N-terminal acetylation of the nascent chains of α-crystallin

Abstract

Labeling of a cell-free extract derived from eye lens tissue with (35S) Met-tRNAfMet results in α-crystallin polypeptides which bear acetyl-(35S)methionine in N-terminal position. An attempt has been made to determine the exact moment of acetylation. For that purpose peptidyl-tRNAs were isolated in a one-step procedure which is based on a specially designed sucrose gradient referred to as strip-gradient. Separation of the nascent chains has been carried out according to their molecular weights. Our previous suggestion that N-terminal acetylation takes place while the peptide is still on the ribosome was confirmed. It is demonstrated that acetylation starts when the polypeptide chain is about 25 amino acids long and is virtually completed when the chains have reached a length of about 50 amino acids. Acetyl-CoA appears to deliver the acetyl group for the N-terminal acetylation.