W) both reduce complex formation between Naa15 and Naa10 as well as association of Naa50 with the NatA complex. Naa15 mutants defective in ribosome interaction, but not NatA/E complex-formation, have growth defects at elevated temperatures and defective N-terminal acetylation of selective NatA substrates. For example, His3 and Chk1 appear unaffected by the loss of ribosome binding of NatA, however, MS-Pdi1 as well as ribosomal proteins Rps12, Rps16A and Rpl16B are deficiently acetylated when NatA binding is lost. Overall, data suggest that Naa15 positions Naa10 at the ribosome to facilitate co-translational N-terminal acetylation. This localization is important for the modification of some but not all substrates.

Keyword(s)

N-terminal acetylation; Nascent chain; NatA; Protein biogenesis; Ribosome; Translation

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