Amino terminal acetylation and protein synthesis in eukaryotes

The amino terminal residue in many proteins is N-acetylated. Available evidence indicates that acetylation occurs during translation whilst the nascent polypeptide is still attached to the ribosome; with some proteins, like the actins, an additional acetylation step occurs which may be catalysed by cytoplasmic enzymes. Studies of protein synthesis in heterologous cell-free systems show that the enzymes which catalyse amino terminal acetylation are ubiquitous and such studies have provided insights into the effects of limitations in the supply of acetyl CoA in vitro. Evidence that availability of acetyl groups in vivo may in part determine the cellular concentration of N-acetylated proteins is examined in terms of the relative amounts of the aceylated fraction of foetal haemoglobin and carbonic anhydrase I in human erythrocytes. Tissue and developmental differences in the concentration of the muscle isoenzyme, carbonic anhydrase III, are considered in terms of changes in the activity and distribution of mitochondria in muscle. Finally, the possibility is discussed that differential effects of thyroid hormone on protein concentration might be partly due to an effect of the hormone on acetyl CoA availability via its action on mitochondria.